10 April 2009

Evolution-proof insecticide?

ResearchBlogging.orgIn this week's issue of PLoS Biology, an essay describes the perfect means for controlling malaria-carrying mosquitoes: an "evolution-proof insecticide." By taking advantage of the life history traits of both mosquitoes and the malaria parasite, Read et al. argue it should be possible to create an insecticide that will cut malaria transmission without selecting for resistance in the mosquitoes.

Malaria remains a major public health problem in much of the world - according the World Health Organization, a child dies of the disease every 30 seconds, and the cost of malaria may cut economic growth by as much as 1.3% in countries with high infection rates. In the absence of a vaccine, the best approach for malaria management is to control the mosquitoes that transmit the malaria parasite. This is usually done with insecticides, but these have a limited useful lifespan, as they create strong selective pressure for mosquito populations to evolve resistance.


Photo by LoreleiRanveig.
As Read et al. point out, it's not that we need to kill off mosquitoes as such; we just need to stop them from transmitting malaria. If this can be accomplished without strongly reducing the mosquitoes' fitness, it would reduce or eliminate selection for resistance. Malaria typically needs a long time to incubate inside a mosquito before it becomes transmissible to humans, and, in what Read et al. call "one of the great ironies of malaria," this incubation time is longer than most mosquitoes live. That is, the mosquitoes who successfully transmit malaria are the small proportion of the population who live long enough to incubate the parasite.

Here's where evolutionary biology interacts with the life history of malaria parasites in a highly convenient way: an insecticide that selectively targets older mosquitoes will have a smaller impact on the mosquito population's fitness. This is because most of a female mosquito's fitness - the total number of offspring she produces - is concentrated in her first one or two egg-laying cycles. Her fitness can increase if she survives to complete more cycles, but it's pretty rare that she does. From natural selection's point of view, that first of eggs counts much more than possible future batches, because they're not very likely.

For that hypothetical female mosquito to transmit malaria, she has to bite an infected human in the course of feeding to fuel one egg-laying cycle, then incubate the malaria parasites for an additional two to six cycles. Therefore, say Read et al., an insecticide that doesn't harm mosquitoes until they complete their first few egg-laying cycles is the "evolution-proof" solution - the only offspring it "steals" from the affected mosquitoes were pretty improbable anyway, and it prevents the malaria parasites from incubating long enough to successfully infect a new human host.

As it happens, the evolution-proof insecticide might not be a chemical agent, but a biological one. A paper I discussed back in January suggested that infecting malaria-carrying mosquitoes with the parasitic Wolbachia bacterium could control mosquito populations [$-a] by, yes, reducing their total lifespan to something less than the malaria parasite's incubation time. In short, it looks like the goal of a malaria-free world is not as improbable as it used to be.

References

McMeniman, C., Lane, R., Cass, B., Fong, A., Sidhu, M., Wang, Y., & O'Neill, S. (2009). Stable introduction of a life-shortening Wolbachia infection into the mosquito Aedes aegypti Science, 323 (5910), 141-144 DOI: 10.1126/science.1165326

Read, A., Lynch, P., & Thomas, M. (2009). How to make evolution-proof insecticides for malaria control PLoS Biology, 7 (4) DOI: 10.1371/journal.pbio.1000058

2 comments:

  1. Fascinating post. I had gotten into an argument w. a coworker once about mosquitoes, with her arguing they die after one feeding and me arguing that they go on for several. We didn't go into egg-laying cycles and the one entomologist I spoke to over the phone couldn't tell me how many feedings were required for a batch of eggs or how many egg-laying cycles an average mosquito went through.

    I've heard a number of conservatives railing against environmentalists and their banning of DDT, speaking of it as the only thing that could save us from malaria. Have any mosquito species developed resistance to it?

    It occurs to me to wonder how likely it is that malaria might evolve an answer to the selective insecticide by reducing its incubation period.

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  2. Hey, Matt - so, yeah, I wasn't aware myself that most mosquitoes only make it through one or two batches of eggs until I read Read et al. I also gather from that article that a female mosquito only needs one feeding to prepare a batch of eggs, although I guess if she's disturbed mid-bite she may end up sampling multiple hosts.

    Mosquitoes have indeed evolved DDT resistance - Read et al. discuss this in a box set apart from the main text, with a graph of resistance rates to several insecticides and strategies over a four-year trial.

    And that's an excellent point about the selective effect on malaria parasites themselves - it would definitely be something to watch for in trials of an insecticide designed to Read et al.'s specifications.

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