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The theory is simple: if you kill the mosquitoes with insecticides, they can't transmit the malaria parasite, and the malaria problem is solved.
Putting this theory into practice has been remarkably successful in some parts of the world, but almost impossible to execute in others. And some of the key weapons in the fight against mosquitoes – insecticides – have become almost useless as the insects gain resistance to the chemicals.
"Resistance is a serious problem," says Professor Janet Hemingway at the Liverpool School of Tropical Medicine. "For example, pyrethroid is an insecticide class used for spraying and in bednets over much of Africa, but we've found pyrethroid resistance all the way down West Africa, from the Ivory coast to South Africa, and in East Africa, from Kenya into Tanzania. South Africa switched to using pyrethroids three years ago, pyrethroid resistance was found very soon after, and the incidence of malaria took off."
With only a relatively small range of insecticides approved for use in vector control – "companies tend to produce compounds that will work on agricultural pests first; vector control is a bonus," points out Professor Hemingway – careful management of the chemicals available may help to slow resistance.
"We've been running a large resistance management programme in Mexico for about six years," says Professor Hemingway. "Resistance is found throughout Mexico's malaria belt – along the coast – so we're testing different ways of slowing down its spread."
Working in close collaboration with Mexico's Anti-Malaria Control Campaign and Centro de Investigacion de Paludismo, who are handling the spraying, the insecticides in some areas are changed every year: organophosphates one year, pyrethroids the next, then carbamates and back to pyrethroids. Other areas use a 'fine-scale mosaic', where a pyrethroid and an organophosphate are used within the same village, but the same houses are always sprayed with the same insecticide.
These systems are being compared to use of DDT alone – which has been used for many years in Mexico – or to the use of a pyrethroid alone.
"We're asking whether we get reasonable control with all the different insecticides, whether the rotations or mosaics slow down the rate of resistance evolution, and how quickly do the mosquitoes lose their resistance to DDT when we stop spraying with it and take the selection pressure off. We're looking closely at the logistics, to see whether these systems are feasible as part of a national campaign, and we've been using questionnaires to find out whether the insecticides we're using are acceptable to the local population."
While such management programmes can help to slow the development and spread of resistance to insecticides, Professor Hemingway sees molecular biology as the key to finding a way to stop it altogether. "Up till now we've only been able to detect and monitor resistance," she says. "Management has to be at a crude scale. But we want to be able to attack the resistance genes directly."
Feature: Mosquito resistance genes
Professor Janet Hemingway is Director of the Liverpool School of Tropical Medicine.
Page of 2; 2/9/04
[WTD023870] The mosquito resistance movement.doc