Outwitted: The fungi reverse insecticide resistance in Anopheles mosquitoes

CDC/ WHO

Insecticide resistance in mosquitoes — a growing problem for malaria control — can be reversed by infecting them with a fungus, new research shows.

Researchers infected insecticide-resistant African Anopheles mosquitoes with two fungus species already known to kill mosquitoes: Beauveria bassiana and Metarhizium anisopliae. Three days after infection, the mosquitoes were exposed to two of the main mosquito insecticides. More fungi-infected mosquitoes died than uninfected controls.

Earlier research has found that mosquitoes are fatally infected when a suspension containing fungal spores is sprayed or painted on surfaces attractive to the insects (see Fungus could be next weapon in war on malaria).

The spores stick to the mosquito and then germinate, eventually penetrating the insect's outer surface, absorbing nutrients, destroying cells and releasing toxins. Infected mosquitoes die within 14 days — before they have a chance to mature and pass on the malaria parasite.

"The bonus effect is that when [an insecticide-resistant mosquito] is infected with fungus it becomes sensitive again to the insecticide," says Marit Farenhorst, a research in the Laboratory of Entomology at Wageningen University in the Netherlands and one of the authors of the paper.

The researchers believe this occurs because being simultaneously bombarded with both a fungus and an insecticide probably overwhelms the enzymes that normally make a mosquito resistant to insecticide.

"Mosquitoes use enzymes to break down the insecticide, to eat it up. We think that the detoxifying enzymes also have to work to get rid of the fungal toxins. They have many more toxins to get rid of all of a sudden and they can't cope," Farenhorst told SciDev.Net.

She added: "It's beneficial to use insecticide alongside the fungus — they increase the effectiveness of each other. Integrated control is always best — you have to use everything you've got against this disease."

Farenhorst and her colleagues are now attempting to create a long-lasting fungal product that can be used in the field in Africa. They are also trying to determine the best locations and delivery methods for the fungus.

"The temperature in the lab is different to in the field," she said. "In the lab, it [the treatment] works really well but in the field it only lasts up to a month before it needs reapplying — and that's not cost effective. We are trying to make a coating for the spores that will protect them from desiccation and allow the treatment to work for much longer."

Farenhorst believes it could be 2–3 years before such a product can be tested on a large scale.

The results were published in PNAS last week (22 September).

Link to full paper in PNAS*

*Free access to users in developing countries