Asymptomatic Plasmodium falciparum carriage and clinical disease: a five-year community-based longitudinal study in The Gambia.

Background: Carriers of persistent asymptomatic Plasmodium falciparum infections constitute an infectious reservoir that maintains malaria transmission. Understanding the extent of carriage and characteristics of carriers specific to endemic areas could guide use of interventions to reduce infectious reservoir. Methods In eastern Gambia, an all-age cohort from four villages was followed up from 2012 to 2016. Each year, cross-sectional surveys were conducted at the start and end of the malaria transmission season to determine asymptomatic P. falciparum carriage by microscopy while passive case detection was conducted during the transmission to determine incidence of clinical malaria. Association between carriage at the end of a season and at the start of the next and risk factors for this were assessed. Effect of carriage at the start of the season on risk of clinical malaria during the season was also examined. Results A total of 1403 individuals − 1154 from a semi-urban village and 249 from three rural villages were enrolled; median age was 12 years (IQR 6, 30) and 12 years (IQR 7, 27) respectively. In adjusted analysis, asymptomatic P. falciparum carriage at the end of a transmission season and at the start of the next were strongly associated (OR = 19.99; 95% CI 12.57–31.77, P < 0.001). The odds of carriage across seasons (i.e carriage at the end of a season and at the start of the next) were higher in rural villages (OR = 13.0; 95% CI 6.33–26.88, P < 0.001) and amongst children aged 5–15 years (OR = 5.03; 95% CI 2.47–10.23, P = < 0.001). In the rural villages, carriage at the start of the transmission season was associated with a lower risk of clinical malaria (IRR 0.48, 95% CI 0.27–0.81, P = 0.007). Conclusions Asymptomatic P. falciparum carriage at the end of a transmission season strongly predicts carriage at the start the next season. Odds of carriage across seasons was higher in villages with relatively higher transmission and in older children. Interventions that reduce persistent asymptomatic infections can be applied in a targeted manner and may supress the initiation of yearly seasonal malaria transmission.