Plasmodium falciparum Carriage in central and northern Mali amid Seasonal Malaria Chemoprevention implementation

Introduction Seasonal malaria chemoprevention (SMC) reduces malaria incidence in children under five in Sahelian countries. However, transmission persists at substantial levels and incidence is re-increasing. Addressing Plasmodium falciparum carriage in asymptomatic individuals could represent a relevant intervention to complement current access to care- and vector-control-based strategies. In this study, we aim to characterise the parasite reservoir dynamics over one year in the general population of two sites of central and northern Mali.

We included members of randomly selected households in four villages (2 in Kati and 2 in Dire districts). At four planned visits, infection was detected using rapid diagnostic tests (RDTs) and qPCR. Clinical malaria incidence was recorded passively at local health facility and Plasmodium positive samples were genotyped. We analyse clinical malaria incidence, P. falciparum PCR prevalence, parasite density and multiplicity of infection (eMOI) at different seasons, as well as factors associated with P. falciparum carriage.

Malaria seasonal dynamics was driven by rainfall in Kati (Central Mali) and by flooding in Dire (Northern Mali). In Kati, prevalence rose from 21.5% at the dry season 2021 baseline to 33.5% in December after the rainy season. Prevalence was lowest in SMC-eligible children across 3 of 4 surveys. Prevalence was highest (40-50%) for 10-24 years old in December. Median parasite densities were generally <10 parasites/µL except for 10-24 years old, who also presented higher eMOI. In Dire, prevalence was high at both baseline and December survey (60%) but dropped in September and in May 22. SMC was not distributed in 2021, and prevalence was homogenously high (50-70%) across groups in December. Parasite densities and eMOI fluctuated strongly across age-groups and survey.

Our findings show that in the “classic” Sahelian, rainfall-driven setting of Kati, a high prevalence of asymptomatic infections among 10-24 year-olds likely represents the reservoir for transmission. In the understudied Northern Mali, our results suggest generalized high burden and complex dynamics. Continuous SMC modifies the age-distribution of the parasite reservoir. Targeting asymptomatic carriers beyond SMC coverage could represent a relevant strategy to further reduce transmission in seasonal settings. Competing Interest Statement The authors have declared no competing interest. Funding Statement This study used data produced within the MARS project (A*Midex International 2018), funded by the Excellence Initiative of Aix Marseille University, A*MIDEX Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study protocol was approved by the Ethics Committee of the University of Sciences, Techniques and Technologies of Bamako, Mali (protocol No. 2020/297/CE/FMOS/ FAPH). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Footnotes This revised version includes minor updates to the Methods, Results and Discussion sections. The laboratory procedures have been clarified and expanded. The description of parasite density categorisation has been standardised. The statistical analysis section has been strengthened with a clearer description of the generalised additive multilevel logistic regression models and the inclusion of behavioural profiles derived from principal component analysis and hierarchical clustering. Three distinct regression models are now explicitly described. Clinical malaria incidence results have been updated following data verification, resulting in revised case counts in both sites. The Results and Discussion sections have been expanded to improve interpretation of age specific prevalence, parasite density and eMOI patterns, and to better contextualise findings within regional transmission settings and previous studies. Public health implications have been further developed, including discussion of targeting asymptomatic carriers beyond current SMC eligibility. Supplementary materials have been reorganised and expanded, with additional figures and tables referenced throughout the manuscript. The reference list has been updated to include recent publications. These revisions improve methodological transparency, analytical clarity, and interpretation of findings. Data Availability All data produced in the present study are available upon reasonable request to the authors