Progress Towards Onchocerciasis Elimination in Senegal’s Historical Transmission Zones, Following Sustained Annual Mass Drug Administration and Multilevel Surveillance

Progress Towards Onchocerciasis Elimination in Senegal’s Historical Transmission Zones, Following Sustained Annual Mass Drug Administration and Multilevel Surveillance | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Progress Towards Onchocerciasis Elimination in Senegal’s Historical Transmission Zones, Following Sustained Annual Mass Drug Administration and Multilevel Surveillance Djiby Sow, Mamadou Alpha Diallo, Rose Monteil, Aita Sene, Achille Kabore, and 10 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-9176663/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 13 You are reading this latest preprint version Abstract Background Onchocerciasis remains a major public health problem in sub-Saharan Africa, despite decades of control efforts. In Senegal, transmission was historically focal but intense along the Gambia and Falémé river basins in the southeast of the country. Building on the legacy of the Onchocerciasis Control Programme and the African Programme for Onchocerciasis Control, Senegal implemented long-term community-directed mass drug administration (MDA) with ivermectin, aiming to interrupt local transmission and progress towards elimination. Methods A retrospective descriptive analysis was conducted based on data from the Senegalese National Onchocerciasis Control Programme between 1998 and 2022. Annual MDA reports were used to estimate geographical and therapeutic coverage across endemic districts. Epidemiological impact was assessed through serial parasitological surveys (skin snips), entomological pool-screen PCR of Simulium blackflies, and Ov16 IgG4 ELISA performed on dried blood spots collected from children aged 5–9 years in the Gambia and Falémé river basins. These data were interpreted against WHO thresholds for stopping MDA and for verification of elimination. Results From 1998 onwards, geographical coverage rapidly reached 100% of identified endemic villages and was maintained throughout the programme. Therapeutic coverage remained around 78–80% of the total population annually, exceeding the WHO minimum target of 65%. Microfilarial prevalence fell from hyperendemic baseline levels (> 15% with nodules in some villages) to near zero within a decade. In the most recent surveys (2020 and 2022), skin snip examinations of more than 1,100 individuals across four historically endemic districts detected no microfilariae. Entomological pool-screening of hundreds of thousands of Simulium flies showed infection rates generally below 0.2%, with several years at zero and values consistently under the WHO threshold for sustainable transmission. In 2022, none of the 6,227 children tested by Ov16 ELISA (3,077 from the Gambia basin and 3,150 from the Falémé basin) was seropositive, indicating the absence of recent exposure to Onchocerca volvulus . Conclusion High and sustained coverage with community-directed ivermectin MDA, combined with rigorous parasitological, entomological and serological surveillance, has led to the apparent interruption of O. volvulus transmission in Senegal’s historical foci. MDA was safely stopped nationwide in 2022, and the country has entered a three-year post-treatment surveillance phase in line with WHO guidelines. Maintaining integrated surveillance and cross-border collaboration will be essential to prevent recrudescence and to support Senegal’s dossier for WHO verification of onchocerciasis elimination. Onchocerciasis Onchocerca volvulus Ivermectin Mass drug administration Elimination Senegal Figures Figure 1 Figure 2 Figure 3 Figure 4 Background Onchocerciasis, or “river blindness,” is a parasitic disease caused by the nematode Onchocerca volvulus and transmitted to humans by the bite of Simulium blackflies that breed along rivers. It causes severe skin lesions (itching, depigmentation) and eye lesions that can lead to irreversible blindness [ 1 ]. More than 99% of infected individuals live in sub-Saharan Africa [ 2 ]. In 2017, an estimated 14.6 million people suffered from onchocerciasis-related skin manifestations and 1.15 million were blind due to the disease, underscoring the public health importance of this neglected tropical disease [ 2 – 4 ]. Recent syntheses continue to document programmatic gains across Africa [ 5 , 6 ]. In West Africa, a large Onchocerciasis Control Program (OCP) was implemented from 1974 to 2002, bringing the endemic disease under control in many historically affected foci. The OCP initially relied on vector control (insecticide spraying by helicopter over blackfly breeding sites), followed by chemoprevention with ivermectin starting in 1989 [ 2 , 4 , 7 , 8 ]. Ivermectin (brand name Mectizan®) is a microfilaricidal antiparasitic drug administered at least once per year for 10 to 15 years to gradually reduce the reservoir of microfilariae in humans. Thanks to the donation of ivermectin by its manufacturer since 1987 and the community-directed treatment strategy promoted by the African Program for Onchocerciasis Control (APOC) from 1995 onward, mass treatment coverage has greatly expanded across Africa. Before the start of control/elimination activities, the endemic foci were delineated using rapid epidemiological approaches/ rapid epidemiological mapping of onchocerciasis (REA/REMO) based on the prevalence of palpable onchocercal nodules in adults living long-term in sentinel communities. In these approaches, 30–50 adults (typically ≥ 20 years old and resident ≥ 10 years) are examined per selected village, and areas where nodule prevalence exceeds 20% are classified as high-risk and prioritized for intervention, an operational threshold adopted for large-scale mapping and programme planning. These baseline mapping results informed the selection of intervention zones and provide the pre-control reference against which the later impact of ivermectin-based strategies can be interpreted [ 9 ]. These coordinated efforts have transformed onchocerciasis from a major cause of blindness into a disease on the path to elimination in several countries [ 8 ]. Following APOC’s closure in 2015, WHO/AFRO launched the Expanded Special Project for Elimination of Neglected Tropical Diseases (ESPEN) in 2016 to sustain and accelerate country programmes targeting the five Preventive Chemotherapy-NTDs, including onchocerciasis [ 10 ]. ESPEN now supports national programmes and hosts the sub-national ESPEN Portal for planning and evaluation [ 6 , 10 ]. Momentum has accelerated in recent years. Meta-analytic evidence indicates that programs achieving and maintaining high therapeutic coverage are most likely to meet elimination-of-transmission (EOT) criteria [ 5 ]. At the regional policy level, the Global Onchocerciasis Network for Elimination (GONE) facilitates data-sharing and coordinated action, and in January 2025 WHO verified Niger as the first country in the African Region to eliminate onchocerciasis—an important proof-point for feasibility at national scale [ 11 , 12 ]. In Senegal, onchocerciasis occurs only in focal areas in the southeastern part of the country. The main endemic foci are located along the Gambia and Falémé river basins (the Falémé being a tributary of the Senegal River) and their tributaries (Fig. 1 ) [ 2 ]. This southeastern zone, corresponding to the former health districts of Kedougou (fully affected), Goudiry, Tambacounda (partially affected), and Vélingara [ 13 ], covers approximately 36,000 km² (about 18% of the national territory). In 2002, 611 at-risk villages were recorded in this region, comprising more than 160,000 inhabitants exposed to onchocerciasis. Before interventions, the region was hyperendemic, with up to 17% of the population presenting with subcutaneous nodules in certain sentinel villages [ 2 , 13 ]. Subsequently, results from Senegal and similar foci in Mali empirically demonstrated that mass treatment of sufficient duration and coverage can eliminate parasite transmission, providing the first proof-of-principle that onchocerciasis elimination was feasible in Africa [ 7 , 14 ]. These findings directly informed the global paradigm shift from onchocerciasis control to elimination, which was formally endorsed in 2016 with the release of the new WHO guidelines for stopping mass drug administration and verifying elimination. Building on this scientific evidence and the revised global policy framework, Senegal adopted elimination as its national objective, defined as the complete cessation of local transmission. In 2022, after more than two decades of mass ivermectin administration, the country halted treatment in all previously endemic communities, which are now considered free of the disease [ 2 , 3 , 13 ]. Senegal’s national onchocerciasis control program began chemoprevention activities in 1988–1989 under the OCP [ 8 , 13 ] and adopted a community-directed treatment strategy on a large scale starting in 1998 [ 2 ]. From that point forward, annual campaigns of free ivermectin distribution were organized in all identified endemic villages, in collaboration with local communities (via community volunteers). The initial goal was to control morbidity (prevent blindness and skin lesions) by treating at least 65% of the at-risk population each year, a threshold considered necessary to interrupt transmission in the long term. The present article provides a scientific overview of this trajectory toward elimination, analyzing data on treatment coverage and epidemiological impact from Senegal’s ivermectin chemoprevention program. Methodology Study Design: This retrospective descriptive study analysed routinely collected programme data from the Senegalese National Onchocerciasis Control/Elimination Programme from 1998 through 2024, covering the scale-up of community-directed ivermectin mass drug administration (MDA), the national stop-MDA decision in 2022, and the early post-treatment surveillance (PTS) period. In addition to annual MDA reports used to quantify geographical and therapeutic coverage, we included multilevel surveillance outputs, parasitological and serological assessments in endemic foci and post-MDA entomological monitoring in 2023–2024 (including O-150 PCR pool screen testing of Simulium vectors), to document progress toward, and confirm, interruption of transmission as required during PTS. Study Area: The activities focused on the southeastern region of Senegal corresponding to the endemic foci of onchocerciasis. Initially, four health districts were involved (Kedougou, Tambacounda, Goudiry, and Vélingara). Over time, due to administrative reorganization and refined mapping of endemicity, the program expanded to operate in a total of eight endemic health districts. These districts cover river basins bordering Mali (Falémé basin) and those bordering Guinea and Gambia (Gambia River basin), where transmission is seasonal (especially during and after the rainy season, when blackfly populations proliferate) [ 14 ]. The total population in the endemic areas was estimated at approximately 600,000 inhabitants by the end of the program (WHO, 2023). For programmatic purposes, Senegal grouped all historically endemic districts into two Operational Transmission Zones (OTZs), aligned with the Gambia and Falémé river basins. Each OTZ encompasses neighbouring districts linked by shared Simulium breeding sites and common onchocerciasis transmission dynamics. Epidemiological and entomological survey results are therefore analyzed and interpreted at this OTZ level. Ivermectin chemoprevention: Annual mass distributions of ivermectin were carried out in all identified at-risk villages. The drug was administered by trained community volunteers under the supervision of health personnel. All community members were targeted for treatment, except for children under 5 years of age (approximately < 90 cm, the height below which ivermectin is not recommended), pregnant women, and severely ill individuals, in accordance with the contraindications. This non-treatable fraction represented approximately 13–15% of the total population, leaving about 85% eligible for treatment. During each annual campaign, a community census was conducted to enumerate the resident population and estimate the number of tablets required. After distribution, the number of individuals who were effectively treated (i.e., those who ingested the medication under direct observation) was reported on a village-by-village basis using standardized forms. Coverage indicators: Two main indicators were monitored annually: Geographical coverage: Defined as the proportion of eligible villages that actually received ivermectin distribution (target: 100%). Therapeutic coverage: Defined as the proportion of the total enumerated population that received treatment within the covered villages. The WHO recommends achieving at least 65% coverage of the total population in each community (which corresponds to approximately 80% of the treatment-eligible population) to sufficiently impact transmission. In our analysis, we calculated the overall annual therapeutic coverage as the ratio of the number of individuals treated to the total enumerated population across all covered villages, expressed as a percentage [ 15 , 16 ]. Epidemiological surveillance: The program established a network of sentinel villages to monitor the infection. In these villages, parasitological surveys were conducted before the initiation of treatment and at regular intervals (every 2 to 3 years). The surveys included clinical examinations and skin snip collections from a sample of voluntary inhabitants aged ≥ 5 years to detect the presence of O. volvulus microfilariae in the skin and determine the prevalence of microfilaridermia (the percentage of infected individuals). For example, 12 sentinel villages were evaluated in 1996 (prior to intervention), 20 in 2000, and up to 31 in 2001 [ 4 , 17 ]. The number of individuals examined per survey ranged from approximately 1,400 in 1996 to around 6,300 in 2001, reflecting the expansion of the surveillance network. In parallel, entomological surveys were conducted to assess vector transmission. Simulium blackflies were captured at biting sites along rivers in the endemic foci and then sent to the reference laboratory (WHO’s Centre for Multidisease Surveillance in Ouagadougou, Burkina Faso) for molecular analyses [ 4 , 18 ]. The heads of the blackflies were tested using PCR amplification with the O-150 marker to detect any infectious O. volvulus larvae [ 19 ]. The vector infectivity rate (i.e., the proportion of flies carrying infectious L3 larvae) was thus estimated and monitored over time, providing a direct indicator of parasite transmission to humans [ 20 ]. Serological testing by ELISA Ov16 The detection of IgG4 antibodies specific to Onchocerca volvulus was performed using the Ov16 ELISA on dried blood spots (DBS), following standardized procedures described by Dusabiman A. et al [ 21 ]. The antigen used was recombinant Ov16-GST, coated onto flat-bottom Immunolon 2HB microplates. After sample elution and blocking with PBST-BSA, samples and standards were applied in duplicate, followed by incubation with biotin-conjugated anti-human IgG4 antibodies and streptavidin-alkaline phosphatase. The colorimetric reaction was developed using the pNPP substrate and read at 404 nm using a microplate reader. A standard curve with 8 dilutions (640 to 5 units) was included on each plate to ensure consistency and allow semi-quantitative interpretation. Positive controls consisted of humanized anti-Ov16 monoclonal antibodies. Only results meeting predefined quality control thresholds were retained for next steps. The analysis threshold was set at 40 arbitrary units. Samples yielding optical density (OD) values ≥ 40 in duplicate were considered putatively positive and retested; confirmed positives were those maintaining OD ≥ 40 for Ov16 and < 40 when tested against GST alone [ 21 , 22 ]. Data analysis: Data analysis was performed using R Studio version 4.3.1 for statistical processing and Python (with the Pandas and Matplotlib libraries) for data visualization. Data were aggregated by year and by district. We constructed a time series of annual geographical and therapeutic coverage and compared these indicators to the recommended targets. The results of the parasitological surveys were compiled to examine the evolution of infection prevalence (particularly among children who had experienced only the treatment era) and the number of villages where transmission persisted. Finally, we interpreted the Senegalese results in comparison with those reported from other regions that implemented similar mass ivermectin treatment strategies, notably in West Africa (neighboring countries) and in the Americas, to identify common success factors and challenges. Results Mass treatment coverage From the inception of the Senegalese mass drug administration (MDA) program in 1998, geographic coverage was optimal. Every village identified as endemic was reached by the annual ivermectin distribution. For example, in 2000, 590 out of 590 eligible villages were treated (100% geographic coverage). This performance was maintained subsequently, with 100% of the 611 targeted villages being effectively covered in 2001–2002. Moreover, the number of villages covered increased during the initial years, rising from 511 villages in 1998 (during the initial mapping) to 611 in 2002, thanks to the inclusion of additional peripheral villages as needed (Fig. 2). Figure 2: Mass treatment coverage by district between 2001 and 2007 Regarding therapeutic coverage, the program successfully treated a high proportion of the at-risk population each year. Figure 3 presents some coverage indicators from 1998 (the first full year of the CDTI program) to 2002. The number of people treated increased from approximately 104,000 in 1998 to nearly 130,000 in 2002, while the recorded population in covered villages rose from 136,000 to 163,000 over the same period (partly due to population growth and the expansion of treatment to new villages). Thus, the overall therapeutic coverage remained around 78–80% annually. This rate reflects the proportion of the entire population in covered villages who received ivermectin. It far exceeds the minimum threshold of 65% set for program efficacy and approaches nearly the entire eligible population (considering ~ 15% of people excluded due to contraindications). Additionally, over 90% of villages individually met the criterion of ≥ 65% coverage within their population by the program’s second year, demonstrating nearly universal community adherence to the treatments. Epidemiological surveillance: Entomology Overall, eight entomological surveys using O‑150 pool‑screen PCR were conducted between 2006 and 2020 in the Falémé and Gambia river basins (Table 1 ). Infection rates were below 0.2% in all these years except Falémé in 2006 (0.79%) and 2010 (0.82%), while Gambia showed a modest increase to 0.31% in 2019. Table 1 Summary of Simulium pool‑screen surveys for the first breeding season Year Falémé n (%) Gambia n (%) 2006 2,266 (0.787) 43,498 (0.158) 2007 25,933 (0.0) 95,767 (0.0) 2008 52,928 (0.0) 89,057 (0.0) 2010 50,172 (0.819) 250,519 (0.0) 2015 12,428 (0.0) 6,066 (0.0) 2016 35,576 (0.0) 36,962 (0.0) 2019 47,371 (0.19) 64,548 (0.308) 2020 20,270 (0.034) 48,925 (0.221) Serology and microfilariae (OTZ level) Historical serological and skin‑snip surveys summarised at OTZ level document the progressive decline of infection from baseline to the 2019 Stop‑MDA assessments (Table 2 ). At baseline in 1987–1988, combined Ov16 seroprevalence exceeded 35% and microfilarial prevalence was above 30%, whereas by 2001–2003 MF prevalence had already fallen below 1% and Ov16 seroreactivity to around 1% in the combined foci. Table 2 Serology and microfilariae (OTZ level) OTZ Year Serology tested Serology positive Serology prevalence (%) MF tested MF prevalence (%) Combined 1987 704 296 42.05 505 33.893 Combined 1988 3095 1176 38.0 1088 35.153 Combined 1996 0 0 NA 135 9.534 Combined 1999 0 0 NA 70 2.464 Combined 2000 0 0 NA 119 3.31 Combined 2001 6300 27 0.43 3 0.066 Combined 2002 4099 44 1.07 5 0.151 Combined 2003 3042 7 0.23 1 0.048 Faleme 2006 0 0 NA 3 0.128 Faleme 2007 0 0 NA 2 0.093 Faleme 2008 0 0 NA 8 0.666 Faleme 2010 0 0 NA 3 0.265 Faleme 2014 1510 20 1.32 4 0.388 Faleme 2019 3077 0 0.0 0 NA Gambie 2006 0 0 NA 1 0.03 Gambie 2007 0 0 NA 0 NA Gambie 2008 0 0 NA 2 0.131 Gambie 2010 1540 0 0.0 0 NA Gambie 2014 3381 46 1.36 14 0.529 Gambie 2019 3152 0 0.0 0 NA In 2014, low‑level Ov16 positivity (1.3–1.4%) persisted in both OTZs, but by 2019 no seropositive children and no microfilariae were detected in either the Gambie or Falémé OTZ, consistent with interruption of recent transmission; these trends in microfilarial prevalence are illustrated in Fig. 4 . Skin‑snip surveys by district Results of the 2020 and 2022 skin-snip surveys in the four historically endemic districts of the Gambia basin (Goudiry, Kedougou, Tambacounda, and Velingara) confirmed the absence of O. volvulus microfilariae. In 2020, a total of 489 individuals were examined across the four districts, and in 2022, a further 623 individuals were examined. No microfilariae were detected in any of the samples, resulting in a crude prevalence of 0.0%, an age-standardised prevalence of 0.0%, and a CMFL of 0.0 mfs/ss in all districts for both survey years. Discussion Over the course of more than two decades, community-directed mass ivermectin administration in Senegal has achieved near-universal coverage and had a profound epidemiological impact. Geographical coverage reached all known endemic villages approaching 100%, and therapeutic coverage consistently remained around 78–80% of the total population each year, well above the World Health Organization (WHO) recommended minimum (approximately 65% of the total population, equivalent to 80% of the eligible population) needed to break transmission [ 15 ]. This scale-up aligns with global elimination goals and reflects the long arc from OCP-era control to elimination-focused strategies in Africa [ 23 ]. At baseline (pre-MDA; 1987–1988 skin-snip surveys), district-level microfilaridermia prevalence indicated one hypoendemic district (Salémata), two mesoendemic districts (Kédougou and Saraya), and no hyperendemic district by district-mean prevalence, although village-level prevalence reached hyperendemic levels (≥ 60%) in some sentinel communities [ 24 ]. In addition to eye and skin issues, cutting down on transmission also helps prevent neurological problems. Studies in Cameroon have shown that children with higher levels of microfilariae are more likely to develop epilepsy later in life. This highlights how important it is for public health efforts to aim for elimination, benefiting communities in many ways. [ 25 ]. Entomological assessments also showed a collapse in transmission: hundreds of thousands of vector blackflies were tested, with either zero infective larvae found or rates far below the threshold for sustainable transmission (< 0.5 infected flies per 1,000 flies) [ 26 ]. These very low infectivity levels are consistent with WHO stop-MDA benchmarks and with programme experiences reporting zero or near-zero infective flies when transmission is no longer sustainable [ 27 ]. Early demonstrations from Mali–Senegal “proof-of-principle” zones previewed these dynamics, showing that sustained high-coverage annual MDA can reduce child infection to zero and meet entomological thresholds for stopping. The Senegalese experience clearly shows that annual mass ivermectin chemoprevention can not only control but actually eliminate onchocerciasis, as long as exceptionally high coverage is achieved and maintained over time. Remarkably, just 4–5 years of consecutive annual treatments reduced microfilarial prevalence to nearly zero in children, effectively stopping transmission much faster than the originally projected 10–15 years [ 28 ]. This quick success is due to three main factors: first, community adherence was high, with 78–80% of the total population treated annually which is above the minimum coverage historically targeted for control (65% of total population) and consistent with elimination frameworks showing that achieving ≥ 80% of the eligible population is strongly associated with interruption of transmission [ 29 ]; second, a strongly seasonal transmission cycle—limited to a few months each year, which reduces chances of re-infection ; and third, the absence of Loa loa co-endemicity, which makes safe ivermectin delivery more difficult in Central Africa [ 30 ]. Where Loa loa is co-endemic or adherence is systematically low, persistent transmission has been documented despite many MDA rounds—highlighting context-specific barriers that Senegal largely avoided [ 31 , 32 ]. Together, these factors enabled Senegal to use ivermectin’s microfilaricidal effect without significant logistical or safety challenges. Mechanistically, ivermectin rapidly clears skin microfilariae but has limited macrofilaricidal activity, implying that elimination depends on repeated rounds until adult worms senesce [ 33 ]. A second pillar of success has been unwavering commitment to long‑term mass drug administration (MDA). Onchocerca volvulus adults can live up to 10–14 years, so interrupting the parasite’s lifecycle requires treatment campaigns that span at least one full generation of worms. Senegal’s program delivered annual MDA for over 15 years, ensuring that all pre-existing adult worms died off naturally while preventing new infections [ 34 ]. The empirical data bear out this strategy: after 15 years at high coverage, not only was morbidity controlled, but sustained transmission interruption was confirmed, mirroring the proof-of-principle findings from parallel Mali and Senegal studies (15–17 years of treatment) where three formerly hyperendemic foci halted MDA without recrudescence [ 14 ]. When compared to other regions, Senegal’s success converges with broader evidence that community-directed ivermectin is a reproducible pathway to elimination. A distinctive operational feature in Senegal, however, was the early decision to treat all areas found endemic—regardless of baseline endemicity—so that hypoendemic districts were included from the outset rather than being deferred. This approach likely reduced the risk that low-intensity transmission “reservoirs” would persist adjacent to treated foci and later seed recrudescence, a challenge that WHO has highlighted for elimination programmes given that historically ivermectin distribution often prioritised only higher-prevalence areas, leaving lower-transmission zones untreated [ 35 ]. In neighboring Mali, similar coverage levels yielded comparable epidemiological outcomes by the early 2010s [ 14 ]. In Nigeria, four southeastern states recently met stop-MDA criteria—representing the largest single stop-treatment decision to date [ 36 ]. Outside West Africa, island and focal settings have reached interruption with integrated packages: Bioko Island combined strong MDA with intensified surveillance to verify interruption [ 27 ]. In northern Uganda, the Obongi focus used integrated chemotherapy and documented interruption of O. volvulus transmission alongside Wuchereria bancrofti . More recently, Niger was verified by WHO in January 2025 as the first country in the African Region to eliminate onchocerciasis [ 12 ]. In Togo, progress toward elimination is substantial, but MDA has not stopped in all foci; stop-MDA has occurred in the southern part of the country, while treatment continues in some northern river basins/foci where evidence indicates residual transmission and the need for intensified/continued interventions [ 37 ]. By contrast, countries with more extensive hyperendemic zones or Loa loa co-endemicity (e.g., the Democratic Republic of the Congo, Cameroon, and the Central African Republic) still face hurdles in scaling MDA safely [ 4 ]. These contrasts reinforce a central lesson: high coverage is necessary but not always sufficient; hotspots may need tailored measures (biannual dosing, moxidectin pilots, or targeted vector control) to overcome persistent transmission [ 31 , 33 ]. These regional differences highlight that while high coverage consistently reduces infection rates, the time to elimination can differ depending on transmission intensity, access to health systems, and co-endemic factors risks. Continued regional coordination—data sharing, synchronized activities, and cross-border surveillance—remains essential given human and vector movement [ 23 ]. In Latin America, semi‑annual ivermectin campaigns have likewise interrupted transmission across six endemic countries, leading to WHO certification between 2013 and 2016 [ 34 ]. For example, Colombia has implemented semi-annual ivermectin treatments along with targeted vector control measures, showcasing their proactive approach [ 38 ]. Although those foci were generally smaller, they demonstrated operational principles—high coverage, frequent rounds, and rigorous verification—that are transferable to African savannah settings [ 33 ]. Senegal’s achievements thus strengthen the global evidence base: even in African hyper-endemic settings, community-led annual MDA can replicate the elimination paths seen elsewhere. In settings with stubborn residual transmission, complementary measures, such as biannual dosing or vector control, may accelerate progress, as seen in Uganda [ 39 ]. However, Senegal’s data confirm that an annual cadence alone sufficed in this case. Several important lessons emerge. First, elimination is a practical goal in sub-Saharan Africa, not just a theoretical idea. Second, the delivery platforms created for onchocerciasis MDA—including trained volunteers, census systems, and supply chains—provide a solid foundation for integrated neglected tropical disease (NTD) control, covering lymphatic filariasis, soil-transmitted helminths, and other diseases. Coverage continues to be the most reliable predictor of success, and maintaining a therapeutic coverage of 80% or more has consistently been linked to interrupting the process effectively [ 5 ]. Finally, as Senegal transitions into post-treatment surveillance, maintaining diligence with parasitological, serological, and entomological monitoring—and collaborating with Mali, Guinea, and Guinea-Bissau—will be crucial to preventing re-emergence and protecting this major public health success. WHO recommends at least three years of post-treatment surveillance using combined indicators to detect recrudescence early [ 40 ]. The skin-snip surveys conducted in 2020 and 2022 across the four Gambie‐basin districts of Goudiry, Kedougou, Tambacounda and Velingara yielded zero microfilarial positives in all age groups. This persistent absence of infection, despite sampling over 150 individuals per district in 2022, confirms the sustained interruption of O. volvulus transmission and underscores the robustness of the community‐directed mass ivermectin campaigns. In line with WHO guidance, skin-snip microscopy and nodule palpation can support monitoring earlier in elimination programmes, but demonstration/confirmation of transmission interruption relies on O-150 PCR (Poolscreen) entomology and Ov16 serology, because skin-snip sensitivity declines markedly at low microfilarial loads [ 41 ]. The consistency of these findings across two separate rounds indicates that residual transmission, if any, is below the detection threshold of approximately 0.1% prevalence, which meets WHO criteria for stopping mass drug administration and moving to post-treatment surveillance. [ 14 , 34 ]. Following stop-MDA in 2022, Senegal entered the WHO-recommended post-treatment surveillance period (minimum 3 years). Entomological surveillance conducted in 2023 and 2024 at sentinel catching/breeding sites, using O-150 PCR pool screening of blackfly heads, detected no infective-stage larvae and remained below WHO transmission thresholds, corroborating the Ov16 serology results in children. Together, these post-2022 entomological data support the statement that the country is on track to complete PTS and submit its verification dossier to WHO [ 42 ]. Together, these complementary surveillance approaches, entomological/parasitological skin-snips and serological Ov16 ELISA, provide convergent evidence that interruption of the transmission of OV has been achieved in Senegal. Post-MDA surveillance for onchocerciasis is fundamentally entomological, relying on periodic O-150 PCR monitoring of blackflies to detect any recrudescence or reintroduction of transmission; Ov16 serology is used mainly as a complementary indicator when entomological sampling is insufficient or as supporting evidence [ 41 ]. Because recrudescence risk is greatest in border zones where neighbouring countries may still have active foci, Senegal’s PTS should be implemented with formal cross-border coordination and data-sharing, particularly with Guinea and Mali (and, where relevant, Guinea-Bissau), including synchronized surveillance activities in shared river basins and agreed contingency responses [ 11 ]. Study Limitations This retrospective analysis of programmatic data has several limitations that should be considered when interpreting the findings. First, the long duration of the program (1998–2022) was occasionally disrupted by external factors beyond the control of the National Onchocerciasis Control Programme. Specifically, social movements and political instability in certain years led to either the withholding of data or the boycott of mass drug administration (MDA) strategies by local communities. These events resulted in temporary interruptions in data collection and, in some instances, incomplete annual coverage reports. Second, the program experienced financial constraints during specific periods, which limited the ability to procure sufficient quantities of ivermectin. These funding shortages meant that, for certain years, MDA could not be implemented at full scale, or specific villages were temporarily excluded from treatment campaigns. Consequently, some annual coverage data are missing or incomplete, creating gaps in the time series analysis. Third, while the surveillance system was robust overall, the frequency and geographic scope of parasitological and entomological surveys varied over time due to logistical and resource limitations. For example, entomological data are not available for every year between 2006 and 2020, and some surveys were conducted only in selected sentinel sites rather than across all transmission zones. This may introduce a degree of uncertainty in the precise timing of transmission interruption. Finally, the reliance on routinely collected program data means that potential inconsistencies in data entry, reporting biases at the community level, or changes in diagnostic methods over time could affect the comparability of results across different phases of the program. Despite these limitations, the consistency of the findings across multiple complementary indicators (parasitological, serological, and entomological) supports the overall conclusion that transmission has been successfully interrupted in Senegal. Conclusion Senegal’s long-term, community-directed ivermectin programme has driven O. volvulus to levels consistent with transmission interruption across its historic foci, enabling the national stop MDA decision in 2022 and entry into WHO recommended post-treatment surveillance. During post-treatment surveillance, entomological monitoring is the backbone for detecting recrudescence or reintroduction, using O-150 PCR pool screening in blackflies, while Ov16 serology in children provides complementary evidence of absent recent exposure. Because the greatest risk of recrudescence occurs in border river basins, Senegal’s verification pathway should explicitly include coordinated cross-border surveillance and data sharing with neighbouring endemic areas, particularly Guinea and Mali. If Senegal completes the required surveillance period with entomological indicators remaining below WHO thresholds and compiles the national dossier, it will be well positioned for formal WHO verification of elimination, in line with recent precedents such as Niger’s WHO verification in January 2025. Finally, the community-directed delivery platform developed for onchocerciasis can be leveraged to strengthen integrated neglected tropical disease delivery and surveillance beyond river blindness. Declarations Clinical trial number Not applicable. Ethics approval and consent to participate This study is a retrospective descriptive analysis of data routinely collected under the Senegalese National Program for the Elimination of Onchocerciasis (PNLO). All data, including parasitological survey results, entomological surveillance data, and dried blood spot (DBS) samples analyzed by Ov16 IgG4 ELISA, were collected as part of the national program’s routine surveillance activities aimed at assessing the impact of treatment and monitoring progress towards elimination. These activities were conducted under the standing ethics clearance of the national program and were not undertaken specifically for this study. Accordingly, separate ethics committee or IRB approval for this retrospective analysis was not required under applicable national regulations in Senegal. This study was conducted in accordance with the principles of the Declaration of Helsinki. As this analysis used only routinely and anonymously collected programmatic data and did not involve new recruitment of participants, a separate informed consent process was not required. Competing interests The authors declare that they have no competing interests. Funding Declaration This study was supported by FHI360 through the Act to End NTDs, in collaboration with the Senegalese National Program for the Elimination of Lymphatic Filariasis and Onchocerciasis. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Data Availability All data supporting the findings of this study are available within the article and its Supplementary Information files. References Gyasi ME, Okonkwo ON, Tripathy K. Onchocerciasis. 2025. Bagcchi S. GONE to combat onchocerciasis. Lancet Microbe. 2024;5:e315. https://doi.org/10.1016/S2666-5247(24)00036-3 Schmidt CA, Cromwell EA, Hill E, Donkers KM, Schipp MF, Johnson KB, et al. The prevalence of onchocerciasis in Africa and Yemen, 2000–2018: a geospatial analysis. BMC Med. 2022;20:293. https://doi.org/10.1186/s12916-022-02486-y Brattig NW, Cheke RA, Garms R. Onchocerciasis (river blindness) – more than a century of research and control. Acta Trop. 2021;218:105677. https://doi.org/10.1016/j.actatropica.2020.105677 Mutono N, Basáñez MG, James A, Stolk WA, Makori A, Kimani TN, et al. Elimination of transmission of onchocerciasis (river blindness) with long-term ivermectin mass drug administration with or without vector control in sub-Saharan Africa: a systematic review and meta-analysis. Lancet Glob Health. 2024;12. https://doi.org/10.1016/S2214-109X(24)00043-3 Ncogo P, Giesen C, Perteguer MJ, Rebollo MP, Nguema R, Benito A, et al. The Impact of Onchocerciasis Elimination Measures in Africa: A Systematic Review. Trop Med Infect Dis [Internet]. 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Progress towards elimination of onchocerciasis in the Region du Sud-Ouest of Burkina Faso which was previously subject to a recrudescence event after vector control. PLoS Negl Trop Dis. 2024;18:e0012118. https://doi.org/10.1371/journal.pntd.0012118 Higazi TB, Katholi CR, Mahmoud BM, Baraka OZ, Mukhtar MM, Qubati Y Al, et al. Onchocerca volvulus: Genetic Diversity of Parasite Isolates from Sudan. Exp Parasitol. 2001;97:24–34. https://doi.org/10.1006/expr.2000.4589 Albers A, Esum ME, Tendongfor N, Enyong P, Klarmann U, Wanji S, et al. Retarded Onchocerca volvulus L1 to L3 larval development in the Simulium damnosum vector after anti-wolbachial treatment of the human host. Parasit Vectors. 2012;5:12. https://doi.org/10.1186/1756-3305-5-12 Dusabimana A, Siewe Fodjo JN, Ndahura MM, Mmbando BP, Jada SR, Boven A, et al. Surveillance for Onchocerciasis-Associated Epilepsy and OV16 IgG4 Testing of Children 6–10 Years Old Should Be Used to Identify Areas Where Onchocerciasis Elimination Programs Need Strengthening. Pathogens. 2022;11:281. https://doi.org/10.3390/pathogens11030281 Rodríguez-Pérez MA, Unnasch TR, Domínguez-Vázquez A, Morales-Castro AL, Peña-Flores GP, Orozco-Algarra ME, et al. Interruption of Transmission of Onchocerca volvulus in the Oaxaca Focus, Mexico. The American Society of Tropical Medicine and Hygiene. 2010;83:21–7. https://doi.org/10.4269/ajtmh.2010.09-0544 Colebunders R, Basáñez MG, Siling K, Post RJ, Rotsaert A, Mmbando B, et al. From river blindness control to elimination: Bridge over troubled water. Infect Dis Poverty. 2018;7. https://doi.org/10.1186/s40249-018-0406-7 Aza’ah RA, Sumo L, Ntonifor NH, Bopda J, Bamou RH, Nana-Djeunga HC. Point prevalence mapping reveals hotspot for onchocerciasis transmission in the Ndikinimeki Health District, Centre Region, Cameroon. Parasites & Vectors 2020 13:1 [Internet]. BioMed Central; 2020 [cited 2026 Feb 22];13:519-. https://doi.org/10.1186/s13071-020-04387-6 Chesnais CB, Bizet C, Campillo JT, Njamnshi WY, Bopda J, Nwane P, et al. A Second Population-Based Cohort Study in Cameroon Confirms the Temporal Relationship Between Onchocerciasis and Epilepsy. Open Forum Infect Dis. 2020;7. https://doi.org/10.1093/ofid/ofaa206 Diawara L, Traoré MO, Badji A, Bissan Y, Doumbia K, others. Feasibility of onchocerciasis elimination with ivermectin treatment in endemic foci in Africa: first evidence from studies in Mali and Senegal. PLoS Negl Trop Dis. 2009;3:e497. Herrador Z, Garcia B, Ncogo P, Perteguer MJ, Rubio JM, Rivas E, et al. Interruption of onchocerciasis transmission in Bioko Island: Accelerating the movement from control to elimination in Equatorial Guinea. PLoS Negl Trop Dis [Internet]. Public Library of Science; 2018 [cited 2025 Aug 10];12:e0006471. https://doi.org/10.1371/JOURNAL.PNTD.0006471 Traore MO, Sarr MD, Badji A, Bissan Y, Diawara L, Doumbia K, et al. Proof-of-Principle of Onchocerciasis Elimination with Ivermectin Treatment in Endemic Foci in Africa: Final Results of a Study in Mali and Senegal. PLoS Negl Trop Dis [Internet]. 2012 [cited 2025 Aug 10];6:e1825. https://doi.org/10.1371/JOURNAL.PNTD.0001825 Brieger WR, Okeibunor JC, Abiose AO, Wanji S, Elhassan E, Ndyomugyenyi R, et al. Compliance with eight years of annual ivermectin treatment of onchocerciasis in Cameroon and Nigeria. Parasites & Vectors 2011 4:1 [Internet]. BioMed Central; 2011 [cited 2026 Feb 22];4:152-. https://doi.org/10.1186/1756-3305-4-152 Boussinesq M, Gardon J, Gardon-Wendel N, Chippaux J-P. Clinical picture, epidemiology and outcome of Loa-associated serious adverse events related to mass ivermectin treatment of onchocerciasis in Cameroon. Filaria J [Internet]. Springer Science and Business Media LLC; 2003 [cited 2026 Feb 22];2:S4. https://doi.org/10.1186/1475-2883-2-s1-s4 Forrer A, Wanji S, Obie ED, Nji TM, Hamill L, Ozano K, et al. Why onchocerciasis transmission persists after 15 annual ivermectin mass drug administrations in South-West Cameroon. BMJ Glob Health [Internet]. BMJ Glob Health; 2021 [cited 2025 Aug 10];6. https://doi.org/10.1136/BMJGH-2020-003248 Komlan K, Vossberg PS, Gantin RG, Solim T, Korbmacher F, Banla M, et al. Onchocerca volvulus infection and serological prevalence, ocular onchocerciasis and parasite transmission in northern and central Togo after decades of Simulium damnosum s.l. vector control and mass drug administration of ivermectin. PLoS Negl Trop Dis [Internet]. PLoS Negl Trop Dis; 2018 [cited 2025 Aug 10];12. https://doi.org/10.1371/JOURNAL.PNTD.0006312 Lakwo T, Oguttu D, Ukety T, Post R, Bakajika D. Onchocerciasis Elimination: Progress and Challenges. Res Rep Trop Med [Internet]. Res Rep Trop Med; 2020 [cited 2025 Aug 10];11:81–95. https://doi.org/10.2147/RRTM.S224364 World Health Organization (WHO). Guidelines for stopping mass drug administration and verifying elimination of human onchocerciasis: criteria and procedures. Geneva: WHO; Licence: CC BY-NC-SA 3.0 IGO. ISBN: 9789241510011. 2016 [cited 2025 Jul 14]; https://apps.who.int/iris/handle/10665/204180. Accessed 14 Jul 2025 Onchocerciasis elimination mapping: a handbook for national elimination programmes [Internet]. [cited 2026 Feb 22]. https://www.who.int/publications/i/item/9789240099227. Accessed 22 Feb 2026 Richards FO, Eigege A, Umaru J, Kahansim B, Adelamo S, Kadimbo J, et al. The Interruption of Transmission of Human Onchocerciasis by an Annual Mass Drug Administration Program in Plateau and Nasarawa States, Nigeria. Am J Trop Med Hyg [Internet]. The American Society of Tropical Medicine and Hygiene; 2020 [cited 2025 Aug 10];102:582–92. https://doi.org/10.4269/AJTMH.19-0577 GONE webinar: Togo on its path towards the elimination of Onchocerciasis [Internet]. [cited 2026 Feb 22]. https://www.who.int/news-room/events/detail/2024/06/11/default-calendar/gone-webinar-togo-on-its-path-towards-the-elimination-of-onchocerciasis. Accessed 22 Feb 2026 Nicholls RS, Duque S, Olaya LA, López MC, Sánchez SB, Morales AL, et al. Elimination of onchocerciasis from Colombia: First proof of concept of river blindness elimination in the world. Parasit Vectors [Internet]. BioMed Central Ltd.; 2018 [cited 2025 Aug 10];11:1–9. https://doi.org/10.1186/S13071-018-2821-9/TABLES/5 Hendy A, Krit M, Pfarr K, Laemmer C, De Witte J, Nwane P, et al. Onchocerca volvulus transmission in the Mbam valley of Cameroon following 16 years of annual community-directed treatment with ivermectin, and the description of a new cytotype of Simulium squamosum. Parasit Vectors. 2021;14:563. https://doi.org/10.1186/s13071-021-05072-y World Health Organization. Ending the Neglect to Attain the Sustainable Development Goals : A Road Map for Neglected Tropical Diseases 2021-2030. Geneva: World Health Organization; 2021. EXECUTIVE SUMMARY - Guidelines for Stopping Mass Drug Administration and Verifying Elimination of Human Onchocerciasis - NCBI Bookshelf [Internet]. [cited 2026 Feb 22]. https://www.ncbi.nlm.nih.gov/books/NBK344104/. Accessed 22 Feb 2026 POST-ELIMINATION SURVEILLANCE (PHASE 3) - Guidelines for Stopping Mass Drug Administration and Verifying Elimination of Human Onchocerciasis - NCBI Bookshelf [Internet]. [cited 2026 Feb 22]. https://www.ncbi.nlm.nih.gov/books/NBK344123/. Accessed 22 Feb 2026 Additional Declarations No competing interests reported. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-9176663","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":640415194,"identity":"343ca9e5-80ac-45d1-9518-e0a18ab30cff","order_by":0,"name":"Djiby Sow","email":"","orcid":"","institution":"Cheikh Anta Diop University","correspondingAuthor":false,"prefix":"","firstName":"Djiby","middleName":"","lastName":"Sow","suffix":""},{"id":640415197,"identity":"fce539e5-be18-4971-8002-fef327682e04","order_by":1,"name":"Mamadou Alpha Diallo","email":"data:image/png;base64,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","orcid":"","institution":"Cheikh Anta Diop University","correspondingAuthor":true,"prefix":"","firstName":"Mamadou","middleName":"Alpha","lastName":"Diallo","suffix":""},{"id":640415201,"identity":"8dcbb0d8-89f8-4be1-a1d9-ade0d77e307b","order_by":2,"name":"Rose Monteil","email":"","orcid":"","institution":"Family Health International 360","correspondingAuthor":false,"prefix":"","firstName":"Rose","middleName":"","lastName":"Monteil","suffix":""},{"id":640415202,"identity":"5f32e6ff-654f-4cc4-af62-67552f8ce465","order_by":3,"name":"Aita Sene","email":"","orcid":"","institution":"Cheikh Anta Diop University","correspondingAuthor":false,"prefix":"","firstName":"Aita","middleName":"","lastName":"Sene","suffix":""},{"id":640415205,"identity":"f16c043c-d751-4971-a793-983f16bb1bbb","order_by":4,"name":"Achille Kabore","email":"","orcid":"","institution":"Family Health International 360","correspondingAuthor":false,"prefix":"","firstName":"Achille","middleName":"","lastName":"Kabore","suffix":""},{"id":640415207,"identity":"f8e40f33-1ae0-4e22-9e21-36ae82f6a6af","order_by":5,"name":"Moussa Dieng Sarr","email":"","orcid":"","institution":"Family Health International 360","correspondingAuthor":false,"prefix":"","firstName":"Moussa","middleName":"Dieng","lastName":"Sarr","suffix":""},{"id":640415210,"identity":"99579d9b-08c9-4e6e-bad8-efae267a5f2d","order_by":6,"name":"Khadim Diongue","email":"","orcid":"","institution":"Cheikh Anta Diop University","correspondingAuthor":false,"prefix":"","firstName":"Khadim","middleName":"","lastName":"Diongue","suffix":""},{"id":640415213,"identity":"aae76061-7762-440a-bedc-253c759f6480","order_by":7,"name":"Mame Cheikh Seck","email":"","orcid":"","institution":"Cheikh Anta Diop University","correspondingAuthor":false,"prefix":"","firstName":"Mame","middleName":"Cheikh","lastName":"Seck","suffix":""},{"id":640415215,"identity":"ec1e52ee-6fa1-4f57-9bd9-32c9b7cd5c41","order_by":8,"name":"Mouhamadou Ndiaye","email":"","orcid":"","institution":"Cheikh Anta Diop University","correspondingAuthor":false,"prefix":"","firstName":"Mouhamadou","middleName":"","lastName":"Ndiaye","suffix":""},{"id":640415216,"identity":"9c56bd67-03ba-41ca-9094-efa7ff4e5588","order_by":9,"name":"Aida Sadikh Badiane","email":"","orcid":"","institution":"Cheikh Anta Diop University","correspondingAuthor":false,"prefix":"","firstName":"Aida","middleName":"Sadikh","lastName":"Badiane","suffix":""},{"id":640415217,"identity":"409c6fcb-ee7f-4fac-96f6-4d3c4a800696","order_by":10,"name":"Mariama Touré","email":"","orcid":"","institution":"Cheikh Anta Diop University","correspondingAuthor":false,"prefix":"","firstName":"Mariama","middleName":"","lastName":"Touré","suffix":""},{"id":640415218,"identity":"00b56974-d212-456d-b25d-d25de1d0d278","order_by":11,"name":"Daniel Boakye","email":"","orcid":"","institution":"Noguchi Memorial Institute for Medical Research","correspondingAuthor":false,"prefix":"","firstName":"Daniel","middleName":"","lastName":"Boakye","suffix":""},{"id":640415219,"identity":"27908207-0e75-4a79-ac0b-242da43a5972","order_by":12,"name":"Didier K. 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It causes severe skin lesions (itching, depigmentation) and eye lesions that can lead to irreversible blindness [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. More than 99% of infected individuals live in sub-Saharan Africa [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In 2017, an estimated 14.6\u0026nbsp;million people suffered from onchocerciasis-related skin manifestations and 1.15\u0026nbsp;million were blind due to the disease, underscoring the public health importance of this neglected tropical disease [\u003cspan additionalcitationids=\"CR3\" citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eRecent syntheses continue to document programmatic gains across Africa [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. In West Africa, a large Onchocerciasis Control Program (OCP) was implemented from 1974 to 2002, bringing the endemic disease under control in many historically affected foci.\u003c/p\u003e \u003cp\u003eThe OCP initially relied on vector control (insecticide spraying by helicopter over blackfly breeding sites), followed by chemoprevention with ivermectin starting in 1989 [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Ivermectin (brand name Mectizan\u0026reg;) is a microfilaricidal antiparasitic drug administered at least once per year for 10 to 15 years to gradually reduce the reservoir of microfilariae in humans. Thanks to the donation of ivermectin by its manufacturer since 1987 and the community-directed treatment strategy promoted by the African Program for Onchocerciasis Control (APOC) from 1995 onward, mass treatment coverage has greatly expanded across Africa. Before the start of control/elimination activities, the endemic foci were delineated using rapid epidemiological approaches/ rapid epidemiological mapping of onchocerciasis (REA/REMO) based on the prevalence of palpable onchocercal nodules in adults living long-term in sentinel communities.\u003c/p\u003e \u003cp\u003eIn these approaches, 30\u0026ndash;50 adults (typically\u0026thinsp;\u0026ge;\u0026thinsp;20 years old and resident\u0026thinsp;\u0026ge;\u0026thinsp;10 years) are examined per selected village, and areas where nodule prevalence exceeds 20% are classified as high-risk and prioritized for intervention, an operational threshold adopted for large-scale mapping and programme planning.\u003c/p\u003e \u003cp\u003eThese baseline mapping results informed the selection of intervention zones and provide the pre-control reference against which the later impact of ivermectin-based strategies can be interpreted [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThese coordinated efforts have transformed onchocerciasis from a major cause of blindness into a disease on the path to elimination in several countries [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. Following APOC\u0026rsquo;s closure in 2015, WHO/AFRO launched the Expanded Special Project for Elimination of Neglected Tropical Diseases (ESPEN) in 2016 to sustain and accelerate country programmes targeting the five Preventive Chemotherapy-NTDs, including onchocerciasis [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. ESPEN now supports national programmes and hosts the sub-national ESPEN Portal for planning and evaluation [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eMomentum has accelerated in recent years. Meta-analytic evidence indicates that programs achieving and maintaining high therapeutic coverage are most likely to meet elimination-of-transmission (EOT) criteria [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. At the regional policy level, the Global Onchocerciasis Network for Elimination (GONE) facilitates data-sharing and coordinated action, and in January 2025 WHO verified Niger as the first country in the African Region to eliminate onchocerciasis\u0026mdash;an important proof-point for feasibility at national scale [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn Senegal, onchocerciasis occurs only in focal areas in the southeastern part of the country. The main endemic foci are located along the Gambia and Fal\u0026eacute;m\u0026eacute; river basins (the Fal\u0026eacute;m\u0026eacute; being a tributary of the Senegal River) and their tributaries (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. This southeastern zone, corresponding to the former health districts of Kedougou (fully affected), Goudiry, Tambacounda (partially affected), and V\u0026eacute;lingara [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e], covers approximately 36,000 km\u0026sup2; (about 18% of the national territory). In 2002, 611 at-risk villages were recorded in this region, comprising more than 160,000 inhabitants exposed to onchocerciasis. Before interventions, the region was hyperendemic, with up to 17% of the population presenting with subcutaneous nodules in certain sentinel villages [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSubsequently, results from Senegal and similar foci in Mali empirically demonstrated that mass treatment of sufficient duration and coverage can eliminate parasite transmission, providing the first proof-of-principle that onchocerciasis elimination was feasible in Africa [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. These findings directly informed the global paradigm shift from onchocerciasis control to elimination, which was formally endorsed in 2016 with the release of the new WHO guidelines for stopping mass drug administration and verifying elimination. Building on this scientific evidence and the revised global policy framework, Senegal adopted elimination as its national objective, defined as the complete cessation of local transmission. In 2022, after more than two decades of mass ivermectin administration, the country halted treatment in all previously endemic communities, which are now considered free of the disease [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eSenegal\u0026rsquo;s national onchocerciasis control program began chemoprevention activities in 1988\u0026ndash;1989 under the OCP [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] and adopted a community-directed treatment strategy on a large scale starting in 1998 [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. From that point forward, annual campaigns of free ivermectin distribution were organized in all identified endemic villages, in collaboration with local communities (via community volunteers). The initial goal was to control morbidity (prevent blindness and skin lesions) by treating at least 65% of the at-risk population each year, a threshold considered necessary to interrupt transmission in the long term.\u003c/p\u003e \u003cp\u003eThe present article provides a scientific overview of this trajectory toward elimination, analyzing data on treatment coverage and epidemiological impact from Senegal\u0026rsquo;s ivermectin chemoprevention program.\u003c/p\u003e"},{"header":"Methodology","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eStudy Design:\u003c/h2\u003e \u003cp\u003eThis retrospective descriptive study analysed routinely collected programme data from the Senegalese National Onchocerciasis Control/Elimination Programme from 1998 through 2024, covering the scale-up of community-directed ivermectin mass drug administration (MDA), the national stop-MDA decision in 2022, and the early post-treatment surveillance (PTS) period. In addition to annual MDA reports used to quantify geographical and therapeutic coverage, we included multilevel surveillance outputs, parasitological and serological assessments in endemic foci and post-MDA entomological monitoring in 2023\u0026ndash;2024 (including O-150 PCR pool screen testing of \u003cem\u003eSimulium\u003c/em\u003e vectors), to document progress toward, and confirm, interruption of transmission as required during PTS.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eStudy Area:\u003c/h3\u003e\n\u003cp\u003eThe activities focused on the southeastern region of Senegal corresponding to the endemic foci of onchocerciasis. Initially, four health districts were involved (Kedougou, Tambacounda, Goudiry, and V\u0026eacute;lingara). Over time, due to administrative reorganization and refined mapping of endemicity, the program expanded to operate in a total of eight endemic health districts. These districts cover river basins bordering Mali (Fal\u0026eacute;m\u0026eacute; basin) and those bordering Guinea and Gambia (Gambia River basin), where transmission is seasonal (especially during and after the rainy season, when blackfly populations proliferate) [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. The total population in the endemic areas was estimated at approximately 600,000 inhabitants by the end of the program (WHO, 2023). For programmatic purposes, Senegal grouped all historically endemic districts into two Operational Transmission Zones (OTZs), aligned with the Gambia and Fal\u0026eacute;m\u0026eacute; river basins. Each OTZ encompasses neighbouring districts linked by shared Simulium breeding sites and common onchocerciasis transmission dynamics. Epidemiological and entomological survey results are therefore analyzed and interpreted at this OTZ level.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e\n\u003ch3\u003eIvermectin chemoprevention:\u003c/h3\u003e\n\u003cp\u003eAnnual mass distributions of ivermectin were carried out in all identified at-risk villages. The drug was administered by trained community volunteers under the supervision of health personnel. All community members were targeted for treatment, except for children under 5 years of age (approximately\u0026thinsp;\u0026lt;\u0026thinsp;90 cm, the height below which ivermectin is not recommended), pregnant women, and severely ill individuals, in accordance with the contraindications. This non-treatable fraction represented approximately 13\u0026ndash;15% of the total population, leaving about 85% eligible for treatment. During each annual campaign, a community census was conducted to enumerate the resident population and estimate the number of tablets required. After distribution, the number of individuals who were effectively treated (i.e., those who ingested the medication under direct observation) was reported on a village-by-village basis using standardized forms.\u003c/p\u003e\n\u003ch3\u003eCoverage indicators:\u003c/h3\u003e\n\u003cp\u003eTwo main indicators were monitored annually:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eGeographical coverage: Defined as the proportion of eligible villages that actually received ivermectin distribution (target: 100%).\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eTherapeutic coverage: Defined as the proportion of the total enumerated population that received treatment within the covered villages. The WHO recommends achieving at least 65% coverage of the total population in each community (which corresponds to approximately 80% of the treatment-eligible population) to sufficiently impact transmission. In our analysis, we calculated the overall annual therapeutic coverage as the ratio of the number of individuals treated to the total enumerated population across all covered villages, expressed as a percentage [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e].\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e\n\u003ch3\u003eEpidemiological surveillance:\u003c/h3\u003e\n\u003cp\u003eThe program established a network of sentinel villages to monitor the infection. In these villages, parasitological surveys were conducted before the initiation of treatment and at regular intervals (every 2 to 3 years). The surveys included clinical examinations and skin snip collections from a sample of voluntary inhabitants aged\u0026thinsp;\u0026ge;\u0026thinsp;5 years to detect the presence of \u003cem\u003eO. volvulus\u003c/em\u003e microfilariae in the skin and determine the prevalence of microfilaridermia (the percentage of infected individuals). For example, 12 sentinel villages were evaluated in 1996 (prior to intervention), 20 in 2000, and up to 31 in 2001 [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The number of individuals examined per survey ranged from approximately 1,400 in 1996 to around 6,300 in 2001, reflecting the expansion of the surveillance network. In parallel, entomological surveys were conducted to assess vector transmission. \u003cem\u003eSimulium\u003c/em\u003e blackflies were captured at biting sites along rivers in the endemic foci and then sent to the reference laboratory (WHO\u0026rsquo;s Centre for Multidisease Surveillance in Ouagadougou, Burkina Faso) for molecular analyses [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The heads of the blackflies were tested using PCR amplification with the O-150 marker to detect any infectious \u003cem\u003eO. volvulus\u003c/em\u003e larvae [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. The vector infectivity rate (i.e., the proportion of flies carrying infectious L3 larvae) was thus estimated and monitored over time, providing a direct indicator of parasite transmission to humans [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eSerological testing by ELISA Ov16\u003c/h2\u003e \u003cp\u003eThe detection of IgG4 antibodies specific to \u003cem\u003eOnchocerca volvulus\u003c/em\u003e was performed using the Ov16 ELISA on dried blood spots (DBS), following standardized procedures described by Dusabiman A. et \u003cem\u003eal\u003c/em\u003e [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The antigen used was recombinant Ov16-GST, coated onto flat-bottom Immunolon 2HB microplates. After sample elution and blocking with PBST-BSA, samples and standards were applied in duplicate, followed by incubation with biotin-conjugated anti-human IgG4 antibodies and streptavidin-alkaline phosphatase. The colorimetric reaction was developed using the pNPP substrate and read at 404 nm using a microplate reader. A standard curve with 8 dilutions (640 to 5 units) was included on each plate to ensure consistency and allow semi-quantitative interpretation. Positive controls consisted of humanized anti-Ov16 monoclonal antibodies. Only results meeting predefined quality control thresholds were retained for next steps. The analysis threshold was set at 40 arbitrary units. Samples yielding optical density (OD) values\u0026thinsp;\u0026ge;\u0026thinsp;40 in duplicate were considered putatively positive and retested; confirmed positives were those maintaining OD\u0026thinsp;\u0026ge;\u0026thinsp;40 for Ov16 and \u0026lt;\u0026thinsp;40 when tested against GST alone [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003eData analysis:\u003c/h2\u003e \u003cp\u003eData analysis was performed using R Studio version 4.3.1 for statistical processing and Python (with the Pandas and Matplotlib libraries) for data visualization. Data were aggregated by year and by district. We constructed a time series of annual geographical and therapeutic coverage and compared these indicators to the recommended targets. The results of the parasitological surveys were compiled to examine the evolution of infection prevalence (particularly among children who had experienced only the treatment era) and the number of villages where transmission persisted. Finally, we interpreted the Senegalese results in comparison with those reported from other regions that implemented similar mass ivermectin treatment strategies, notably in West Africa (neighboring countries) and in the Americas, to identify common success factors and challenges.\u003c/p\u003e \u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003e \u003cb\u003eMass treatment coverage\u003c/b\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFrom the inception of the Senegalese mass drug administration (MDA) program in 1998, geographic coverage was optimal. Every village identified as endemic was reached by the annual ivermectin distribution. For example, in 2000, 590 out of 590 eligible villages were treated (100% geographic coverage). This performance was maintained subsequently, with 100% of the 611 targeted villages being effectively covered in 2001\u0026ndash;2002. Moreover, the number of villages covered increased during the initial years, rising from 511 villages in 1998 (during the initial mapping) to 611 in 2002, thanks to the inclusion of additional peripheral villages as needed (Fig.\u0026nbsp;2).\u003c/p\u003e \u003cp\u003e \u003cb\u003eFigure 2: Mass treatment coverage by district between 2001 and 2007\u003c/b\u003e \u003c/p\u003e \u003cp\u003eRegarding therapeutic coverage, the program successfully treated a high proportion of the at-risk population each year. Figure\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e3\u003c/span\u003e presents some coverage indicators from 1998 (the first full year of the CDTI program) to 2002. The number of people treated increased from approximately 104,000 in 1998 to nearly 130,000 in 2002, while the recorded population in covered villages rose from 136,000 to 163,000 over the same period (partly due to population growth and the expansion of treatment to new villages). Thus, the overall therapeutic coverage remained around 78\u0026ndash;80% annually. This rate reflects the proportion of the entire population in covered villages who received ivermectin. It far exceeds the minimum threshold of 65% set for program efficacy and approaches nearly the entire eligible population (considering\u0026thinsp;~\u0026thinsp;15% of people excluded due to contraindications). Additionally, over 90% of villages individually met the criterion of \u0026ge;\u0026thinsp;65% coverage within their population by the program\u0026rsquo;s second year, demonstrating nearly universal community adherence to the treatments.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eEpidemiological surveillance:\u003c/h2\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003eEntomology\u003c/h2\u003e \u003cp\u003eOverall, eight entomological surveys using O‑150 pool‑screen PCR were conducted between 2006 and 2020 in the Fal\u0026eacute;m\u0026eacute; and Gambia river basins (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Infection rates were below 0.2% in all these years except Fal\u0026eacute;m\u0026eacute; in 2006 (0.79%) and 2010 (0.82%), while Gambia showed a modest increase to 0.31% in 2019.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSummary of Simulium pool‑screen surveys for the first breeding season\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eYear\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eFal\u0026eacute;m\u0026eacute; n (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eGambia n (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2006\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2,266 (0.787)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e43,498 (0.158)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2007\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e25,933 (0.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e95,767 (0.0)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2008\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e52,928 (0.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e89,057 (0.0)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2010\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e50,172 (0.819)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e250,519 (0.0)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2015\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e12,428 (0.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6,066 (0.0)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2016\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e35,576 (0.0)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e36,962 (0.0)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2019\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e47,371 (0.19)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e64,548 (0.308)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2020\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e20,270 (0.034)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e48,925 (0.221)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003eSerology and microfilariae (OTZ level)\u003c/h2\u003e \u003cp\u003eHistorical serological and skin‑snip surveys summarised at OTZ level document the progressive decline of infection from baseline to the 2019 Stop‑MDA assessments (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). At baseline in 1987\u0026ndash;1988, combined Ov16 seroprevalence exceeded 35% and microfilarial prevalence was above 30%, whereas by 2001\u0026ndash;2003 MF prevalence had already fallen below 1% and Ov16 seroreactivity to around 1% in the combined foci.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eSerology and microfilariae (OTZ level)\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eOTZ\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eYear\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSerology tested\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSerology positive\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eSerology prevalence (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eMF tested\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eMF prevalence (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCombined\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1987\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e704\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e296\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e42.05\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e505\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e33.893\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCombined\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1988\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3095\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1176\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e38.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1088\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e35.153\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCombined\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1996\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e135\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e9.534\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCombined\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e1999\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e70\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e2.464\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCombined\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e119\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e3.31\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCombined\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2001\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e27\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.43\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.066\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCombined\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2002\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4099\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.151\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCombined\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2003\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3042\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.048\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFaleme\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2006\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.128\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFaleme\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2007\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.093\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFaleme\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2008\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.666\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFaleme\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2010\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.265\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFaleme\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2014\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1510\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.388\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFaleme\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2019\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3077\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGambie\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2006\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGambie\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2007\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGambie\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2008\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.131\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGambie\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2010\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1540\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGambie\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2014\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3381\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003e0.529\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eGambie\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e2019\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e3152\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e0.0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c7\"\u003e \u003cp\u003eNA\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eIn 2014, low‑level Ov16 positivity (1.3\u0026ndash;1.4%) persisted in both OTZs, but by 2019 no seropositive children and no microfilariae were detected in either the Gambie or Fal\u0026eacute;m\u0026eacute; OTZ, consistent with interruption of recent transmission; these trends in microfilarial prevalence are illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003eSkin‑snip surveys by district\u003c/h2\u003e \u003cp\u003eResults of the 2020 and 2022 skin-snip surveys in the four historically endemic districts of the Gambia basin (Goudiry, Kedougou, Tambacounda, and Velingara) confirmed the absence of \u003cem\u003eO. volvulus\u003c/em\u003e microfilariae. In 2020, a total of 489 individuals were examined across the four districts, and in 2022, a further 623 individuals were examined. No microfilariae were detected in any of the samples, resulting in a crude prevalence of 0.0%, an age-standardised prevalence of 0.0%, and a CMFL of 0.0 mfs/ss in all districts for both survey years.\u003c/p\u003e \u003c/div\u003e"},{"header":"Discussion","content":"\u003cp\u003eOver the course of more than two decades, community-directed mass ivermectin administration in Senegal has achieved near-universal coverage and had a profound epidemiological impact. Geographical coverage reached all known endemic villages approaching 100%, and therapeutic coverage consistently remained around \u003cem\u003e78\u0026ndash;80%\u003c/em\u003e of the total population each year, well above the World Health Organization (WHO) recommended minimum (approximately 65% of the total population, equivalent to 80% of the eligible population) needed to break transmission [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. This scale-up aligns with global elimination goals and reflects the long arc from OCP-era control to elimination-focused strategies in Africa [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. At baseline (pre-MDA; 1987\u0026ndash;1988 skin-snip surveys), district-level microfilaridermia prevalence indicated one hypoendemic district (Sal\u0026eacute;mata), two mesoendemic districts (K\u0026eacute;dougou and Saraya), and no hyperendemic district by district-mean prevalence, although village-level prevalence reached hyperendemic levels (\u0026ge;\u0026thinsp;60%) in some sentinel communities [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. In addition to eye and skin issues, cutting down on transmission also helps prevent neurological problems. Studies in Cameroon have shown that children with higher levels of microfilariae are more likely to develop epilepsy later in life. This highlights how important it is for public health efforts to aim for elimination, benefiting communities in many ways. [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. Entomological assessments also showed a collapse in transmission: hundreds of thousands of vector blackflies were tested, with either zero infective larvae found or rates far below the threshold for sustainable transmission (\u0026lt;\u0026thinsp;0.5 infected flies per 1,000 flies) [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. These very low infectivity levels are consistent with WHO stop-MDA benchmarks and with programme experiences reporting zero or near-zero infective flies when transmission is no longer sustainable [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eEarly demonstrations from Mali\u0026ndash;Senegal \u0026ldquo;proof-of-principle\u0026rdquo; zones previewed these dynamics, showing that sustained high-coverage annual MDA can reduce child infection to zero and meet entomological thresholds for stopping. The Senegalese experience clearly shows that annual mass ivermectin chemoprevention can not only control but actually eliminate onchocerciasis, as long as exceptionally high coverage is achieved and maintained over time. Remarkably, just 4\u0026ndash;5 years of consecutive annual treatments reduced microfilarial prevalence to nearly zero in children, effectively stopping transmission much faster than the originally projected 10\u0026ndash;15 years [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. This quick success is due to three main factors: first, community adherence was high, with 78\u0026ndash;80% of the total population treated annually which is above the minimum coverage historically targeted for control (65% of total population) and consistent with elimination frameworks showing that achieving\u0026thinsp;\u0026ge;\u0026thinsp;80% of the eligible population is strongly associated with interruption of transmission [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]; second, a strongly seasonal transmission cycle\u0026mdash;limited to a few months each year, which reduces chances of re-infection ; and third, the absence of \u003cem\u003eLoa loa\u003c/em\u003e co-endemicity, which makes safe ivermectin delivery more difficult in Central Africa [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]. Where \u003cem\u003eLoa loa\u003c/em\u003e is co-endemic or adherence is systematically low, persistent transmission has been documented despite many MDA rounds\u0026mdash;highlighting context-specific barriers that Senegal largely avoided [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]. Together, these factors enabled Senegal to use ivermectin\u0026rsquo;s microfilaricidal effect without significant logistical or safety challenges. Mechanistically, ivermectin rapidly clears skin microfilariae but has limited macrofilaricidal activity, implying that elimination depends on repeated rounds until adult worms senesce [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA second pillar of success has been unwavering commitment to long‑term mass drug administration (MDA). \u003cem\u003eOnchocerca volvulus\u003c/em\u003e adults can live up to 10\u0026ndash;14 years, so interrupting the parasite\u0026rsquo;s lifecycle requires treatment campaigns that span at least one full generation of worms. Senegal\u0026rsquo;s program delivered annual MDA for over 15 years, ensuring that all pre-existing adult worms died off naturally while preventing new infections [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. The empirical data bear out this strategy: after 15 years at high coverage, not only was morbidity controlled, but sustained transmission interruption was confirmed, mirroring the proof-of-principle findings from parallel Mali and Senegal studies (15\u0026ndash;17 years of treatment) where three formerly hyperendemic foci halted MDA without recrudescence [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eWhen compared to other regions, Senegal\u0026rsquo;s success converges with broader evidence that community-directed ivermectin is a reproducible pathway to elimination. A distinctive operational feature in Senegal, however, was the early decision to treat all areas found endemic\u0026mdash;regardless of baseline endemicity\u0026mdash;so that hypoendemic districts were included from the outset rather than being deferred. This approach likely reduced the risk that low-intensity transmission \u0026ldquo;reservoirs\u0026rdquo; would persist adjacent to treated foci and later seed recrudescence, a challenge that WHO has highlighted for elimination programmes given that historically ivermectin distribution often prioritised only higher-prevalence areas, leaving lower-transmission zones untreated [\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. In neighboring Mali, similar coverage levels yielded comparable epidemiological outcomes by the early 2010s [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. In Nigeria, four southeastern states recently met stop-MDA criteria\u0026mdash;representing the largest single stop-treatment decision to date [\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e]. Outside West Africa, island and focal settings have reached interruption with integrated packages: Bioko Island combined strong MDA with intensified surveillance to verify interruption [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]. In northern Uganda, the Obongi focus used integrated chemotherapy and documented interruption of O. volvulus transmission alongside \u003cem\u003eWuchereria bancrofti\u003c/em\u003e. More recently, Niger was verified by WHO in January 2025 as the first country in the African Region to eliminate onchocerciasis [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]. In Togo, progress toward elimination is substantial, but MDA has not stopped in all foci; stop-MDA has occurred in the southern part of the country, while treatment continues in some northern river basins/foci where evidence indicates residual transmission and the need for intensified/continued interventions [\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. By contrast, countries with more extensive hyperendemic zones or \u003cem\u003eLoa loa\u003c/em\u003e co-endemicity (e.g., the Democratic Republic of the Congo, Cameroon, and the Central African Republic) still face hurdles in scaling MDA safely [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. These contrasts reinforce a central lesson: high coverage is necessary but not always sufficient; hotspots may need tailored measures (biannual dosing, moxidectin pilots, or targeted vector control) to overcome persistent transmission [\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. These regional differences highlight that while high coverage consistently reduces infection rates, the time to elimination can differ depending on transmission intensity, access to health systems, and co-endemic factors risks. Continued regional coordination\u0026mdash;data sharing, synchronized activities, and cross-border surveillance\u0026mdash;remains essential given human and vector movement [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn Latin America, semi‑annual ivermectin campaigns have likewise interrupted transmission across six endemic countries, leading to WHO certification between 2013 and 2016 [\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e]. For example, Colombia has implemented semi-annual ivermectin treatments along with targeted vector control measures, showcasing their proactive approach [\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e]. Although those foci were generally smaller, they demonstrated operational principles\u0026mdash;high coverage, frequent rounds, and rigorous verification\u0026mdash;that are transferable to African savannah settings [\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. Senegal\u0026rsquo;s achievements thus strengthen the global evidence base: even in African hyper-endemic settings, community-led annual MDA can replicate the elimination paths seen elsewhere. In settings with stubborn residual transmission, complementary measures, such as biannual dosing or vector control, may accelerate progress, as seen in Uganda [\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e]. However, Senegal\u0026rsquo;s data confirm that an annual cadence alone sufficed in this case.\u003c/p\u003e \u003cp\u003eSeveral important lessons emerge. First, elimination is a practical goal in sub-Saharan Africa, not just a theoretical idea. Second, the delivery platforms created for onchocerciasis MDA\u0026mdash;including trained volunteers, census systems, and supply chains\u0026mdash;provide a solid foundation for integrated neglected tropical disease (NTD) control, covering lymphatic filariasis, soil-transmitted helminths, and other diseases. Coverage continues to be the most reliable predictor of success, and maintaining a therapeutic coverage of 80% or more has consistently been linked to interrupting the process effectively [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Finally, as Senegal transitions into post-treatment surveillance, maintaining diligence with parasitological, serological, and entomological monitoring\u0026mdash;and collaborating with Mali, Guinea, and Guinea-Bissau\u0026mdash;will be crucial to preventing re-emergence and protecting this major public health success. WHO recommends at least three years of post-treatment surveillance using combined indicators to detect recrudescence early [\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe skin-snip surveys conducted in 2020 and 2022 across the four Gambie‐basin districts of Goudiry, Kedougou, Tambacounda and Velingara yielded zero microfilarial positives in all age groups. This persistent absence of infection, despite sampling over 150 individuals per district in 2022, confirms the sustained interruption of \u003cem\u003eO. volvulus\u003c/em\u003e transmission and underscores the robustness of the community‐directed mass ivermectin campaigns. In line with WHO guidance, skin-snip microscopy and nodule palpation can support monitoring earlier in elimination programmes, but demonstration/confirmation of transmission interruption relies on O-150 PCR (Poolscreen) entomology and Ov16 serology, because skin-snip sensitivity declines markedly at low microfilarial loads [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. The consistency of these findings across two separate rounds indicates that residual transmission, if any, is below the detection threshold of approximately 0.1% prevalence, which meets WHO criteria for stopping mass drug administration and moving to post-treatment surveillance. [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eFollowing stop-MDA in 2022, Senegal entered the WHO-recommended post-treatment surveillance period (minimum 3 years). Entomological surveillance conducted in 2023 and 2024 at sentinel catching/breeding sites, using O-150 PCR pool screening of blackfly heads, detected no infective-stage larvae and remained below WHO transmission thresholds, corroborating the Ov16 serology results in children. Together, these post-2022 entomological data support the statement that the country is on track to complete PTS and submit its verification dossier to WHO [\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eTogether, these complementary surveillance approaches, entomological/parasitological skin-snips and serological Ov16 ELISA, provide convergent evidence that interruption of the transmission of OV has been achieved in Senegal. Post-MDA surveillance for onchocerciasis is fundamentally entomological, relying on periodic O-150 PCR monitoring of blackflies to detect any recrudescence or reintroduction of transmission; Ov16 serology is used mainly as a complementary indicator when entomological sampling is insufficient or as supporting evidence [\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e]. Because recrudescence risk is greatest in border zones where neighbouring countries may still have active foci, Senegal\u0026rsquo;s PTS should be implemented with formal cross-border coordination and data-sharing, particularly with Guinea and Mali (and, where relevant, Guinea-Bissau), including synchronized surveillance activities in shared river basins and agreed contingency responses [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e].\u003c/p\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003eStudy Limitations\u003c/h2\u003e \u003cp\u003eThis retrospective analysis of programmatic data has several limitations that should be considered when interpreting the findings. First, the long duration of the program (1998\u0026ndash;2022) was occasionally disrupted by external factors beyond the control of the National Onchocerciasis Control Programme. Specifically, social movements and political instability in certain years led to either the withholding of data or the boycott of mass drug administration (MDA) strategies by local communities. These events resulted in temporary interruptions in data collection and, in some instances, incomplete annual coverage reports.\u003c/p\u003e \u003cp\u003eSecond, the program experienced financial constraints during specific periods, which limited the ability to procure sufficient quantities of ivermectin. These funding shortages meant that, for certain years, MDA could not be implemented at full scale, or specific villages were temporarily excluded from treatment campaigns. Consequently, some annual coverage data are missing or incomplete, creating gaps in the time series analysis.\u003c/p\u003e \u003cp\u003eThird, while the surveillance system was robust overall, the frequency and geographic scope of parasitological and entomological surveys varied over time due to logistical and resource limitations. For example, entomological data are not available for every year between 2006 and 2020, and some surveys were conducted only in selected sentinel sites rather than across all transmission zones. This may introduce a degree of uncertainty in the precise timing of transmission interruption.\u003c/p\u003e \u003cp\u003eFinally, the reliance on routinely collected program data means that potential inconsistencies in data entry, reporting biases at the community level, or changes in diagnostic methods over time could affect the comparability of results across different phases of the program. Despite these limitations, the consistency of the findings across multiple complementary indicators (parasitological, serological, and entomological) supports the overall conclusion that transmission has been successfully interrupted in Senegal.\u003c/p\u003e \u003c/div\u003e"},{"header":"Conclusion","content":"\u003cp\u003eSenegal\u0026rsquo;s long-term, community-directed ivermectin programme has driven \u003cem\u003eO. volvulus\u003c/em\u003e to levels consistent with transmission interruption across its historic foci, enabling the national stop MDA decision in 2022 and entry into WHO recommended post-treatment surveillance. During post-treatment surveillance, entomological monitoring is the backbone for detecting recrudescence or reintroduction, using O-150 PCR pool screening in blackflies, while Ov16 serology in children provides complementary evidence of absent recent exposure. Because the greatest risk of recrudescence occurs in border river basins, Senegal\u0026rsquo;s verification pathway should explicitly include coordinated cross-border surveillance and data sharing with neighbouring endemic areas, particularly Guinea and Mali. If Senegal completes the required surveillance period with entomological indicators remaining below WHO thresholds and compiles the national dossier, it will be well positioned for formal WHO verification of elimination, in line with recent precedents such as Niger\u0026rsquo;s WHO verification in January 2025. Finally, the community-directed delivery platform developed for onchocerciasis can be leveraged to strengthen integrated neglected tropical disease delivery and surveillance beyond river blindness.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eClinical trial number\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eNot applicable.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study is a retrospective descriptive analysis of data routinely collected under the Senegalese National Program for the Elimination of Onchocerciasis (PNLO). All data, including parasitological survey results, entomological surveillance data, and dried blood spot (DBS) samples analyzed by Ov16 IgG4 ELISA, were collected as part of the national program\u0026rsquo;s routine surveillance activities aimed at assessing the impact of treatment and monitoring progress towards elimination. These activities were conducted under the standing ethics clearance of the national program and were not undertaken specifically for this study. Accordingly, separate ethics committee or IRB approval for this retrospective analysis was not required under applicable national regulations in Senegal. This study was conducted in accordance with the principles of the Declaration of Helsinki. As this analysis used only routinely and anonymously collected programmatic data and did not involve new recruitment of participants, a separate informed consent process was not required.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare that they have no competing interests.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding Declaration\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study was supported by FHI360 through the Act to End NTDs, in collaboration with the Senegalese National Program for the Elimination of Lymphatic Filariasis and Onchocerciasis. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data supporting the findings of this study are available within the article and its Supplementary Information files.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eGyasi ME, Okonkwo ON, Tripathy K. Onchocerciasis. 2025. \u003c/li\u003e\n\u003cli\u003eBagcchi S. GONE to combat onchocerciasis. Lancet Microbe. 2024;5:e315. https://doi.org/10.1016/S2666-5247(24)00036-3\u003c/li\u003e\n\u003cli\u003eSchmidt CA, Cromwell EA, Hill E, Donkers KM, Schipp MF, Johnson KB, et al. The prevalence of onchocerciasis in Africa and Yemen, 2000\u0026ndash;2018: a geospatial analysis. BMC Med. 2022;20:293. https://doi.org/10.1186/s12916-022-02486-y\u003c/li\u003e\n\u003cli\u003eBrattig NW, Cheke RA, Garms R. 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Exp Parasitol. 2001;97:24\u0026ndash;34. https://doi.org/10.1006/expr.2000.4589\u003c/li\u003e\n\u003cli\u003eAlbers A, Esum ME, Tendongfor N, Enyong P, Klarmann U, Wanji S, et al. Retarded Onchocerca volvulus L1 to L3 larval development in the Simulium damnosum vector after anti-wolbachial treatment of the human host. Parasit Vectors. 2012;5:12. https://doi.org/10.1186/1756-3305-5-12\u003c/li\u003e\n\u003cli\u003eDusabimana A, Siewe Fodjo JN, Ndahura MM, Mmbando BP, Jada SR, Boven A, et al. Surveillance for Onchocerciasis-Associated Epilepsy and OV16 IgG4 Testing of Children 6\u0026ndash;10 Years Old Should Be Used to Identify Areas Where Onchocerciasis Elimination Programs Need Strengthening. Pathogens. 2022;11:281. https://doi.org/10.3390/pathogens11030281\u003c/li\u003e\n\u003cli\u003eRodr\u0026iacute;guez-P\u0026eacute;rez MA, Unnasch TR, Dom\u0026iacute;nguez-V\u0026aacute;zquez A, Morales-Castro AL, Pe\u0026ntilde;a-Flores GP, Orozco-Algarra ME, et al. Interruption of Transmission of Onchocerca volvulus in the Oaxaca Focus, Mexico. The American Society of Tropical Medicine and Hygiene. 2010;83:21\u0026ndash;7. https://doi.org/10.4269/ajtmh.2010.09-0544\u003c/li\u003e\n\u003cli\u003eColebunders R, Bas\u0026aacute;\u0026ntilde;ez MG, Siling K, Post RJ, Rotsaert A, Mmbando B, et al. From river blindness control to elimination: Bridge over troubled water. Infect Dis Poverty. 2018;7. https://doi.org/10.1186/s40249-018-0406-7\u003c/li\u003e\n\u003cli\u003eAza\u0026rsquo;ah RA, Sumo L, Ntonifor NH, Bopda J, Bamou RH, Nana-Djeunga HC. Point prevalence mapping reveals hotspot for onchocerciasis transmission in the Ndikinimeki Health District, Centre Region, Cameroon. Parasites \u0026amp; Vectors 2020 13:1 [Internet]. BioMed Central; 2020 [cited 2026 Feb 22];13:519-. https://doi.org/10.1186/s13071-020-04387-6\u003c/li\u003e\n\u003cli\u003eChesnais CB, Bizet C, Campillo JT, Njamnshi WY, Bopda J, Nwane P, et al. A Second Population-Based Cohort Study in Cameroon Confirms the Temporal Relationship Between Onchocerciasis and Epilepsy. Open Forum Infect Dis. 2020;7. https://doi.org/10.1093/ofid/ofaa206\u003c/li\u003e\n\u003cli\u003eDiawara L, Traor\u0026eacute; MO, Badji A, Bissan Y, Doumbia K, others. Feasibility of onchocerciasis elimination with ivermectin treatment in endemic foci in Africa: first evidence from studies in Mali and Senegal. PLoS Negl Trop Dis. 2009;3:e497. \u003c/li\u003e\n\u003cli\u003eHerrador Z, Garcia B, Ncogo P, Perteguer MJ, Rubio JM, Rivas E, et al. Interruption of onchocerciasis transmission in Bioko Island: Accelerating the movement from control to elimination in Equatorial Guinea. PLoS Negl Trop Dis [Internet]. Public Library of Science; 2018 [cited 2025 Aug 10];12:e0006471. https://doi.org/10.1371/JOURNAL.PNTD.0006471\u003c/li\u003e\n\u003cli\u003eTraore MO, Sarr MD, Badji A, Bissan Y, Diawara L, Doumbia K, et al. Proof-of-Principle of Onchocerciasis Elimination with Ivermectin Treatment in Endemic Foci in Africa: Final Results of a Study in Mali and Senegal. PLoS Negl Trop Dis [Internet]. 2012 [cited 2025 Aug 10];6:e1825. https://doi.org/10.1371/JOURNAL.PNTD.0001825\u003c/li\u003e\n\u003cli\u003eBrieger WR, Okeibunor JC, Abiose AO, Wanji S, Elhassan E, Ndyomugyenyi R, et al. Compliance with eight years of annual ivermectin treatment of onchocerciasis in Cameroon and Nigeria. Parasites \u0026amp; Vectors 2011 4:1 [Internet]. BioMed Central; 2011 [cited 2026 Feb 22];4:152-. https://doi.org/10.1186/1756-3305-4-152\u003c/li\u003e\n\u003cli\u003eBoussinesq M, Gardon J, Gardon-Wendel N, Chippaux J-P. Clinical picture, epidemiology and outcome of Loa-associated serious adverse events related to mass ivermectin treatment of onchocerciasis in Cameroon. Filaria J [Internet]. Springer Science and Business Media LLC; 2003 [cited 2026 Feb 22];2:S4. https://doi.org/10.1186/1475-2883-2-s1-s4\u003c/li\u003e\n\u003cli\u003eForrer A, Wanji S, Obie ED, Nji TM, Hamill L, Ozano K, et al. Why onchocerciasis transmission persists after 15 annual ivermectin mass drug administrations in South-West Cameroon. BMJ Glob Health [Internet]. BMJ Glob Health; 2021 [cited 2025 Aug 10];6. https://doi.org/10.1136/BMJGH-2020-003248\u003c/li\u003e\n\u003cli\u003eKomlan K, Vossberg PS, Gantin RG, Solim T, Korbmacher F, Banla M, et al. Onchocerca volvulus infection and serological prevalence, ocular onchocerciasis and parasite transmission in northern and central Togo after decades of Simulium damnosum s.l. vector control and mass drug administration of ivermectin. PLoS Negl Trop Dis [Internet]. PLoS Negl Trop Dis; 2018 [cited 2025 Aug 10];12. https://doi.org/10.1371/JOURNAL.PNTD.0006312\u003c/li\u003e\n\u003cli\u003eLakwo T, Oguttu D, Ukety T, Post R, Bakajika D. Onchocerciasis Elimination: Progress and Challenges. Res Rep Trop Med [Internet]. Res Rep Trop Med; 2020 [cited 2025 Aug 10];11:81\u0026ndash;95. https://doi.org/10.2147/RRTM.S224364\u003c/li\u003e\n\u003cli\u003eWorld Health Organization (WHO). Guidelines for stopping mass drug administration and verifying elimination of human onchocerciasis: criteria and procedures. Geneva: WHO; Licence: CC BY-NC-SA 3.0 IGO. ISBN: 9789241510011. 2016 [cited 2025 Jul 14]; https://apps.who.int/iris/handle/10665/204180. Accessed 14 Jul 2025\u003c/li\u003e\n\u003cli\u003eOnchocerciasis elimination mapping: a handbook for national elimination programmes [Internet]. [cited 2026 Feb 22]. https://www.who.int/publications/i/item/9789240099227. Accessed 22 Feb 2026\u003c/li\u003e\n\u003cli\u003eRichards FO, Eigege A, Umaru J, Kahansim B, Adelamo S, Kadimbo J, et al. The Interruption of Transmission of Human Onchocerciasis by an Annual Mass Drug Administration Program in Plateau and Nasarawa States, Nigeria. Am J Trop Med Hyg [Internet]. The American Society of Tropical Medicine and Hygiene; 2020 [cited 2025 Aug 10];102:582\u0026ndash;92. https://doi.org/10.4269/AJTMH.19-0577\u003c/li\u003e\n\u003cli\u003eGONE webinar: Togo on its path towards the elimination of Onchocerciasis [Internet]. [cited 2026 Feb 22]. https://www.who.int/news-room/events/detail/2024/06/11/default-calendar/gone-webinar-togo-on-its-path-towards-the-elimination-of-onchocerciasis. Accessed 22 Feb 2026\u003c/li\u003e\n\u003cli\u003eNicholls RS, Duque S, Olaya LA, L\u0026oacute;pez MC, S\u0026aacute;nchez SB, Morales AL, et al. Elimination of onchocerciasis from Colombia: First proof of concept of river blindness elimination in the world. Parasit Vectors [Internet]. BioMed Central Ltd.; 2018 [cited 2025 Aug 10];11:1\u0026ndash;9. https://doi.org/10.1186/S13071-018-2821-9/TABLES/5\u003c/li\u003e\n\u003cli\u003eHendy A, Krit M, Pfarr K, Laemmer C, De Witte J, Nwane P, et al. Onchocerca volvulus transmission in the Mbam valley of Cameroon following 16 years of annual community-directed treatment with ivermectin, and the description of a new cytotype of Simulium squamosum. Parasit Vectors. 2021;14:563. https://doi.org/10.1186/s13071-021-05072-y\u003c/li\u003e\n\u003cli\u003eWorld Health Organization. Ending the Neglect to Attain the Sustainable Development Goals : A Road Map for Neglected Tropical Diseases 2021-2030. Geneva: World Health Organization; 2021. \u003c/li\u003e\n\u003cli\u003eEXECUTIVE SUMMARY - Guidelines for Stopping Mass Drug Administration and Verifying Elimination of Human Onchocerciasis - NCBI Bookshelf [Internet]. [cited 2026 Feb 22]. https://www.ncbi.nlm.nih.gov/books/NBK344104/. Accessed 22 Feb 2026\u003c/li\u003e\n\u003cli\u003ePOST-ELIMINATION SURVEILLANCE (PHASE 3) - Guidelines for Stopping Mass Drug Administration and Verifying Elimination of Human Onchocerciasis - NCBI Bookshelf [Internet]. [cited 2026 Feb 22]. https://www.ncbi.nlm.nih.gov/books/NBK344123/. Accessed 22 Feb 2026\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"bmc-infectious-diseases","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"infd","sideBox":"Learn more about [BMC Infectious Diseases](http://bmcinfectdis.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/infd","title":"BMC Infectious Diseases","twitterHandle":"#bmcinfectdis","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Onchocerciasis, Onchocerca volvulus, Ivermectin, Mass drug administration, Elimination, Senegal","lastPublishedDoi":"10.21203/rs.3.rs-9176663/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-9176663/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cb\u003eBackground\u003c/b\u003e\u003c/p\u003e \u003cp\u003eOnchocerciasis remains a major public health problem in sub-Saharan Africa, despite decades of control efforts. In Senegal, transmission was historically focal but intense along the Gambia and Fal\u0026eacute;m\u0026eacute; river basins in the southeast of the country. Building on the legacy of the Onchocerciasis Control Programme and the African Programme for Onchocerciasis Control, Senegal implemented long-term community-directed mass drug administration (MDA) with ivermectin, aiming to interrupt local transmission and progress towards elimination.\u003c/p\u003e\u003cp\u003e\u003cb\u003eMethods\u003c/b\u003e\u003c/p\u003e \u003cp\u003eA retrospective descriptive analysis was conducted based on data from the Senegalese National Onchocerciasis Control Programme between 1998 and 2022. Annual MDA reports were used to estimate geographical and therapeutic coverage across endemic districts. Epidemiological impact was assessed through serial parasitological surveys (skin snips), entomological pool-screen PCR of Simulium blackflies, and Ov16 IgG4 ELISA performed on dried blood spots collected from children aged 5\u0026ndash;9 years in the Gambia and Fal\u0026eacute;m\u0026eacute; river basins. These data were interpreted against WHO thresholds for stopping MDA and for verification of elimination.\u003c/p\u003e\u003cp\u003e\u003cb\u003eResults\u003c/b\u003e\u003c/p\u003e \u003cp\u003eFrom 1998 onwards, geographical coverage rapidly reached 100% of identified endemic villages and was maintained throughout the programme. Therapeutic coverage remained around 78\u0026ndash;80% of the total population annually, exceeding the WHO minimum target of 65%. Microfilarial prevalence fell from hyperendemic baseline levels (\u0026gt;\u0026thinsp;15% with nodules in some villages) to near zero within a decade. In the most recent surveys (2020 and 2022), skin snip examinations of more than 1,100 individuals across four historically endemic districts detected no microfilariae. Entomological pool-screening of hundreds of thousands of Simulium flies showed infection rates generally below 0.2%, with several years at zero and values consistently under the WHO threshold for sustainable transmission. In 2022, none of the 6,227 children tested by Ov16 ELISA (3,077 from the Gambia basin and 3,150 from the Fal\u0026eacute;m\u0026eacute; basin) was seropositive, indicating the absence of recent exposure to \u003cem\u003eOnchocerca volvulus\u003c/em\u003e.\u003c/p\u003e\u003cp\u003e\u003cb\u003eConclusion\u003c/b\u003e\u003c/p\u003e \u003cp\u003eHigh and sustained coverage with community-directed ivermectin MDA, combined with rigorous parasitological, entomological and serological surveillance, has led to the apparent interruption of \u003cem\u003eO. volvulus\u003c/em\u003e transmission in Senegal\u0026rsquo;s historical foci. MDA was safely stopped nationwide in 2022, and the country has entered a three-year post-treatment surveillance phase in line with WHO guidelines. Maintaining integrated surveillance and cross-border collaboration will be essential to prevent recrudescence and to support Senegal\u0026rsquo;s dossier for WHO verification of onchocerciasis elimination.\u003c/p\u003e","manuscriptTitle":"Progress Towards Onchocerciasis Elimination in Senegal’s Historical Transmission Zones, Following Sustained Annual Mass Drug Administration and Multilevel Surveillance","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-05-15 16:11:33","doi":"10.21203/rs.3.rs-9176663/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"editorInvitedReview","content":"","date":"2026-05-14T15:56:00+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-10T15:53:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"44207397368658167936048910676688951037","date":"2026-05-09T06:37:46+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"126350425740201470478877320534424286292","date":"2026-05-08T14:57:52+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"249714619842123251175685137557966983237","date":"2026-05-08T09:35:54+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"125479265847339644635128310860234713298","date":"2026-05-07T09:13:06+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-07T09:12:24+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"132167115309187727889897692683647603818","date":"2026-05-07T07:20:58+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-05-06T14:15:07+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-04-17T15:12:06+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2026-04-10T10:28:38+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-04-10T08:51:17+00:00","index":"","fulltext":""},{"type":"submitted","content":"BMC Infectious Diseases","date":"2026-04-10T08:19:22+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"bmc-infectious-diseases","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"infd","sideBox":"Learn more about [BMC Infectious Diseases](http://bmcinfectdis.biomedcentral.com/)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/infd","title":"BMC Infectious Diseases","twitterHandle":"#bmcinfectdis","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"em","reportingPortfolio":"BMC Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"e31e6196-6a1f-43fe-a0bd-48264bb8e002","owner":[],"postedDate":"May 15th, 2026","published":true,"recentEditorialEvents":[{"type":"editorInvitedReview","content":"","date":"2026-05-14T15:56:00+00:00","index":60,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-10T15:53:48+00:00","index":57,"fulltext":""},{"type":"reviewerAgreed","content":"44207397368658167936048910676688951037","date":"2026-05-09T06:37:46+00:00","index":56,"fulltext":""},{"type":"reviewerAgreed","content":"126350425740201470478877320534424286292","date":"2026-05-08T14:57:52+00:00","index":55,"fulltext":""},{"type":"reviewerAgreed","content":"249714619842123251175685137557966983237","date":"2026-05-08T09:35:54+00:00","index":54,"fulltext":""},{"type":"reviewerAgreed","content":"125479265847339644635128310860234713298","date":"2026-05-07T09:13:06+00:00","index":53,"fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-05-07T09:12:24+00:00","index":52,"fulltext":""},{"type":"reviewerAgreed","content":"132167115309187727889897692683647603818","date":"2026-05-07T07:20:58+00:00","index":51,"fulltext":""},{"type":"reviewersInvited","content":"15","date":"2026-05-06T14:15:07+00:00","index":"","fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-15T16:11:33+00:00","versionOfRecord":[],"versionCreatedAt":"2026-05-15 16:11:33","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-9176663","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-9176663","identity":"rs-9176663","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}