Female mosquitoes, which can transmit deadly diseases like malaria, dengue fever, West Nile virus and filariasis, 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 toward exposed areas such as ankles and feet, being drawn there by skin odors（气味）. Why does the mosquito change its track and fly towards skin? How does it detect our skin? What are the odors from skin that it detects? And can we block the mosquito skin odor sensors and reduce attractiveness?

Recent research done by scientists at the University of California, Riverside can now help address these questions. They report on Dec. 5 in the journal Cell that the very receptors in the mosquito's maxillary palp（下颚须） that detect carbon dioxide are ones that detect skin odors as well, thus explaining why mosquitoes are attracted to skin odor -- 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 odor molecules as compared to CO2," said Anandasankar Ray, an associate professor in the Department of Entomology and the project's principal investigator. "For many years we had primarily focused on the complex antennae of mosquitoes for our search for human-skin odor receptors, and ignored the simpler maxillary palp organs."

Until now, which mosquito olfactory neurons were required for attraction to skin odor remained a mystery. The new finding -- that the CO2-sensitive olfactory neuron is also a sensitive detector of human skin -- is critical not only for understanding the basis of the mosquito's host attraction and host preference, but also because it identifies this dual receptor of CO2 and skin-odorants as a key target that could be useful to disrupt host-seeking behavior and thus aid in the control of disease transmission.

To test whether cpA activation by human odor 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 behavior on human foot odor -- specifically, on a dish of foot odor-laden beads placed in an experimental wind tunnel -- and found the mosquito's attraction to the odor was greatly reduced.

Next, using a chemical computational method they developed, the researchers screened nearly half a million compounds and identified thousands of predicted ligands（配体）. 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.