By Sharon Oosthoek

Most mosquitoes don’t discriminate between human and animal prey – blood is blood after all. Yet some strains of Aedes aegypti, the mosquito that carries Zika, malaria and dengue fever, have evolved to feast almost exclusively on us.

Until recently, the chemical odors they use to differentiate us from dogs, rats, pigs or birds were a mystery. A team of researchers led by Princeton University neuroscientist and evolutionary biologist Carolyn “Lindy” McBride, has now figured out what it is about human odor that they cue in on, and what part of their brain allows them to pick out those signals. They published their research in May in the journal Nature.

University of British Colombia zoologist Ben Matthews, one of the paper’s authors, explains that human odor is made up of dozens of different volatile compounds produced by microbes on our skin. Those same compounds, in slightly different ratios, are present in most mammal odors.

None of those compounds is attractive to mosquitoes by itself, says Matthews, so the challenge was to determine the exact blend of components that mosquitos use to recognize our smell.

The team concluded that two chemicals, decanal and undecanal, are enriched in human odor. They patented a blend featuring decanal that they hope could lead to baits attracting mosquitoes to lethal traps, or repellants that interrupt the signal.

“It’s not just the chemicals but the ratio of the chemicals that’s important,” says Matthews. “So rather than using DEET, you could make yourself smell like a sheep or a dog or a bird with the right dose.”

To provide comparison mammals to test, Princeton graduate student Jessica Zung worked with former research specialists Alexis Kriete and Azwad Iqbal to collect hair, fur and wool samples. They used odor from sixteen humans, two rats, two guinea pigs, two quail, one sheep and four dogs.

“For the human samples, we had a bunch of great volunteers,” Zung says in a news release. “We had them not shower for a few days, then strip down naked and lie down in a Teflon bag.” The volunteers had to remove their clothes because cotton, polyester and other clothing fibers have their own smells that would distort human odors.

The team genetically engineered mosquitoes whose brains lit up when active, before delivering human- and animal-flavoured air in ways that the mosquitos could detect while inside the team’s custom-built imaging equipment.

Jane Hill, who studies volatile molecules in breath and the transcripts in blood at the University of British Columbia, called the team’s evidence compelling. It also begs some followup questions, says Hill, who was not involved in the research.

“For example, how do these findings translate to non-human primates or other species not tested but who still get bitten by mosquitoes? And, what influence do perfumes, soaps, antibacterial compounds, antibiotics have?”

“More pressingly,” adds Hill, “as summer is upon us, how can we change that ratio so that the mosquito swarms go elsewhere? I, for one, would like to know.”