Infecting mosquitoes with a transgenic fungus could drastically cut their ability to transmit malaria, according to research published today in Science (25 February).
Existing efforts to develop fungal malaria control focus on slowly killing mosquitoes before they have the chance to pass on Plasmodium, the malaria parasite. But they rely on mosquitoes being inoculated with parasitic fungus soon after Plasmodium infection, which limits their use.
In this latest research, scientists have changed tactics and instead focused on reducing the infectiousness of the mosquitoes, so that fungus could be applied later in a mosquito's lifecycle but still cut malaria transmission.
The researchers genetically modified (GM) the fungus Metarhizium anisopliae, which infects mosquitoes on contact, to express molecules which impede the entrance of sporozoites — the cells that malaria parasites produce to infect new hosts — to the salivary gland of the mosquitoes, reducing the number that can be passed to humans through a bite.
The GM fungi reduced the number of sporozoites in mosquito salivary glands by up to 98 per cent compared to those infected with the non-GM fungi. Within just two days of infection 80 per cent of mosquitoes could not transmit malaria anymore compared to only 14 per cent of fungi-free mosquitoes and 32 per cent of those infected with non-GM fungi.
The results confirmed that such transgenic fungi can block transmission of the disease even by mosquitoes with advanced malaria infections.
The strategy would be no more expensive than current chemical insecticides, said Raymond St. Leger, an author of the study from the University of Maryland, United States. The fungi could be used indoors or outdoors like a contact insecticide, but would not kill or decrease the lifespan of the mosquitoes.
"This is a clear advantage over other similar methodologies," Hernando del Portillo, a malaria researcher from the Barcelona Centre for International Health Research, Spain, told SciDev.Net. Shortening the lifespan of mosquitoes could promote development of resistance to fungi, rendering the control measurements useless.
Currently, fungi such as M. anisopliae are produced on a large scale in Africa, Australia and China to control locust swarms, and are used in South America to control sugar cane and coffee pests, said St. Leger.
He added that his team plans to test the fungus in contained trials in Africa once they have obtained biosafety permission.
But del Portillo said that there was still a lot of work to be done to convince ethical committees that such GM organisms are safe.
And Willem Takken, from Wageningen University in the Netherlands and a pioneer of the use of fungi in mosquito control, told SciDev.Net: "I do not see the need to use transgenic fungi as long as we have excellent natural fungi to kill the mosquitoes".
His team published a paper in Malaria Journal this month (2 February) showing that non-GM fungi can kill insecticide-resistant mosquitoes under field conditions in Africa, and another paper in Parasite & Vectors (22 February) describing a new way of applying fungal spores in the field.
He added that transgenic fungi are not species-specific, and could affect beneficial insect species.
Science doi: 10.1126/science.1199115 (2011)
Parasites & Vectors doi: 10.1186/1756-3305-4-23 (2011)
Malaria Journal doi: 10.1186/1475-2875-10-24 (2011)