Scientists have discovered why a key insecticide used to fight malaria is no longer effective at controlling mosquitoes, identifying the genes responsible for resistance.

A mainstay of malaria control is the use of pyrethroids in bednets and indoor spraying to control the number of mosquitoes that transmit the disease.

But in recent years mosquitoes have evolved to overcome the effects of these chemicals. Scientists have been trying to understand the genetic basis of this resistance.

Researchers studying the common mosquito Anopheles funestus found a family of genes that code for enzymes known as cytochrome P450s, which can soak up the pyrethroids, making them ineffective.

The same genes were recently identified in another key malaria-transmitting mosquito Anopheles gambiae — also associated with resistance to pyrethroids.

"We expected to find that different species and populations would have different groups of genes responsible but they are very similar," said Hilary Ranson of the Liverpool School of Tropical Medicine in the United Kingdom, who was an author of the study.

"This study shows that it might mean this insecticide can no longer be used to control these mosquitoes — removing one of the most important weapons used to fight malaria,'' Ranson told SciDev.Net.

One implication of the results is that countries infested by Anopheles funestus could switch to the insecticide DDT (Dichloro-Diphenyl-Trichloroethane) for eradicating mosquitoes. Mozambique and South Africa have already re-introduced DDT because of mosquito resistance.

One of the most exciting implications of the discovery is that it may be possible to test wild mosquitoes in different parts of Africa for resistance. This could provide early warning of insecticide resistance and "greatly enhance our ability to mitigate the potentially devastating effects of resistance on malaria control", say the researchers.

In the long term it may be possible to produce "smart" insecticides to minimise the mosquitoes' potential for resistance ahead of a spraying programme. An area might be sprayed with a substance that re-sensitises the mosquitoes to pyrethroids before the insecticide itself is applied.

"By understanding the mechanisms by which the mosquitoes have developed this resistance, we can develop ways to reverse this resistance and beat the mosquitoes at their own game," Ranson said.

Source: SciDev net
Date: 9 February 2009