Pool game. For the study, larvae of Anopheles gambiae (shown as adults, inset) were collected at small water pools in Burkina Faso.

Kenneth Vernick; (inset) CDC

'Outdoor Mosquito' May Have Unexpected Role in Malaria

Most malaria researchers believe that the world's most dangerous mosquito—a malaria-parasite-carrying species called Anopheles gambiae—is a homebody that prefers biting people indoors. But a new study challenges that view. Researchers have found a large subpopulation of the insect that prefers to live outside. These bugs may play an important, but so far hidden, role in malaria transmission.

"This is fantastically interesting," says Willem Takken, a medical entomologist at Wageningen University in the Netherlands. The "dogma" that malaria is primarily transmitted indoors may not be correct, he says. If so, indoor control methods—such as bed nets and insecticide spray—may need to be supplemented with outdoor-based strategies.

One reason the newly discovered mosquitoes may have flown under the radar is that collecting mosquitoes outside is hard and therefore rarely done, says Kenneth Vernick of the Pasteur Institute in Paris. Indoors, you put a white sheet on the ground, spray some insecticide on walls and ceilings, and the mosquitoes literally drop at your feet. Traps that use light to attract mosquitoes also work much better indoors, says Takken.

But all A. gambiae mosquitoes begin their lives outdoors—as larvae living in pools of stagnant water. Vernick reasoned that some of these mosquitoes might stay outside when they become adults. His team collected A. gambiae larvae at sites along a 400-kilometer line in Burkina Faso and then checked them for a series of genetic markers commonly used to type A. gambiae. The team also trapped adult mosquitoes indoors at nearby villages.

They found that the puddles contained two different subpopulations. Some 43% of the larvae were genetically identical to the adult mosquitoes trapped inside homes. The remaining 57%, however, were genetically very distinct and were not found inside homes, suggesting that they spend their adult lives outside. The team named them GOUNDRY, after a local village. The width of the genetic divide suggests there is very little or no interbreeding between the groups, the team reports in the 4 February issue of Science.

GOUNDRY mosquitoes are much more susceptible to becoming infected with malaria: When fed on blood spiked with malaria parasites, some 58% became infected, versus only 35% of the home dwellers. That suggests that GOUNDRY may contribute considerably to the transmission of malaria to humans, Vernick says—and intense indoor malaria control may actually give them an evolutionary leg up.

The researchers didn't actually catch adult GOUNDRY, however, and they don't yet know whether they bite humans. In theory, they could feed on cattle, goats, sheep, or donkeys rather than people, says Vernick, in which case they wouldn't be important for human malaria transmission.

"We have to know what this group is doing in terms of biting, but also in other aspects of its ecology," says Jacob Koella, an evolutionary epidemiologist at Imperial College London. The fact that indoor-based control methods work well in many places suggests that outdoors mosquitoes aren't major contributors to malaria, he says. Nonetheless, "this is clearly an important study."

Takken's group, meanwhile, has developed a tool that could help shed more light on the discovery: a trap that uses human or animal scents as bait and that is more efficient than current methods. (A paper about these traps is currently under review, Takken says.) With the new traps, Vernick's team has now caught many adult mosquitoes in the same areas in Burkina Faso. He hopes that the strategy will reveal much more about where exactly GOUNDRY mosquitoes spend their adult lives, which species they feed on, and how much they contribute to malaria's deadly cycle.