It's all in the genes ...

A malaria epidemic in South Africa 10 years ago inspired research leading to the discovery of two genes that make a mosquito resistant to some insecticides.

University of Witwatersrand and National Institute for Communicable Diseases entomologist Maureen Coetzee, a South African contributor to the study and international authority on insecticide-resistant mosquitoes, said scientists were stymied by a 1999-2000 malaria outbreak on the east coast of the country despite a reliable anti-malaria spraying programme.

“We found that Anopheles funestus had returned to South Africa after an absence of 50 years,” she said.

The entomologists suspected the cause of the epidemic was the ineffectiveness of pyrethroid, an insecticide used widely in malaria spraying programmes, and that the Anopheles funestus’ s genes were playing a role. The South African health authorities moved quickly, spraying traditional homesteads with DDT and controlling the outbreak.

At the time University of the Witwatersrand entomologist Richard Hunt, another co-author of the latest study, successfully colonised Anopheles funestos, enabling scientists worldwide to begin researching the malaria carrier in a controlled way.

Last month Charles Wondji and colleagues at the Liverpool School of Tropical Medicine (LSTM) announced they now know why the mosquito is no longer killed by pyrethroid-based insecticides. Its genetic make-up has evolved, making it resistant to the insecticide.

LSTM researcher Hilary Ranson, a co-author of the study, said that pyrethroids are the most common anti-malaria and agricultural insecticides in Africa.

“On the positive side, 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.

The study, published in the journal Genome Research (February 4 2009), identified two P450 genes that produce an enzyme that soaks up the insecticide, making it ineffective.

The mosquitoes for the study were collected in Angola and Mozambique.

In the short term this means countries infested by Anopheles funestos can change to guaranteed-to-kill DDT. 

One of the most exciting implications of the discovery, which is being investigated by the LSTM team is to test wild mosquitoes in different parts of Africa for the resistant genes.—www.SciDev.NetCarol Campbell

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