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  • DDT 'repels, rather than kills' mosquitoes


The insecticide DDT prevents the spread of mosquito-borne diseases primarily by repelling mosquitoes rather than killing them, according to a study conducted in Thailand.

The study was published this month in the journal PLoS ONE (8 August).

American and Thai researchers analysed the way mosquitoes responded to DDT (Dichloro-Diphenyl-Trichloroethane) in experimental huts in a village in Thailand.

The researchers sprayed DDT inside the huts and released Aedes aegypti mosquitoes — the species that carries Chikungunya, dengue and yellow fever — ten metres away from the huts.

Nearly 60 per cent fewer mosquitoes entered the DDT-sprayed hut compared with the control hut that was not sprayed with DDT, suggesting that the chemical primarily repels mosquitoes.

The researchers also found that DDT acts as an irritant, with 31 per cent more mosquitoes leaving the DDT-sprayed hut after they had entered it compared with a control hut. DDT was not found to be particularly toxic.

The researchers also released malaria-carrying mosquitoes, with 70 per cent of them avoiding the DDT-sprayed hut. These results are yet to published.

Current malaria control emphasises the toxic effect of insecticides and the necessity of killing mosquitoes. But toxic chemicals carry the chance "for rapid build up of resistance", write the authors.

They assert that by simply preventing contact between people and mosquitoes, disease transmission can be stopped.

DDT does this by creating a "spatial repellent barrier" that stops mosquitoes from entering people's homes, or drives them out again before they take blood, write the authors.

"If we could prevent the insects from entering houses, a lot of disease could be prevented," said Theeraphap Charoenviriyaphap, a researcher on the study from Kasetsart University in Thailand.

Thailand phased out the use of DDT in 2000. The country has since seen a resurgence of malaria.

In 2006, the WHO recommended that DDT should be central to malaria control measures "where appropriate".

But Wichai Fatimai, director of Thailand's Office of Vector Borne Disease Control, said the country would not bring back DDT because its likely negative impacts outweighed its benefits.

"We are still worried about its impact on the environment and contamination of our farm produce," said Fatimai.

Link to full paper in PLoS ONE

PLoS ONE 8, e716 (2007)
doi:10.1371/journal.pone.0000716 (2007)

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