How malaria-spreading mosquitoes can tell you're home

Females of the malaria-spreading mosquito tend to obtain their blood meals within human dwellings. Indeed, this mosquito, Anopheles gambiae, spends much of its adult life indoors where it is constantly exposed to human odor - from used clothing, bedding, etc. - even when people are absent.

But is human odor enough as a reliable cue for the mosquitoes in finding humans to bite?

Not quite, reports a team of entomologists at the University of California, Riverside in a research paper published online earlier this month in the Journal of Chemical Ecology. The researchers' experiments with female Anopheles gambiae show that the mosquitoes respond very weakly to human skin odor alone. The researchers found that the mosquitoes' landing on a source of skin odor was dramatically increased when carbon dioxide was also present, even at levels that barely exceed its background level. The researchers suggest, too, was that the mosquitoes use a "sit-and-wait" ambush strategy during which they ignore persistent human odor until a living human is present.

"Responding strongly to human skin odor alone once inside a dwelling where human odor is ubiquitous is a highly inefficient means for the mosquito of locating a feeding site," said Ring Cardé, a distinguished professor of entomology, whose lab conducted the research.

"We already know that mosquitoes will readily fly upwind towards human skin odor but landing, the final stage of host location, which typically takes place indoors, does not occur unless a fluctuating concentration of carbon dioxide indicates that a human host is present. It may be that upwind flight towards human odor has more to do with locating a human dwelling, which emits human odor even when its occupants are absent, than locating a feeding site per se."

Cardé explained that mosquitoes, once indoors, conserve their energy by ignoring omnipresent human odor in an unoccupied room. Small increases in carbon dioxide indicate to the mosquitoes the probable presence of a human. This then triggers the mosquitoes to land on human skin.

Why Some Mosquitoes Spread Malaria and Others Don't

Sequencing the genomes of numerous Anopheles mosquito species from locations around the world has shown why some can carry and spread the deadly disease malaria while others don't, researchers say.

Evolution among more than 500 different Anopheles species has left certain varieties genetically better equipped for transmitting the disease than even some close cousins, they say.

Only about 60 of those species can transmit the malaria parasite Plasmodium and infect humans, an international research team reports in the current issue of Science.

Those parasites cause an estimated 200 million cases of malaria each year, resulting in than 600,000 deaths annually.