Five years of post-validation surveillance of lymphatic filariasis in Thailand

Abstract Background: Eighteen countries and territory (Cambodia, The Cook Islands, Egypt, Kiribati, Maldives, Malawi, Marshall Islands, Niue, Palau, Sri Lanka, Thailand, Togo, Tonga, Vanuatu, Viet Nam, Wallis and Futuna,Yemen and recently Bangladesh are now acknowledged as achieving elimination of lymphatic filariasis (LF) as a public health problem (WHO web). Work on lymphatic filariasis (LF) in Thailand started in 1949 with the first survey to identify lymphoedema cases. The Division of Lymphatic Filariasis under the Department of Health, Ministry of Public Health (MoPH) Thailand was established in 1961. The National Programme for Elimination on of Lymphatic Filariasis (NPELF) was launched in 2001. As a result of intensive interventions with the NPELF over more than 15 years, WHO certified Thailand in September 2017 as having eliminated LF as a public health problem with recommendations that continued surveillance would be required and people affected with chronic filariasis would have full care and access to support systems for treatment and care. This article describes measures and progress made in Thailand with post elimination surveillance of LF from 2018 till 2022. Methods: The implementing unit (IU) is a sub-village in 11 former LF endemic provinces. Human blood surveys are targeted in 10% of sub villages each year. In Wuchereria bancrofti areas, filaria antigen test kits (FTS) are used and in B. malayi areas, antibody test kits (FilariaDIAG RAPID) are used and positive cases confirmed by thick blood film (TBF) and PCR. Vector surveys are conducted in 1% of sub-villages where human blood surveys are conducted in that year. Selection is guided by sub villages where the current LF vectors are predominant and known environmental reservoirs (B. malayi areas). Three nights of mosquito trapping are done where mosquito species identification and Mf detection is done. Brugia spp. are sent for PCR confirmation. Blood surveys using FTS are conducted among migrants in five provinces that record the highest number of labour migrants registered in the previous year and target 300 - 500 samples per province among migrants who are non-registered. Additional migrant health screening data for registered migrants surveillance are through health check-up conducted by various agencies, including Provincial Health Office, the Division of International Disease Control Ports”, the Department of Medical Service of the Bangkok Metropolitan Authority (BMA) and the One Stop Service (OSS), Office of the Permanent Secretary of MOPH. In the four provinces previously endemic for B. malayi, active surveillance of cats are done in areas that previously recorded > 1.0% mf rate among cats, using thick blood film (TBF) and positive TBF are confirmed with PCR. LF chronic disease survey and management is done every 2 years and includes MMDP and health facility assessments in LF endemic areas where a chronic disease patients reside under the care of a sub-district health facility in each province. The register of chronic patients is updated every year by the province and DVBD. The primary tools are the MMDP kit for chronic patients and the WHO Direct Inspection Protocol version 1.1 for health facilities and includes trainings for health facility staff. Results: Of a total of 357 former endemic IUs in 11 provinces, over the period of 2018-2022, human blood surveys were conducted in145 IUs (41%) with an average population coverage of 81%. 24 Mf positive cases detected in 10 IUs of Narathiwat province only. An additional 2 cases were detected in one province through routine public health surveillance. Over 2018-2022, vector surveys were done in all 11 provinces in 47 IUs with Mf (all B. malayi) detected only in Narathiwat province in 2018 and 2019 in dissected Ma. annulata and Ma. bonneae species. There was a reduction in the cohort of LF patients with lymphoedema/elephantiasis from 114 in 2017 to 76 in 2022 with all patients under the care of 42 health facilities. Surveillance among unregistered migrants yielded 12 antigen positive cases in 6 provinces over 2018-2022. However no Mf in mosquitoes were detected in vector surveys in these same areas. Survey among domestic/peri-domestic cats in Narathiwat province yielded a decline in MPR from 1.9% in 2018 to 0.7% in 2022. All Brugia species are confirmed by PCR. In IUs where Mf positive cats were detected, new human Mf cases were also detected. MMDP assessments revealed gaps in health care provider’s management of chronic cases due to staff turnover. Conclusions: Current WHO recommendations are for countries to continue with PVS for 10 years after validation of LF elimination. In 2022, after 5 years of PVS, Thailand has re-surveyed 41% of all its 357 previously endemic IUs in 11 provinces and demonstrating on-going transmission in only one province of Narathiwat where Mf prevalence was below the WHO transmission threshold of 1% for 2018-2022 except for 2019 where it was 1.44%. It is envisaged Thailand will recover from program setbacks in 2020-21 due to COVID-19 and achieve 100% coverage of its IUs by 2027. Passive surveillance nation wide, targeted migrant screening in specified provinces, MMDP refresher training especially for newly stationed health facility staff and interventions for possible zoonotic transmission in Narathiwat province needs to continue in the PVS phase.