Science - September 24, 2009

Fungus is sting of death for malaria mosquito

Mosquitoes don't develop resistance to the deadly fungus.
Infected mosquitoes are more susceptible to insecticides.
- Field tests in preparation

Fungus-infected malaria mosquito
Mosquito-killing fungi are an effective weapon in the fight against insecticide-resistant malaria mosquitoes. The fungi do not just destroy the mosquitoes, they also make them more susceptible to chemical treatment. That is what Marit Farenhorst of the Entomology Lab reported this week in the journal PNAS. The PhD student and her colleagues at the University of Johannesburg in South Africa discovered that the fungus Beauveria bassiana is an extremely effective mosquito killer. 'After ten days all the mosquitoes that had been infected with the fungus were dead while 80 percent of the mosquitoes without the fungus were still alive', says Farenhorst. 'You won't believe how many mosquitoes I've killed. Quite sad, really.'
Penetrating
'There are a number of major pluses to using fungi to kill mosquitoes', maintains Farenhorst. 'It is environmentally friendly, but the main thing is that it is a long-lasting method as the mosquitoes are slow to build up immunity to the fungus.' One reason they do not develop resistance is because the fungi attack the mosquito on several fronts. They penetrate the mosquito from its outer layer, destroying cells and organs and secreting toxic compounds.
On top of that, the mosquito takes a long time to die. 'The fact that they survive for quite a while after being infected means they are still able to reproduce. As a result, there is less selection among mosquitoes for the ability to survive the fungus than would be the case if they died immediately.' There is another bonus in the intervening period: fewer malaria parasites survive in the sick mosquitoes, the sick mosquitoes bite less and they lay fewer eggs.   
Overloading the enzymes
The fungus seems to be launching a two-pronged attack in the fight against malaria: the icing on the cake was that the insecticide-resistant mosquitoes that had been infected with the fungus turned out to be suddenly much worse at coping with pesticides. The PhD student has an explanation for the reduced resistance: 'The enzymes the mosquitoes use to break down pesticides are the same ones they use to tackle the toxic components secreted by the fungus. It could be that this is causing the enzymes to be overloaded.' 
Farenhorst thinks her findings will have important consequences for society. In some areas all the mosquitoes are resistant to both DDT and the more environmentally friendly pyrethroids.  That is why her research is now looking at how to effectively transfer the fungus onto the mosquitoes. Farenhorst: 'Everything works really well in the lab. Now we're going to do field experiments to see how we can get the fungus onto the mosquitoes as efficiently as possible.'
Malaria
Malaria is an infectious disease prevalent in the tropics and subtropics. It is caused by a single-cell parasite that exploits the red blood cells of its victims to reproduce. Symptoms vary from anaemia to 'flu-like symptoms but it can also be fatal. Each year the death toll is several million, primarily young children in Africa.

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