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Older malaria mosquitoes targeted

9th April 2009

Scientists say the key to the long-term eradication of malaria may lie with older mosquitoes, or "grandmothers", with insecticides which target the older generation and spare their offspring the most effective.

Mosquito

The malaria-carrying Anopheles mosquito has long been the target of campaigns to spray homes with insecticide.

However, mosquitoes soon develop resistance to the sprays because some of their offspring have natural resistance. They survive the insecticides and then go on to have offspring which are also immune to its effects.

It is older mosquitoes that pass on malaria, however, because it takes time for the malaria parasite to develop within the mosquito.

Meanwhile the younger generations are developing resistance to the insecticides used in prevention campaigns, highlighting the need for an insecticide specifically targeted at "grandmother" mosquitoes.

Andrew Read of the US-based Pennsylvania State University, lead author of a recent study published in the online open access journal PLoS Biology, said that older mosquitoes had already done most of their reproducing before they became dangerous to humans.

Therefore, insecticides targeted at older mosquitoes would mean that future generations had very little opportunity to develop resistance.

Read said campaigns should target the "grandmothers" and leave the younger ones alone.

Using a mathematical model to follow the evolution of resistance in Anopheles gambiae, Read and his colleagues were able to predict that the number of infectious mosquito bites could be slashed by 95% by killing only mosquitoes that have completed at least four cycles of egg production.

Fungal biopesticides that take 10–12 days to grow inside the insects, killing them towards the end of their lives, are now in development.

Read said researchers had been working with the fungus for five years, and knew that it had the right properties to reduce the transmission of malaria.

It was only recently, however, that they realised that it could stop the evolution of resistance. Now, the ultimate goal will be to provide a single malaria control that will last forever.

People who are bitten by non-dangerous mosquitoes, however, will believe that the insecticides are not working.

This could provide a major obstacle to a change in current eradication campaigns.

 

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