MOSQUITO-slaying fungi could help cut malaria鈥檚 toll of misery and death. Two studies have shown that impregnating household surfaces with fungal spores can slash the number of mosquitoes capable of transmitting the parasite.
While simple measures such as using insecticide-impregnated mosquito nets can help prevent malaria, in many areas mosquitoes have developed resistance to the chemicals, so alternatives are urgently needed.
To find out if fungi could be effective, a British team at Imperial College London and the University of Edinburgh fed one mosquito species blood infected with a malaria parasite. After two weeks, nearly a third of the insects carried sporozoites, the infectious form of the parasite. But when the mosquitoes were kept in boxes sprayed with an oil containing fungal spores, 90 per cent died after two weeks and only 0.4 per cent of the survivors carried sporozoites 鈥� an 80-fold reduction (Science, vol 308, p 1638 and p 1641).
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One reason the method works so well is that malaria infection appears to make mosquitoes far more vulnerable to fungal infections, which enter the insect鈥檚 bodies through their feet. The team found only 6 hours of exposure to a sprayed surface was needed, and the fungus was effective for at least 12 days after spraying. The fungus used was Beauveria bassiania, chosen because it has already been approved for use as a biopesticide.
But can the method work in practice? A team led by Bart Knols of the International Atomic Energy Agency in Seibersdorf, Austria, asked five households in a village in Tanzania to hang up black sheets impregnated with fungal spores. They used a related fungus, Metarhizium anisopliae, which is the main component of the 鈥淕reen Muscle鈥� pesticide developed in South Africa to kill locusts and grasshoppers.
The team collected nearly 3000 mosquitoes from the houses during the study. They found that 23 per cent of Anopheles gambiae mosquitoes, one of the species that carries the malaria parasite, were infected with the fungus. They died after less than four days in captivity, whereas non-infected female mosquitoes survived nine days.
While the study was too small to show if this reduced malaria infections, modelling suggests the method could cut the average number of potentially infective mosquito bites people receive in a year by three-quarters, from 262 to just 64. The team hopes the method will be more effective if adopted by all villagers.
Both teams say that there is no evidence that the fungi harm humans, non-target insects or the environment. The approach could be put into practice quickly if it is funded, they say. 鈥淚鈥檓 extremely optimistic that in three to five years we could develop this into a tool,鈥� Knols says. But only with funding, he adds. 鈥淲e maybe need a couple of million dollars to push this down the road to where governments can really apply it as part of national malaria control programmes.鈥�
The main challenge will be to develop fungal formulations that last longer than the month or so achieved so far. 鈥淚t鈥檚 a significant logistical obstacle,鈥� says Knols. It is already a challenge to ensure that pesticide-impregnated bed nets are replaced every six months, he points out.
