The role of submicroscopic parasitemia in malaria transmission: what is the evidence?

Abstract

Achieving malaria elimination requires targeting the human reservoir of infection, including those with asymptomatic infection. Smear-positive asymptomatic infections detectable by microscopy are an important reservoir because they often persist for months and harbor gametocytes, the parasite stage infectious to mosquitoes. However, many asymptomatic infections are submicroscopic and can only be detected by molecular methods. Although there is some evidence that individuals with submicroscopic malaria can infect mosquitoes, transmission is much less likely to occur at submicroscopic gametocyte levels. As malaria elimination programs pursue mass screening and treatment of asymptomatic individuals, further research should strive to define the degree to which submicroscopic malaria contributes to the infectious reservoir and, in turn, what diagnostic detection threshold is needed to effectively interrupt transmission.

Keywords: asymptomatic infection; diagnostic; gametocyte; malaria elimination; reservoir.

Figure I

Figure 1. Submicroscopic gametocytemia and the mosquito infectivity threshold

Mosquito feeding assays, whereby Anopheles mosquitoes are fed blood-stage parasites, either by direct skin feeding or via a membrane feeding apparatus, and subsequently dissected to look for malaria parasites, represent the standard way of assessing human-to-mosquito infectious potential [33,34,35,36]. Depicted are approximate curves of human-to-mosquito infectivity as a function of density of gametocytes in the circulating blood drawn from data depicting natural human malaria infections in West Africa (blue) [39,40] and children post-treatment in Burkina Faso and Kenya (red) [41]. While the range of the older series is much greater, both demonstrate that the probability of mosquito infection is very low until gametocyte densities well within the range of parasitemia detectable by microscopy.