Asymptomatic school children and adults are important for the human infectious reservoir for Plasmodium falciparum malaria in an area of low endemicity in The Gambia

Objectives

In The Gambia, the scale-up of malaria control interventions in the past decades resulted in a substantial decrease of the malaria burden. However, low levels of malaria transmission persist.

Methods

We conducted an observational cohort study in eastern Gambia to better understand the relative contribution of symptomatic and asymptomatic malaria infections to the infectious reservoir. Parasite and gametocyte carriage were determined by molecular methods. Infectiousness to mosquitoes was assessed by mosquito membrane feeding assays on a subset of symptomatic and asymptomatic individuals identified by passive case detection and community surveys.

Results

Incidence of clinical malaria was 1.46 episodes/100 person-months. Prevalence of malaria infection as determined by PCR in community surveys was 10.5%. Among asymptomatic malaria-infected individuals, total parasite density was positively associated with gametocyte density (β = 0.40; P < .0001). Mosquito infection rates in membrane feeding experiments were positively associated with gametocyte density (β = 2.81; P < 0.0001). More than 84% of mosquito infections occurred in asymptomatic individuals with patent infections, with the highest contribution from older children (40.3%), and adolescents and adults (45.5%). Clinical malaria cases identified by passive case detection were responsible for only 1% of mosquito infections; if the definition of clinical malaria included infected individuals identified by community surveys with a history of fever in the preceding week, the contribution of clinical cases to mosquito infections increased to 16%.

Conclusions

In eastern Gambia, malaria transmission is maintained by asymptomatic malaria-infected individuals, mostly adults, adolescents and school-age children, while clinical cases are comparatively less important for transmission.