Plasmodium falciparum carriage in central and northern Mali amid seasonal malaria chemoprevention implementation

Abstract

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 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 in 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 quantitative polymerase chain reaction (qPCR). Clinical malaria incidence was recorded passively at local health facilities, and Plasmodium- positive samples were genotyped. We analysed 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 were 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 33.3% 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-year-olds in December. Median parasite densities were generally <10 parasites/µL except for 10-24-year-olds, who also presented higher eMOI. In Dire, prevalence was high at both the baseline and December surveys (60%) in 2021 but dropped in September 2021 and in May 2022. SMC was not distributed in 2021, and prevalence was homogeneously high (50–70%) across age groups in December. Parasite densities and eMOI fluctuated strongly across age groups and surveys. Our findings show that in the "classic" Sahelian, rainfall-driven setting of Kati, a high prevalence of P. falciparum carriage among 10–24 year-olds likely represents the reservoir for transmission. In the understudied Northern Mali, our results suggest a generalized high burden and complex dynamics. Continuous SMC modifies the age distribution of the parasite reservoir. Targeting P. falciparum carriage beyond SMC coverage could represent a relevant strategy to further reduce transmission in seasonal settings.