BioEd Online

A mouldy house could help combat malaria by killing off the mosquitoes that carry the disease. Researchers have found that an oil-based fungal treatment could be a viable alternative to the insecticides to which mosquitoes have grown resistant.

At least 300 million acute cases of malaria occur each year and cause at least a million deaths, according to the World Health Organization. A recent report in Nature estimated that there are at least twice that number of cases¹.

Much has been done to try to reduce the spread of malaria, including spraying insecticides such as DDT (dichlorodiphenyltrichloroethane), distributing bednets, and investigating the possibility of releasing genetically modified mosquitoes that cannot carry the parasite-borne illness.

But, when it comes to insecticides, the mosquitoes have fought back. After DDT was used extensively against mosquitoes following the Second World War, the insects started to develop resistance to the chemical. Today, some mosquitoes are resistant to a host of more modern insecticides, including those known as pyrethroids, which are used to spray bednets in much of Africa.

Spore war

Researchers are instead turning to the insecticide powers of fungi, some of which are already used to keep bugs from damaging fields. Mosquitoes have not had much contact with such deterrents and so have not yet developed resistance to them.

To bring the insect-killing powers of fungi into the home, Matt Thomas of Imperial College London and colleagues made an oil-based spray of the Beauveria bassiana fungus. The oil helps to keep moisture locked in and allows the fungus to thrive. When the team exposed mosquitoes to a surface sprayed with this concoction, not only did many of the insects die, but those that survived were less able to transmit the malaria parasite to humans. This made for a striking 80-fold reduction in the number of mosquitoes able to transmit malaria, they report in Science².

Similarly, Bart Knols of Wageningen University and Research Centre in the Netherlands and his colleagues looked at cotton sheets treated with a mixture of fungi and oil. In a field study in rural Tanzanian houses, the researchers found that mosquitoes that landed on the sheets had a shortened lifespan³ . This could significantly reduce the number of deaths due to malaria, says Knols.

The idea of using fungus as disease control has the advantage of being cheap. "It would possibly cost as little as 20 US cents to treat a house," Thomas estimates. Swapping between chemicals and fungi-based treatments should help to keep resistance down, he adds.

But fungal treatments typically do not last as long as chemical equivalents, experts point out. "Finding out how long the treatments last is hugely important," says Janet Hemingway of the Liverpool School of Tropical Medicine in Britain.