A new study shows that human odor is not reliable enough to guide the malaria mosquito to its next blood meal. It seems that another trigger – exhaled carbon dioxide – is what guides the flying insect toward its next bite victim.

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The researchers used a wind tunnel and a night vision camera to observe the mosquitoes searching for their feeding sites.
Image credit: Carrie Rosema/UCR

This was the result of a study by entomologists at the University of California-Riverside (UCR), who report their findings in the Journal of Chemical Ecology.

Malaria is a disease that develops from the bite of a mosquito carrying the Plasmodium parasite. Once the parasite enters the bloodstream, it invades and lives in the new host’s red blood cells.

Malaria kills over 600,000 people a year – most of whom are young children in sub-Saharan Africa.

The malaria parasite is carried by the female mosquito, the most common one being Anopheles gambiae. Studies have shown that this species predominantly gets its human blood meals inside human dwellings, which it locates through detecting human odors. It feeds on blood to help it produce eggs.

The female mosquito enters houses throughout the night. After a blood meal, she often remains in the home until she is ready to lay eggs. She may also seek refuge inside human dwellings during the day because of the unbearably hot temperatures outside.

But once she takes up residence, how exactly does the female mosquito go about locating her next blood meal?

Human odor is all around all the time – for instance, in used clothing and bedding – even when there are no humans present. So does she keep flying around and waste valuable energy in the hope of finding a feeding site? Or does she wait until she “knows” when a live victim is around so she can reliably locate her landing point and take her next blood meal?

These were the questions that puzzled Ring Cardé, a distinguished professor of entomology at UCR, who conducted the study with colleagues in his lab.

The team carried out a series of experiments that showed female A. gambiae respond very weakly to human skin odor alone when locating their next blood meal.

They found the mosquitoes’ landing on a source of skin odor was dramatically increased when carbon dioxide was also present, even at concentrations that barely exceeded background levels.

Humans – like other animals – exhale carbon dioxide. Thus, when humans are present, the background level of carbon dioxide increases. This study suggests the female mosquito has adapted to detect even slight changes in ambient carbon dioxide.

The researchers suggest the mosquitoes use an ambush strategy based on “sit and wait,” where they ignore the persistent background human odor until this other signal – change in ambient carbon dioxide – tells them a living human is present. Prof. Cardé explains:

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.”

For their experiments – which they conducted in an “assay cage” that resembles a wind tunnel – the researchers used A. gambiae mosquitoes collected in Cameroon.

They collected samples of skin odor from pieces of cloth that one of the team wore inside his socks for a few hours. They observed the mosquitoes’ landing behavior using videos recorded with the help of a night vision camera.

The authors note how the female mosquitoes’ landing on the samples “was dramatically increased by addition of carbon dioxide at a range of concentrations above ambient. Indeed, this effect was seen even when carbon dioxide was just 0.015 % above ambient within the assay cage.”

The researchers hope their findings will help in the design of new types of mosquito control. They also want to find out if the mosquitoes’ response to skin odor is affected by carbon dioxide outdoors.

Meanwhile, Medical News Today recently learned how researchers at the University of California-Davis discovered that a hybrid malaria mosquito that is resistant to insecticides used in bed-nets has emerged in Mali. The hybrid is a cross between two species of malaria-carrying mosquitoes – Anopheles gambiae and Anopheles coluzzii.