A Cohort Study on the Duration of Plasmodium falciparum Infections During the Dry Season in The Gambia

Abstract

Background In areas where Plasmodium falciparum malaria is seasonal, a dry season reservoir of blood-stage infection is essential for initiating transmission during the following wet season. Methods In The Gambia, a cohort of 42 individuals with qPCR positive P. falciparum infections at the end of the transmission season (December) were followed monthly until the end of the dry season (May) to evaluate infection persistence. The influence of human host and parasitological factors was investigated. Results A large proportion of individuals infected at the end of the wet season had detectable infections until the end of the dry season (40.0%; 16/40). At the start of the dry season, the majority of these persistent infections (82%) had parasite densities >10 p/µL compared to only 5.9% of short-lived infections. Persistent infections (59%) were also more likely to be multi-clonal than short-lived infections (5.9%) and were associated with individuals having higher levels of P. falciparum-specific antibodies (p = 0.02). Conclusion Asymptomatic persistent infections were multi-clonal with higher parasite densities at the beginning of the dry season. Screening and treating asymptomatic infections during the dry season may reduce the human reservoir of malaria responsible for initiating transmission in the wet season.

Strong isolation by distance and evidence of population microstructure reflect ongoingPlasmodium falciparumtransmission in Zanzibar

“…Similar to a few previous studies, we found that asymptomatic infections had a higher COI than symptomatic infections across both the coastal mainland and Zanzibar parasite populations (Sarah-Matio et al, 2022). Potential reasons for this phenomenon include low level immune response resulting in chronic infection with multiple clones over time in asymptomatic individuals (Felger et al, 2012), and dominance of more virulent clones in shorter symptomatic infections resulting in lower COI (Collins et al, 2022). In Zambia, one study suggested that infections that cause asymptomatic infection may be genetically different than those that cause symptomatic infection (Searle et al, 2017).…”

Plasmodium falciparum gametocyte production in longitudinally monitored incident infections is associated with duration of infection and human host factors

“…The exact duration of these infections was impossible to estimate with precision, given the 4-week interval between scheduled visits, but it is evident that the majority of infections were brief and spontaneously cleared. While naturally acquired immunity has been suggested as determinant of infection clearance 32 , recent analyses suggest that infection duration may be shortest in young children 30 and acquired immunity may actually be a prerequisite to sustain chronic infections 33 , rendering a role of immunity-dependent infection clearance less obvious. Importantly, we provide the first evidence that these short-duration infections do not result in measurable gametocyte densities.…”

“…33 gametocytes per µl (IQR 2.56-97.68) among incident asymptomatic infections. Analyzing dynamics of gametocyte densities over the course of infections, 28.1% (9/32) of peak gametocyte densities occurred at the initial visit, 50%(16/32) occurred after the initial visit but by week 4, and 21.9% (7/32) occurred at a timepoint beyond week 4.…”

Gametocyte production in incident P. falciparum infections: a longitudinal study in a low transmission setting under intensive vector control

“…The exact duration of these infections was impossible to estimate with precision, given the 4-week interval between scheduled visits, but it is evident that the majority of infections were brief and spontaneously cleared. While naturally acquired immunity has been suggested as determinant of infection clearance (Bruce et al, 2000; Felger et al, 2012; Nguyen et al, 2018), recent analyses suggest that infection duration may be shortest in young children (Briggs et al, 2020) and acquired immunity may actually be a prerequisite to sustain chronic infections (Collins et al, 2022), rendering a role of immunity-dependent infection clearance less obvious. Importantly, we provide the first evidence that these short-duration infections do not result in measurable gametocyte densities.…”