Science - June 16, 2005

Biological control of malaria possible

The headline in De Gelderlander newspaper last week was ‘Breakthrough in the fight against malaria’. Malaria expert Dr Willem Takken is realistic enough, however, not to expect miracles from the new form of control described in an article last week’s Science. ‘It is a promising measure that works well locally. It deals with both the mosquito and the parasite, a big advantage as the existing means are not enough.’

The idea of using fungi to combat malaria mosquitoes is not new. ‘Two years ago already we published an article about this fungus, and we discovered that there were groups at Edinburgh University and Imperial College London researching the same subject. Fortunately we decided to cooperate closely; they are doing lab work and we are concentrating on the field. We are already reaping the rewards,’ says Takken, who is the coordinator on the Wageningen side.

Many anti-malarial medicines have become ineffective as a result of resistance, and research has not yet come up with a good vaccine. That is why, according to Takken, it is of vital importance to deal with the mosquito, which is the vector of the malaria parasite. In the tropics malaria is still responsible for over a million deaths a year.

Dr Bart Knols and Dr Ernst-Jan Scholte, researchers at the Entomology group, carried out a field study in a traditional rural village in Tanzania together with the Swiss Tropical Institute in Basel and Ifakara Health and Development Centre in Tanzania. Black cotton cloths impregnated with the spores of the fungus Metarhizium anisopliae were suspended from the ceiling of five village huts in an area badly affected by malaria. As a control, five huts were monitored where cloths were not used.

In the huts that were treated, almost a quarter of the mosquitoes were infected by the fungus. Takken: ‘After they have bitten and sucked blood, mosquitoes sometimes remain motionless for as long as 48 hours on the ceiling. A satiated female develops her eggs there, so that is a good moment to tackle them.’

As a result of the fungus treatment, the number of infectious bites decreased by about 75 percent, from 264 to 64 people per year. Takken: ‘We also see the effect in the mosquito catches. Without fungus the mosquitoes live for about another three weeks, but when they ingest fungus that drops to about eleven days. And these are the results from experiments where only a quarter of the ceiling and walls were covered with cloth.’

The entomologists calculated that, with a fungus infection rate of fifty percent, the number of mosquito bites declines by 96 percent. These are very promising figures, but the British researchers also have good news. Under laboratory conditions the mosquitoes infected by the fungus do not develop any more parasites. ‘The fungus spreads throughout the whole body of the living mosquitoes. They start to display different behaviour, stop sucking blood, and apparently as a result of these developments, the parasites become so weak that they no longer make it into the mosquitoes’ saliva,’ explains Takken.

He emphasises, however, that this new form of biological control of malaria needs further development. ‘The fungus is easy to cultivate under local conditions on an agar medium. We scrape the fungus off and suspend it in a vegetable oil. Then we spray it on to the black cloths. At present this treatment works for about a month, but we want to extend this to three months.’ It is a simple and cheap method that complements existing methods of control. Takken: ‘It’s about time for some good news, as methods have been losing their effectiveness for some time now.’ / GvM

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