Mosquitoes hunt humans by smelling odours

Mosquitoes track down humans from a distance by the smell of carbon dioxide on our breath and then find exposed areas through skin odours, researchers, including an Indian-origin scientist, have found.
Female mosquitoes, which can transmit deadly diseases such as malaria and dengue fever are attracted to us by smelling the carbon dioxide we exhale, being capable of tracking us down even from a distance.
But once they get close to us, they often steer away towards exposed areas such as ankles and feet, being drawn there by skin odours.
Scientists at the University of California, Riverside found the very receptors in the mosquito's maxillary palp that detect carbon dioxide are ones that detect skin odours as well, thus explaining why mosquitoes are attracted to skin odour - smelly socks, worn clothes, bedding - even in the absence of CO2.

"It was a real surprise when we found that the mosquito's CO2 receptor neuron, designated cpA, is an extremely sensitive detector of several skin odorants as well, and is, in fact, far more sensitive to some of these odour molecules as compared to CO2," said Anandasankar Ray, project's principal investigator.
Until now, which mosquito olfactory neurons were required for attraction to skin odour remained a mystery.

To test whether cpA activation by human odour is important for attraction, the researchers devised a novel chemical-based strategy to shut down the activity of cpA in Aedes aegypti, the dengue-spreading mosquito.
They then tested the mosquito's behaviour on human foot odour - specifically, on a dish of foot odour-laden beads placed in an experimental wind tunnel - and found the mosquito's attraction to the odour was greatly reduced.

Next, using a chemical computational method, the researchers screened nearly half a million compounds and identified thousands of predicted ligands.
They then short-listed 138 compounds based on desirable characteristics such as smell, safety, cost and whether these occurred naturally.
Several compounds either inhibited or activated cpA neurons of which nearly 85 percent were already approved for use as flavor, fragrance or cosmetic agents.
Better still, several were pleasant-smelling, such as minty, raspberry, chocolate, etc, increasing their value for practical use in mosquito control.
The researchers then zeroed in on two compounds: ethyl pyruvate, a fruity-scented cpA inhibitor approved as a flavor agent in food; and cyclopentanone, a minty-smelling cpA activator approved as a flavour and fragrance agent.

By inhibiting the cpA neuron, ethyl pyruvate was found in their experiments to substantially reduce the mosquito's attraction towards a human arm.
By activating the cpA neuron, cyclopentanone served as a powerful lure, like CO2, attracting mosquitoes to a trap.
They study was published in the journal Cell.