Modelling the relative abundance of the primary African vectors of malaria before and after the implementation of indoor, insecticide-based vector control.

Marianne E Sinka,Nick Golding,N Claire Massey,Antoinette Wiebe,Simon I Hay,Zhi Huang,Catherine L Moyes

doi:10.1186/s12936-016-1187-8

Abstract

BackgroundMalaria remains a heavy burden across sub-Saharan Africa where transmission is maintained by some of the world’s most efficient vectors. Indoor insecticide-based control measures have significantly reduced transmission, yet elimination remains a distant target. Knowing the relative abundance of the primary vector species can provide transmission models with much needed information to guide targeted control measures. Moreover, understanding how existing interventions are impacting on these relative abundances highlights where alternative control (e.g., larval source management) is needed.MethodsUsing the habitat suitability probabilities generated by predictive species distribution models combined with data collated from the literature, a multinomial generalized additive model was applied to produce relative abundance estimates for Anopheles arabiensis, Anopheles funestus and Anopheles gambiae/Anopheles coluzzii. Using pre- and post-intervention abundance data, estimates of the effect of indoor insecticide-based interventions on these relative abundances were made and are illustrated in post-intervention maps.ResultsConditional effect plots and relative abundance maps illustrate the individual species’ predicted habitat suitability and how they interact when in sympatry. Anopheles arabiensis and An. funestus show an affinity in habitat preference at the expense of An. gambiae/An. coluzzii, whereas increasing habitat suitability for An. gambiae/An. coluzzii is conversely less suitable for An. arabiensis but has little effect on An. funestus. Indoor insecticide-based interventions had a negative impact on the relative abundance of An. funestus, and a lesser effect on An. arabiensis. Indoor residual spraying had the greatest impact on the relative abundance of An. funestus, and a lesser effect on An. gambiae/An. coluzzii. Insecticide-treated bed nets reduced the relative abundance of both species equally. These results do not indicate changes in the absolute abundance of these species, which may be reduced for all species overall.ConclusionsThe maps presented here highlight the interactions between the primary vector species in sub-Saharan Africa and demonstrate that An. funestus is more susceptible to certain indoor-based insecticide interventions than An. gambiae/An. coluzzii, which in turn, is more susceptible than An. arabiensis. This may provide An. arabiensis with a competitive advantage where it is found in sympatry with other more endophilic vectors, and potentially increase the need for outdoor-based vector interventions to deal with any residual transmission barring the way to malaria elimination.Electronic supplementary materialThe online version of this article (doi:10.1186/s12936-016-1187-8) contains supplementary material, which is available to authorized users.

Highlights

Malaria remains a heavy burden across sub-Saharan Africa where transmission is maintained by some of the world’s most efficient vectors
Anopheles funestus is thought to have been the first African species to exploit humans as a food source [3] and is regarded as a more efficient vector than An. gambiae/An. coluzzii in some parts of its range [4]. Anopheles arabiensis is another member of the An. gambiae complex and is considered less anthropophilic than either An. gambiae, An. coluzzii or An. funestus [5]. It tends to maintain a life cycle outdoors and is more likely to avoid the two primary methods of vector control used across Africa (long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS)) than the other species [6]
The final analysis for relative abundance used only those data where adult abundance of all three species were reported and where members of the An. gambiae complex were identified to species

Summary

Introduction

Malaria remains a heavy burden across sub-Saharan Africa where transmission is maintained by some of the world’s most efficient vectors. Anopheles arabiensis is another member of the An. gambiae complex and is considered less anthropophilic than either An. gambiae, An. coluzzii or An. funestus [5] It tends to maintain a life cycle outdoors and is more likely to avoid the two primary methods of vector control used across Africa (long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS)) than the other species [6]. This allows malaria transmission to continue even where the abundance of the other two species has been significantly lowered [7]. By estimating the relative abundance of multiple species in a location, predictions can be made about the continuing impact of specific control measures where the species’ biology or behaviour are known [7]

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Conclusion