April 14, 2009. Copyright 2009, Graphic News. All rights reserved

Late-acting insecticides could stop malaria

LONDON, April 14, Graphic News: Kill ’em fast and kill ’em young. For decades the main weapon in the battle against malaria, which claims the lives of a million people each year, most of them children, has been fast-acting insecticides that kill mosquitoes early in their life cycle to keep them from reproducing.

This strategy has provoked an evolutionary response. Mosquitoes that survive long enough to lay eggs breed a new generation of progeny with an insecticide-resistant gene. And fast reproduction cycles -- female mosquitoes produce and lay eggs in two to four days -- encourage the gene to spread quickly among the insect population.

Scientists now suggest evolutionary theory may help in the fight against malaria and lead to resistance-proof insecticides that never become obsolete.

Andrew Read, professor of biology and entomology at Pennsylvania State University, and his colleagues Matthew Thomas, professor of entomology, Penn State, and Penelope Lynch, doctoral student at Britain’s Open University, argue that chemical or biological insecticides that kill only older mosquitoes are a more sustainable way to fight the deadly disease.

Of the 3,500 species of mosquito, the malaria parasite, Plasmodium, is only transmitted by elderly females of the genus Anopheles. Mosquitoes are not born with the parasites inside their bodies, but have instead to acquire them from human hosts already carrying the disease, and that takes time.

Once a female does feed on infected blood (males suck nectar, not blood) the parasites she ingests require a further 10 to 14 days -- or two to six cycles of egg production -- to mature and migrate to her salivary glands, from where they can be transmitted to another human when she next feeds.

“It is one of the great ironies of malaria,” explained Read, whose team’s findings appear in the April issue of PLoS Biology. “Most mosquitoes do not live long enough to transmit the disease. To stop malaria, we only need to kill the old mosquitoes.”

Read et al are working on a late-acting fungal pesticide with spores that take 10 to 12 days to become lethal, achieving the benefit of killing the older, infectious mosquitoes, while dramatically reducing the evolution of resistance in offspring. In addition, fungus-infected females bite less and lay fewer eggs.

To study the impact of late-acting insecticides the researchers constructed a mathematical model of the mosquito’s life-cycle. Using data collected from malaria hotspots in Africa and Papua New Guinea, the model 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 at all.

A trial of this idea, spraying fungal spores on to bed nets and house walls is currently being set up in Tanzania. If successful, evolutionary theory may help solve a biological problem. People will still get bitten, but the bites will not be life-threatening.
/ENDS