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Treating bednets with insecticide

[DENVER] A team of British and Tanzanian researchers have discovered that bedbugs are developing resistance to the insecticide used to treat the bednets that provide protection against malaria-transmitting mosquitoes.

The researchers are concerned that the spread of such resistance could reduce the willingness of villagers to use bednets, which have been found to be a highly effective and cost-efficient way of controlling malaria, particularly among small children.

The discovery was reported this week by Chris Curtis, a researcher at the London School of Hygiene and Tropical Medicine, to the annual meeting of the American Association for the Advancement of Science (AAAS) in Denver, Colorado.

Curtis told the meeting that when bednets were first introduced, one of their attractions to villagers in Africa was that the pyrethroid insecticide with which they are treated not only killed mosquitoes that came into contact with them, but also eradicated bedbugs (Cimex hemipterus).

A recent study carried out in Tanzania, however, whose results are published in the a recent issue of the journal Medical and Veterinary Entomology, has found that bedbugs are developing resistance to the insecticide in villages where treated bednets have been in use for several years.

“We are now testing low toxicity organophosphate compounds against the bedbugs, which kill them effectively,” said Curtis. “But we are very worried that the reappearance of these insects may seriously reduce the enthusiasm of villagers to use treated nets, and to bring them annually for re-treatment.”

The study, which was carried out in collaboration with researchers from the Tanzanian National Institute of Medical Research, found circumstantial evidence that bedbug resistance to pyrethroids may spread more rapidly in villages where not all bednets have been treated.

Conversely it found that bedbugs have not reappeared in a village in which nearly all the beds have been provided with treated nets for 14 years, suggesting that in certain circumstances it is possible to permanently eradicate the problem.

Curtis also reported to the AAAS meeting on the results of a separate survey of 28 Tanzanian villages, which had been provided with free bednets, as well as free annual re-treatment with pyrethroids. These villages have shown consistent reductions in the number of infective mosquitoes and the prevalence of both malaria fever and anaemia in babies and children between the ages of three and four.

Although benefits were less clear in older children — perhaps because of lower immunity to malaria resulting from fewer infections when they were younger — such children were “no worse off” then those without bednets. Curtis said that these results appear to meet the concerns of those worried of a ‘rebound’ effect of reducing — but not eradicating — malaria transmission in highly endemic parts of Africa.

Curtis also challenged those who claimed that bednets were only effective if they were sold to villagers, who would therefore see them as an investment, and thus be more inclined to use them than if they were distributed and re-treated free of charge.

“We believe that marketing bednets and insecticides to villagers is neither equitable nor efficient, and that it is much preferable that organised, pro-active teams visit villages to provide treated nets for all beds free of charge, and to visit annually to re-treat the nets,” he said.

He pointed out that with bednets costing just a few dollars each, it would cost US$500 million a year to provide and regularly re-treat bednets in the whole of rural Africa, where about 90 per cent of the world’s child deaths from malaria occur. Curtis added: “That is less then the amount spent on insecticides for cat flea control in the United States.”

© SciDev.Net 2003

Photo credit: Sara A. Holtz, Peace Corps Volunteer