While bed nets, insecticides, and drug treatments have helped to combat malaria, scientists would really like to cut the tropical disease off at its source: the mosquito Anopheles stephensi, whichtransmits the malaria parasite Plasmodium falciparum to humans.
From there, the steps to malaria eradication seem surprisingly simple: First, release Wolbachia-infected mosquitoes, which can't harbor large numbers of P. falciparum, into the wild. The Wolbachia infection is then transmitted from mother mosquitos to their offspring. In an ideal scenario, the entire population of wild mosquitoes would eventually be infected with Wolbachia and thus be unable to transmit malaria to humans.
For years, researchers attempting to infect mosquitoes with Wolbachia have been unable to stabilize the infection in the A. stephensi species that carries human malaria. And no one understood why these attempts didn't work.
But now, Zhiyong Xi, an entomologist at Michigan State, and his group have succeeded in stably infecting A. stephensi so that Wolbachia bacteria are transferred to offspring. "I think we chose the right strain of Wolbachia and the right mosquito species to try it in," said Xi. "I chose this strain because the first step is to start with something easier to control and to manipulate, then we'll move to a more difficult one."
Wolbachia is a symbiotic bacterium that occurs naturally. It isn't genetically modified and it is not a human pathogen. Because offspring of infected mosquitoes are also infected, the bacterium could be the basis for a logistically simple, low-cost strategy for malaria control.
"And it doesn't significantly impair the fitness of the mosquito, so you don't interfere with the food chain or the ecology of the system," said George Dimopoulos, a microbiologist at the Johns Hopkins Malaria Research Institute and coauthor on the paper.
There are still some hurdles to overcome. For example, the particular Wolbachia strain used reduces the number of eggs that infected mosquitoes lay. The next goal is to develop a strain that allows the mosquitos to be more competitive with wild-type mosquitoes, so that if and when the Wolbachia-infected mosquitoes are released, they would begin to increase in number in the wild population. Since bacteria are genetically quite flexible, this is something that isn't a complete roadblock.
Although researchers have been trying to engineer malaria-proof mosquitoes in other ways, such as by directly modifying their DNA, the Wolbachia infection approach may be an easier sell to the public since it doesn't involve genetic modification.
"And at the end of the day," said Dimopoulos, "none of this is a silver bullet for controlling malaria. The most effective way to suppress malaria infection is probably by using multiple approaches in parallel. But Wolbachia does have the potential to become a very important contribution toward malaria control."
- Bian, G., D. Joshi, Y. Dong, P. Lu, G. Zhou, X. Pan, Y. Xu, G. Dimopoulos, and Z. Xi. 2013. Wolbachia invades anopheles stephensi populations and induces refractoriness to plasmodium infection. Science 340(6133):748-751.