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

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.

Mapping Trends in Insecticide Resistance Phenotypes in African Malaria Vectors

“…gambiae complex sample that was tested. Our analysis only includes test results that are representative of the original sample collected 8, 27 , and our predictions cannot directly represent variation in the prevalence of resistance due to variation in the composition of sibling species 23,38 . Routine identification of the composition of sibling species in tested samples, and the provision of species-specific mortality values, would improve the capacity of susceptibility test data to inform prediction of resistance.…”

“…gambiae and An. coluzzii 38 . For all variables, we obtained spatially explicit data on a grid with a 2.5 arc-minute resolution (which is approximately 5 km at the equator) covering Sub-Saharan Africa.…”

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“…In many elimination settings, residual transmission exists due to anopheline species that exhibit outdoor feeding and resting behaviour (exophagy and exophily), which often coincides with outdoor human behaviours during biting hours [23, 24]. While long-lasting insecticide-treated nets (LLINs) and indoor residual spraying (IRS) are effective for more endophagic and endophilic anopheline species, where high coverage of LLINs and IRS has been achieved, programmes often observe behavioural shifts towards increased outdoor feeding and resting [25–27], as well as increased proportional abundance of more exophilic species such as Anopheles arabiensis [28]. In much of Asia, and southeast Asia in particular, malaria transmission persists due to forest-adapted vectors, such as Anopheles dirus, which exhibit outdoor biting behaviour, transmit Plasmodium knowlesi between macaque monkeys and humans, and are very difficult to control [29, 30].…”

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“…There is no disease importation from outside the modelled area. Transmission intensity and seasonality reflect southern Africa, with two vector species: Anopheles arabiensis (80% of vectors) and Anopheles funestus (20% of vectors) [26,27]. Both vector species are assumed to have 65% anthropophily, with 50% indoor biting for An.…”