BioEd Online

Researchers have discovered a subspecies of malaria-carrying mosquito that lives outside, rather than in human homes as previously known types do. The finding, reported today in Science¹, points to a potential gap in methods of malaria control.

In sub-Saharan Africa, malaria kills some 710,000 people each year. The most dangerous form of the disease is caused by a parasite, Plasmodium falciparum, which is transmitted by mosquitoes, mainly of the species Anopheles gambiae. The mosquito larvae live in water, but the adult insects are thought to spend much of their time in buildings, where they feast on human blood.

Indoor insecticide sprays, bed nets and access to malaria medication have helped to decrease the number of deaths from malaria in the area over the past decade. But the disease is far from gone.

The newly discovered mosquito group — a subtype of A. gambiae dubbed Goundry after a village in Burkina Faso, western Africa, where it was found — could be one reason why eradication attempts have not been entirely successful. Unlike other A. gambiae, adult Goundry mosquitoes spend their time outside, thus avoiding indoor sprays. Furthermore, when fed blood carrying Plasmodium falciparum, the group acquired the parasite more easily than did its relatives.

Inside out

"For decades, we've made the assumption that A. gambiae rests indoors and bites people indoors," says Ken Vernick, a parasitologist at the Pasteur Institute in Paris and lead author of the study. "What we've found is this assumption was biased by collection methods."

Most people with malaria seem to catch it from mosquito bites in homes, so researchers have focused on studying adult mosquitoes captured indoors.

Vernick and his colleagues have found that indoor captures don't give an accurate picture. Collecting adult mosquitoes outside is notoriously difficult: traps enticing mosquitos with artificial bait are inefficient, and using human bait is banned. The aquatic larvae are easier to catch, and they reveal details such as the individual's place of birth. So the team collected larvae from ephemeral puddles in hoof-prints and tire ruts around villages during Burkina Faso's rainy season, and raised them to adulthood in the lab.

The researchers tested the insects for genetic markers and mutations across the genome, and compared them with adults caught inside. They found that the indoor-caught insects consisted of two previously known, genetically distinct types of A. gambiae, which look the same but rarely interbreed. But the outdoor-caught mosquitoes had three distinct types — the two known ones and the previously unknown group, Goundry. And 58% of Goundry mosquitoes that were fed malaria-infected blood picked up the parasite, compared with 35% of the indoor mosquitoes.

A gap in the defences

Concern about the possible existence of an outdoor malaria-transmitting mosquito first emerged in the 1970s, after the failure of a US$6-million eradication programme conducted by the WHO in Garki, Nigeria. The best household pesticides and medicines had been used in full force in the area for several years, but malaria persisted.

In hindsight, researchers speculated that the Garki project was doomed by elusive mosquitoes resting outside. Early studies on genetic differences underlying indoor and outdoor preferences in A. gambiae lent credence to the idea².

"But no one has ever found the smoking gun," says Jo Lines, a coordinator for the WHO's Global Malaria Programme. He notes that there is still no solid evidence of outdoor-living mosquitoes that infect people with malaria: although Vernick and his colleagues found larvae and reared them into adults in the lab, infected adult Goundry have not been caught in the wild.

"For the past four decades, we've been discovering new species of mosquitoes capable of getting the malaria parasite," says Lines, "but many are of zero importance because they don't bite humans." Proof that an outdoor mosquito is a vector could mean that future eradication efforts will be terribly difficult.

Vernick's team is now trying to catch adults of the elusive pests by placing cages over pools of water. "This is the toughest problem we've faced," he says. "But nobody else can do it."