Asymptomatic Plasmodium falciparum Carriage at the End of the Dry Season is Associated with Subsequent Infection and Clinical Malaria in Eastern Gambia

Background Chronic carriage of asymptomatic low-density Plasmodium falciparum parasitaemia in the dry season may support maintenance of acquired immunity that protects against clinical malaria. However, the relationship between chronic low-density infections and subsequent risk of clinical malaria episodes remains unclear. Methods In a 2-years study (December 2014 to December 2016) in eastern Gambia, nine cross-sectional surveys using molecular parasite detection were performed in the dry and wet season. During the 2016 malaria transmission season, passive case detection identified episodes of clinical malaria. Results Among the 5,256 samples collected, 444 (8.4%) were positive for P. falciparum . A multivariate model identified village of residence, male sex, age ≥5 years old, anaemia, and fever as independent factors associated with P. falciparum parasite carriage. Infections did not cluster over time within the same households or recurred among neighbouring households. Asymptomatic parasite carriage at the end of dry season was associated with a higher risk of infection (Hazard Ratio, HR= 3.0, p< 0.0001) and clinical malaria (HR= 1.561, p= 0.057) during the following transmission season. Age and village of residence were additional predictors of infection and clinical malaria during the transmission season. Conclusion Chronic parasite carriage during the dry season is associated with an increased risk of malaria infection and clinical malaria. It is unclear whether this is due to environmental exposure or to other factors. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This work was funded by grants from the Netherlands Organization for Scientific Research (Vidi fellowship NWO 016.158.306) and the Bill & Melinda Gates Foundation (INDIE OPP1173572), the joint MRC/LSHTM fellowship, CNRS, the French National Research Agency (18-CE15-0009-01). The SPF- Post-doc en France programme of the Fondation pour la Recherche Medicale funded the author Balotin Fogang (SPF202209015889). Our thanks go to Julia Mwesigwa and all fieldworkers involved in the study. We would like to thank the inhabitants of the four villages (Madina Samako, Njayel, Sendebu, and Karanbada) in eastern part of The Gambia who took part in this study. ### 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 reviewed and approved by the Gambia Government/MRC Joint Ethics Committee (SCC 1476, SCC 1318, L2015.50) and by the London School of Hygiene & Tropical Medicine ethics committee (Ref 10982). 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 All data produced in the present study are available upon reasonable request to the authors