Spraying walls or cloths with spores of a fungus that kills mosquitoes could greatly reduce malaria transmission, according to a report on the non-profit Science and Development Network website.
Two studies were published on Friday on the issue in the reputable peer-reviewed journal Science. One study tested the approach in rural African field conditions, with important implications for eastern portions of South Africa, where malaria is on the rise.
Gerry Killeen, of the Ifakara Health Research and Development Centre in Tanzania, and Bart Knols, of Wageningen University in The Netherlands, hung cotton sheets impregnated with a specific fungus from the ceilings of traditional houses in a Tanzanian village.
”The model estimates show that fungus-impregnated sheets would have a significant impact on parasite transmission,” the team of scientists says.
Mosquitoes have become increasingly resistant to the normal methods of control, such as spraying pesticides on walls or using insecticide-treated bed nets.
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 it.
The Metarhizium anisopliae fungus infected 23% 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 with data about malaria transmission from a nearby village to assess how effective their approach would be at stopping the spread of the disease.
”Even with just 23% of the mosquitoes in houses acquiring an infection,” they say, the intensity of malaria transmission ”could be reduced …. [by] 75%”.
The researchers say that refining the method used could further reduce the risk.
If 50% of mosquitoes were infected, the intensity of malaria transmission would fall by 96%, 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.”
Meanwhile, Matt Thomas, of Imperial College, and Andrew Read, of the University of Edinburgh, both in the United Kingdom, 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.
The findings are ”highly significant in terms of malaria control”, the team says. 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% 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.
The researchers assessed eight strains of two fungal species. The team sprayed cardboard pots with fungal spores suspended in oil, and 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% 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. 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. — SciDev.Net