Wind direction and proximity to larval sites determines malaria risk in Kilifi District in Kenya

Abstract

Studies of the fine-scale spatial epidemiology of malaria consistently identify malaria hotspots, comprising clusters of homesteads at high transmission intensity. These hotspots sustain transmission, and may be targeted by malaria-control programmes. Here we describe the spatial relationship between the location of Anopheles larval sites and human malaria infection in a cohort study of 642 children, aged 1-10-years-old. Our data suggest that proximity to larval sites predict human malaria infection, when homesteads are upwind of larval sites, but not when homesteads are downwind of larval sites. We conclude that following oviposition, female Anophelines fly upwind in search for human hosts and, thus, malaria transmission may be disrupted by targeting vector larval sites in close proximity, and downwind to malaria hotspots.

Figure 1. Location of larval sites and homesteads showing malaria incidence.

Plots of homesteads (circles) and larval sites (triangles) by longitude and latitude. (a) shows prevalence of asymptomatic parasitaemia by homestead, (b), hotspots of asymptomatic parasitaemia, (c), incidence of febrile malaria, and (d), hotspots of febrile malaria.

Figure 2. Effect of bearing and distance between larval site and homestead on febrile malaria incidence.

Each point on the graph is aggregated for a 180-degree arc of bearings, 90 degrees either side of the central direction indicated on the y axis. The x axis indicates the effect size in a linear regression model of distance from breeding site on febrile malaria incidence.

Figure 3. Effect of bearing and distance between larval site and homestead on prevalence of asymptomatic parasitaemia.

Each point on the graph is aggregated for a 180-degree arc of bearings, 90 degrees either side of the central direction indicated on the y axis. The x axis indicates the odds ratio from a logistic regression model of distance from larval site on asymptomatic parasitaemia.

Figure 4. Elevation and normalized vegetation index in the study area.

Variation in elevation (a) and variance in normalized vegetation index with overlain homestead and larval site locations (b,c).