Topographic mapping of the interfaces between human and aquatic mosquito habitats to enable barrier targeting of interventions against malaria vectors.

Victoria M Mwakalinga,Daniel F Msellemu,Alex J Limwagu,Maureen Coetzee,Gerry F Killeen,Nicodem J Govella,Prosper P Chaki,Yeromin P Mlacha,Benn K D Sartorius,Stefan Dongus

doi:10.1098/rsos.161055

Abstract

Geophysical topographic metrics of local water accumulation potential are freely available and have long been known as high-resolution predictors of where aquatic habitats for immature Anopheles mosquitoes are most abundant, resulting in elevated densities of adult malaria vectors and human infection burden. Using existing entomological and epidemiological survey data, here we illustrate how topography can also be used to map out the interfaces between wet, unoccupied valleys and dry, densely populated uplands, where malaria vector densities and infection risk are focally exacerbated. These topographically identifiable geophysical boundaries experience disproportionately high vector densities and malaria transmission risk, because this is where Anopheles mosquitoes first encounter humans when they search for blood after emerging or ovipositing in the valleys. Geophysical topographic indicators accounted for 67% of variance for vector density but for only 43% for infection prevalence, so they could enable very selective targeting of interventions against the former but not the latter (targeting ratios of 5.7 versus 1.5 to 1, respectively). So, in addition to being useful for targeting larval source management to wet valleys, geophysical topographic indicators may also be used to selectively target adult Anopheles mosquitoes with insecticidal residual sprays, fencing, vapour emanators or space sprays to barrier areas along their fringes.

Highlights

Recent attempts to demonstrate the value of targeting malaria transmission hotspots with elevated vector densities and human infection burden yielded disappointing results in a rural setting with dispersed settlement patterns and variable but ubiquitous transmission [1]
It has been suggested that this geographically selective approach might be more effective in settings with more aggregated populations, because this will result in less dispersal of Anopheles mosquitoes and dispersion of malaria transmission across the landscape [1]
Consistent with results from previous analyses [26], the application of larvicide by means of granular Bacillus thuringiensis var. israelensis (Bti), after but not before the Ministry of Health and Social Welfare (MoHSW) took over the management of all delivery activities from an external contractor, resulted in a reduction of both densities of adult A. gambiae and human malaria infection predictor relative ratea or odds proportion relative ratiob [95% CI]

Summary

Introduction

Recent attempts to demonstrate the value of targeting malaria transmission hotspots with elevated vector densities and human infection burden yielded disappointing results in a rural setting with dispersed settlement patterns and variable but ubiquitous transmission [1]. It has been suggested that this geographically selective approach might be more effective in settings with more aggregated populations, because this will result in less dispersal of Anopheles mosquitoes and dispersion of malaria transmission across the landscape [1]. Taking this rationale further, the urban contexts of towns and cities may perhaps offer the most ideal settings for geographical targeting of supplementary interventions: dense human populations surrounding aquatic larval habitats allow Anopheles mosquitoes to feed nearby and return to oviposit, limiting their dispersal and the diffusion of malaria transmission across the landscape [2,3,4]. Hotspots can occur at scales of less than 100 m, and even at the level of single households [1,13,14], so mapping these out at sufficiently high resolution may not be realistically feasible across large programmatic scales with existing entomological and epidemiological survey techniques [1,14,17]

Methods

Results

Discussion

Conclusion