Abstract
The evolution of insecticide-resistance in malaria vectors is emerging as a serious challenge for the control of malaria. Modelling the spread of insecticide-resistance is an essential tool to understand the evolutionary pressures and dynamics caused by the application of insecticides. We developed a population-genetic model of the spread of insecticide-resistance in a population of Anopheles vectors in response to insecticides used either as adulticides (focussing on insecticide-treated bed nets (ITNs)) or as larvicides (either for the control of malaria or, as an inadvertent side-product, in agriculture). We show that indoor use of insecticides leads to considerably less selection pressure than their use as larvicides, supporting the idea that most resistance of malaria vectors is due to the agricultural use of the insecticides that are also used for malaria control. The reasons for the relatively low selection pressure posed by adulticides are (i) that males are not affected by the ITNs and, in particular, (ii) that the insecticides are also repellents, keeping mosquitoes at bay from contacting the insecticide but also driving them to bite either people who do not use the insecticide or alternative hosts. We conclude by discussing the opposing public health benefits of high repellency at an epidemiological and an evolutionary timescale: whereas repellency is beneficial to delay the evolution of resistance, other models have shown that it decreases the population-level protection of the insecticide.
Highlights
Long-lasting insecticidal nets and indoor residual spraying (IRS) have dramatically reduced malaria transmission, for they protect users from being bitten by the mosquito vectors of malaria [1,2,3,4] and, by decreasing the longevity of mosquitoes, offer additional protection at the level of the community [5, 6]
We considered two pressures selecting for IR: insecticidetreated bed nets (ITNs), to which only adult females are exposed, and larvicides, which affect larvae of both sexes
ITN coverage proportion of mosquitoes exposed to agriculturally used insecticide feeding rate on humans repellency rate probability of surviving ITN insecticide exposure survival of risk of feeding-induced death length of gonotrophic cycle daily mortality rate of vector mortality in one gonotrophic cycle additional mortality if repelled once dominance of IR allele level of resistance conferred by IR allele cost of resistance female fecundity
Summary
Long-lasting insecticidal nets and indoor residual spraying (IRS) have dramatically reduced malaria transmission, for they protect users from being bitten by the mosquito vectors of malaria [1,2,3,4] and, by decreasing the longevity of mosquitoes, offer additional protection at the level of the community [5, 6] This success is being eroded by the evolution of various mechanisms of resistance, including behavioural resistance (e.g. failure to be repelled or shifting from indoor-biting to outdoorbiting) [7, 8] and the focus of this article: insecticide-resistance (IR) rendering mosquitoes less sensitive to the insecticide used on the insecticidetreated bed nets (ITNs) [9, 10]. Conclusion: We conclude by discussing the opposing public health benefits of high repellency at an epidemiological and an evolutionary timescale: whereas repellency is beneficial to delay the evolution of resistance, other models have shown that it decreases the population-level protection of the insecticide
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