AFP

LIKE many other activities, global health has fashions. For the past couple of decades AIDS has captured both the imagination and the research dollars. Recently, though, the focus has shifted towards malaria, which kills a million people a year, most of them children, and debilitates hundreds of millions more. Insecticide-impregnated bednets designed to stop people being bitten by infected mosquitoes are being scattered throughout Africa. New drugs based on a Chinese herb called Artemisia have been introduced. And researchers are vying with one another to be the first to devise an effective vaccine. But the traditional first line of attack on malaria, killing the mosquitoes themselves, has yet to have a serious makeover.

One reason is that time and again chemical insecticides have produced the same dreary pattern. They prove wonderfully effective at first, only to dwindle into uselessness. This is because evolution quickly throws up resistant strains. Indeed, spraying campaigns, which generally aim to kill mosquitoes before they can breed, might have been devised as textbook examples of how to provoke an evolutionary response. With their competitors all dead, the progeny of a mosquito carrying a mutation that can neutralise the insecticide in question have the world to themselves.

The upshot is that discovering a way to retain the anti-malarial benefits of insecticides without provoking an evolutionary response would be a significant breakthrough. And that is what Andrew Read of Pennsylvania State University and his colleagues think they have done. They have rethought the logic of insecticides, putting evolutionary theory at the centre, instead of a simple desire to destroy the enemy. The result is a modest proposal to deal with the problem of resistance.

Euthanasia for mosquitoes

Dr Read started from the observation that it is old, rather than young, mosquitoes that are infectious. Only females can transmit malaria (males suck plant juices, not blood) but they are not born with the parasites inside their bodies. They have instead to acquire them from humans already carrying the disease, and that takes time. Once a female does feed on infected blood, the parasites she ingests require a further 10 to 14 days to mature and migrate to her salivary glands, whence they can be transmitted to another host when she next feeds. In theory, then, killing only the oldest female mosquitoes—those at significant risk of being infectious—could stop the transmission of the disease. Since these females would have had plenty of time to reproduce before they died, the evolutionary pressure imposed by killing them would be much lower.

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.