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

Abstract

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.

AsymptomaticPlasmodium falciparumCarriage at the End of the Dry Season is Associated with Subsequent Infection and Clinical Malaria in Eastern Gambia

“…Consistent with previous studies in The Gambia [6,13,20], the prevalence of infection was lower in children under 5 years of age than in older children and adults, possibly due to the seasonal malaria chemoprophylaxis programme (monthly antimalarial treatment during the transmission season) [28,29]. The higher prevalence of infection and high parasite densities in older children than in adults may be the result of acquired immunity that makes adults better protected and able to better control parasite growth when infected [30].…”

“…The discrepancy between asymptomatic infections being a protection or a risk for reinfection/clinical malaria could be explained by the large range of malaria transmission intensity between the cited studies, as well as local environmental conditions. For example, a recent study conducted in The Gambia revealed that in the semi-urban villages dry season carriage was associated with an increased risk of clinical malaria during the following transmission season, whereas such risk was lower in rural villages [6]. Finally, a pooled analysis also demonstrated the importance of transmission intensity with risks of subsequent clinical malaria, as well as host age and polyclonality of infection [19] although this meta-analysis encompassed studies that evaluated clinical infections within the same asymptomatic infection.…”

Higher multiplication rates ofPlasmodium falciparumin isolates from hospital cases compared with community infections

“…Malaria In The Gambia remains endemic, with almost all cases being caused by P. falciparum, although the incidence is lower than experienced two decades ago [16][17][18][19]. To analyse intrinsic multiplication rates of parasites, isolates from cryopreserved blood samples of hospital cases and community infections have been newly established and assayed in under exponential growth conditions in culture.…”

Prevalence and drivers of malaria infections among asymptomatic individuals from selected communities in five regions of Mainland Tanzania with varying transmission intensities

“…Asymptomatic individuals serve as reservoirs of infections and contribute to the persistence of transmission impending malaria reduction and elimination efforts [27]. The scarcity of data on the scope and prevalence of asymptomatic malaria presents a critical gap in malaria control efforts.…”