Using evolution to control malaria

We’ve posted a number of articles over the years – see here and here – that deal with ways in which evolution can be used to improve malaria eradication. And we’d like to thank Rob Boyd for bringing another story on an evolutionary approach to malaria control to our attention. Penelope Anne Lynch and Mike Boots, from the University of Exeter in Cornwall, UK, have been model-building to come up with new ideas to get around the persistent problem with malaria eradication – the evolution of resistance to insecticides. They have come up with what they call a “double dip” system of disease control which involves insecticides and mosquito repellents.

A key aim of any anti-malarial program, they argue, is to keep disease-bearing mosquitoes out of houses, because the vast majority of malaria is contracted via mosquito bites indoors between dusk and dawn. Historically, DDT has acted as both an insecticide and a spatial repellent but nowadays the most common spatial repellents are burning chemical coils. Their models combine treating house interiors with highly toxic (to mosquitoes) insecticides and a spatial insect repellent that is only effective against a proportion of mosquitoes. Mosquitoes capable of evading the repellent encounter the insecticide and are killed. Over time this selects for mosquito phenotypes that are successfully repelled, thus protecting house interiors where most of these vectors do their dirty work.

This is how they explain their approach: “We propose that there is an opportunity to exploit the evolutionary processes generated by using lethal insecticides to create effective new spatial repellents from compounds that initially repel only a fraction of the vector population. Specifically, we propose a combination of a spatial repellent, which deters mosquitoes from entering buildings, with a low-contact-repellence high-toxicity insecticide, which kills those which do enter, leading to a ‘double-dip’ system of disease control. If failure to enter buildings is seen as a method of transmission reduction in its own right, then the repellent provides transmission reduction by deflecting mosquitoes. For mosquitoes that are not deflected, the mortality imposed by contact with the insecticide will provide transmission reduction in the same manner as current insecticide-only control methods. Provided that, in a given context, the fitness cost of being deflected is less than the fitness cost of being susceptible to the insecticide, there is potential for selection to favour an increase in the proportion of deflected individuals in the treated population. If deflection is viewed as a form of ‘behavioural resistance’, this system actively exploits ‘resistance’ evolution, as mortality generated by the insecticide serves to select for phenotypes that are deflected by the repellent. Candidates for use as spatial repellents thus only need initially to repel a small proportion of a mosquito population, since in this instance, for once, evolution will work to enhance the efficacy of a disease-control measure.”