Fungus could be next weapon in war on malaria

The fungi spread through the mosquitoes, eventually killing them Copyright: Hugh Sturrock

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Spraying walls or cloths with spores of a fungus that kills mosquitoes could greatly reduce malaria transmission, according to two studies published today (10 June) in Science.

Certain fungi not only kill mosquitoes but also can reduce the likelihood of them transmitting malaria before they die, according to research led by Matt Thomas of Imperial College and Andrew Read of the University of Edinburgh, United Kingdom.

When they exposed Anopheles stephensi mosquitoes to the fungi, the researchers found that the mosquitoes were 80 times less likely to transmit malaria. The mosquito transmits malaria to humans but in this study carried a form of malaria that infects rodents

Mosquitoes have become increasingly resistant to the normal methods of controls, such as spraying pesticides on walls or using insecticide-treated bednets.

So researchers are investigating biological control agents, such as bacteria, fungi and viruses. While some bacteria can kill mosquito larvae in water, biological control agents that can kill adults are not yet available.

One advantage of using fungi instead of bacteria or viruses is that mosquitoes need only come into contact with the fungal spores — they do not need to ingest them. The spores germinate and penetrate the insect’s outer surface, before spreading inside them.

The researchers assessed eight strains of two fungal species: Beauveria bassiana and Metarhizium anisopliae.

The fungi kill malarial
mosquitoes before they
can transmit the parasite

The team sprayed cardboard pots with fungal spores suspended in oil, then 24 hours later introduced mosquitoes that had taken a blood meal and left them for two weeks.

In six of the eight tests, more than 80 per cent of the mosquitoes died.

Death is not always quick, but it does not need to be. Mosquitoes cannot transmit malaria until about two weeks after picking up the malaria parasites with a blood meal.

The fungus also affects mosquitoes’ ability to feed, making them less likely to spread malaria.

Thomas and colleagues showed that after exposure to one of the fungal strains, mosquitoes were 80 times less likely to transmit malaria to mice.

The team describes the findings as “highly significant in terms of malaria control”.

Mosquitoes tend to rest on walls or ceilings after feeding. This makes them easy targets for control. Thomas’s team showed that even a brief six-hour exposure to a 12-day old spray killed 89 per cent of mosquitoes.

They say that spraying fungal ‘biopesticides’ could replace or supplement chemical-based control strategies, especially in areas where mosquitoes are developing resistance to pesticides.

Of the fungi studied, Beauveria bassiana is particularly promising as it is already used as an agricultural biopesticide. This means that if further studies show that it is safe, it might rapidly be approved for use.

The second study in Science tested the approach in rural African field conditions. Bart Knols of Wageningen University, in the Netherlands, and Gerry Killeen of the Ifakara Health Research and Development Centre in Tanzania, hung cotton sheets impregnated with the fungus Metarhizium anisopliae from the ceilings of traditional houses in a Tanzanian village.

The fungus infected 23 per cent of the 580 female Anopheles gambiae mosquitoes collected in five houses during the treatment period. Infected females lived on average 3.5 days, uninfected ones for 9.3 days.

The researchers compared their findings to data about malaria transmission from a nearby village to assess how effective their approach would be at stopping the spread of the disease.

“The model estimates show that fungus-impregnated sheets would have a significant impact on parasite transmission,” write Knols and colleagues.

“Even with just 23 per cent of the mosquitoes in houses acquiring an infection, the [intensity of malaria transmission] could be reduced from 262 infective bites per year to 64 — a drop of 75 per cent.”

The researchers say that refining the method used could further reduce the risk.

If 50 per cent of mosquitoes were infected, the intensity of malaria transmission would fall by 96 per cent, they say.

“We need to do more work on specific formulations and increasing the longevity of fungal spores,” says Knols. “Even if we got the funds we need, it would be a couple of years before the research can be commercialised.”

Link to paper by Blanford et al

Link to paper by Scholte et al

Reference: Science 308, 1638
Reference: Science 308, 1641