Abstract
BackgroundFor malaria control in Africa it is crucial to characterise the dispersal of its most efficient vector, Anopheles gambiae, in order to target interventions and assess their impact spatially. Our study is, we believe, the first to present a statistical model of dispersal probability against distance from breeding habitat to human settlements for this important disease vector.Methods/Principal FindingsWe undertook post-hoc analyses of mosquito catches made in The Gambia to derive statistical dispersal functions for An. gambiae sensu lato collected in 48 villages at varying distances to alluvial larval habitat along the River Gambia. The proportion dispersing declined exponentially with distance, and we estimated that 90% of movements were within 1.7 km. Although a ‘heavy-tailed’ distribution is considered biologically more plausible due to active dispersal by mosquitoes seeking blood meals, there was no statistical basis for choosing it over a negative exponential distribution. Using a simple random walk model with daily survival and movements previously recorded in Burkina Faso, we were able to reproduce the dispersal probabilities observed in The Gambia.Conclusions/SignificanceOur results provide an important quantification of the probability of An. gambiae s.l. dispersal in a rural African setting typical of many parts of the continent. However, dispersal will be landscape specific and in order to generalise to other spatial configurations of habitat and hosts it will be necessary to produce tractable models of mosquito movements for operational use. We show that simple random walk models have potential. Consequently, there is a pressing need for new empirical studies of An. gambiae survival and movements in different settings to drive this development.
Highlights
Knowledge of the movements of female vector mosquitoes between water bodies and human hosts is fundamental to understanding spatial variation in malaria transmission rates [1] and for planning and evaluating the impact of vector control [2,3,4]
Conclusions/Significance: Our results provide an important quantification of the probability of An. gambiae s.l. dispersal in a rural African setting typical of many parts of the continent
Non-linear regression produced a highly significant negative exponential model (a = 190.250, SEa = 2.862; b = 20.172, SEb = 0.037; F = 32.9, p,0.001), in which distance from the breeding sites explained over 40% of the variance in geometric mean (GM) female Anopheles gambiae s.l. trapped in villages (Figure 3a; R2 = 0.417)
Summary
Knowledge of the movements (dispersal) of female vector mosquitoes between water bodies (emergence and subsequent egg laying) and human hosts (for blood meals) is fundamental to understanding spatial variation in malaria transmission rates [1] and for planning and evaluating the impact of vector control [2,3,4]. We conducted a retrospective analysis of mosquitoes sampled in villages found at a range of distances from mapped larval habitat in The Gambia [7] This geographic approach allowed us to statistically estimate the probability of female An.gambiae s.l. dispersing a given Euclidean distance between breeding sites and villages, quantifying an important parameter for spatial approaches to malaria control in this kind of landscape. Negative exponential decay functions of declining dispersal probability with increasing distance are generally suitable for modelling passive dispersal, but they may underestimate long-distance dispersal events in actively dispersing organisms [13], such as mosquitoes, that will keep moving until they satisfy the objective of their search Relatively rare, these longer movements may be of ecological importance [14], in this case for both malaria transmission and gene flow among populations of vector and parasite. We believe, the first to present a statistical model of dispersal probability against distance from breeding habitat to human settlements for this important disease vector
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