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Fungus Opens New Front Against Dengue

By Stephen Luntz

The soil fungus Beauvaria bassiana could prove a potent weapon against mosquito-borne disease, testing at the Queensland Institute of Medical Research suggests.

B. bassiana normally feasts on beetles and termites, with spores penetrating the insect’s exoskeleton and growing inside.

Dr Jonathan Darbro of QIMR says the spores cause physical disruption to the insect, as well as releasing toxins, and the combination eventually kills it. Spores subsequently grow on the outside and are picked up by other creatures that crawl over the corpse.

“Mosquitoes need to land on the fungal spores to get infected. Spores can be sprayed onto surfaces such as cloth, but they need to be surfaces where these mosquitoes would land in the wild. The most likely candidates would probably be places where mosquitoes would land to rest or lay eggs,” Darbro says.

While there are numerous chemical sprays capable of killing mosquitoes that land on them, Darbro describes his research on turning B. bassiana against the dengue-carrying Aedes aegypti mosquitoes as “very promising”. Mosquitoes’ short life spans have enabled them to rapidly develop resistance against DDT and other chemical sprays.

“The fungus doesn’t kill the mosquitoes as quickly as a chemical product but because it kills slowly, mosquitoes are less likely to evolve a defence against the fungus,” Darbro says. “They’ll still live long enough to reproduce, so natural selection isn’t pushing as hard for the mosquitoes to resist the fungus as they would for a chemical insecticide.”

In Darbro’s experiments the fungus took 7–8 days to kill the hosts, although he says this varies with temperature and the fungal strain. “In work done in Africa on the two main malarial hosts it generally took 4 days,” Darbro says. Both malaria and dengue take longer than this to develop within a mosquito, so one infected with spores around the time it bites a human carrier will never get to transmit the disease. “Before infected mosquitos die they also show reduced biting behaviour,” Darbro adds.

Spores of Darbro’s strain of B. bassiana have a half-life of 30 days, making him think that sites will need to be retreated once or twice during a mosquito season to prevent outbreaks.

Having reported the success of trials conducted in outdoor cages in The American Journal of Tropical Medicine and Hygiene, Darbro is seeking to conduct mass mosquito infections.