April 14, 2009 The Pump Handle 0Comment

The latest issue of the Economist highlights a new idea in malaria prevention. Traditional prevention efforts emphasize spraying, but mosquitoes evolve resistance to insecticides. Now, Penn State University’s Andrew Read offers this insight, which can help avoid the resistance problem:

To stop malaria, we only need to kill the old mosquitoes.

Once an adult female Anopheles mosquito feeds on a human already infected with malaria, it still takes 10-14 days for the parasite to mature and migrate to the mosquito’s salivary glands, at which point she can infect another human. Since most female Anopheles mosquitoes live only 1-2 weeks (with some surviving up to one month), many of them will die before becoming capable of transmitting malaria.

The Economist explains what the researchers studied and how their findings could be applied:

To test this insight, the researchers constructed a mathematical model of the mosquito’s life-cycle. They then plugged in data, collected from malaria hotspots in Africa and Papua New Guinea, that describe the insect’s lifespan and egg-laying cycles in those parts of the world and the way that malaria parasites grow inside mosquitoes. The model, which they have just published in the Public Library of Science, reveals that selectively killing elderly mosquitoes would reduce the number of infectious bites by 95% and that resistance to such a tactic would spread very slowly, if it spread at all, because mosquitoes vulnerable to a post-breeding insecticide would have had a chance to pass on their vulnerable genes to future generations.

The problem, of course, is to find an insecticide that kills only the elderly. One option is to use existing chemicals, but at greater dilutions. That could work because older mosquitoes are more vulnerable to insecticides than younger ones.

A more radical answer, though, may be to use a completely different sort of insecticide: a fungus. The team are working with fungi that take 10 to 12 days to become lethal. That is short enough to kill parasite-infected insects before they become infectious, but long enough to allow them to breed. A trial of this idea, spraying fungal spores on to bednets and house walls in Tanzania, is being set up at the moment. If it works, it will be a good example of the value of thinking about biological problems from an evolutionary perspective. People will still get bitten, but the bites will merely be irritating, not life-threatening.

Read et al’s paper, published last week in PLoS Biology, notes that insecticide (or fungicide) applications that don’t reduce insect bites will be less popular than conventional insecticide applications, which can rid homes of mosquitoes and bedbugs. Households might not use it as readily, and high rates of coverage are still necessary for this new type of insecticide to work. Even if it seems less attractive in the short term, though, such a product would have a significant long-term benefit: it could keep working, year after year.

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