This week, Yale researchers published findings concerning how mosquitoes respond to the smell of human sweat, aiding them in detecting human hosts. This discovery will be useful in developing insect repellents and insect traps.

John Carlson, professor of molecular, cellular and developmental biology, and Elissa Hallem GRD ’05, created a means of identifying specific smell receptors of mosquitoes, and testing which odorants elicit the strongest response from them.

Carlson and Hallem cloned mosquito smell receptor genes and expressed them in the nerve cell of a mutant fruit fly that otherwise lacked this receptor. Researchers could then test specific compounds to see which ones activated or inhibited the smell receptors.

4-methyl phenol, a component of human sweat, elicited the strongest response in the nerve cell holding one of the mosquito smell receptor.

“We tried a number of odors, and [the nerve cell] didn’t respond to any of them, and we were quite disappointed. But we tried 4-methyl phenol, and it elicited a strong response,” Carlson said. “4-methyl phenol <2011> it smells like a horse barn.”

Insects use smell to find their prey. Odor receptors are carried on the antennae of insects. The odor receptor that detects 4-methyl phenol is specific to females, and only females have the ability to bite humans. However, the production of these receptors ceases after a female has had a blood meal.

“Basically, mosquitoes find human hosts through their sense of smell… but we know very little about how they do that,” Carlson said. “One of the nice things about this– it all fits together with the possibility that this receptor plays a role with the means by which a mosquito finds a host.”

These findings are relevant to human health, as mosquitoes are an international health problem. Annually, more than a million people die of malaria, a disease transmitted by a bite from an Anopheles mosquito. An estimated 300 to 500 million people are infected yearly, according to the Center for Disease Control Web site.

“Ten percent of the world suffers from malaria,” Carlson said. “Anopheles mosquitoes have been called the most dangerous animals in the world.”

Further research may provide means of controlling mosquitoes’ responses to odorant stimuli, thereby preventing mosquitoes from smelling out their prey. Compounds that elicit or prevent a response in mosquito smell receptors can be used in repellents and traps.

“The thought is if this receptor plays an important role in the process of finding human hosts, and if we can find components that block that receptor, it might jam that receptor system,” Carlson said. “I suspect there will be other [mosquito receptors] that will be sensitive to human sweat, and we will need a cocktail of repellents.”

Insects also destroy many agricultural crops yearly, and while there are repellents available, they are imperfect.

Future studies will concentrate on different receptors in mosquitoes.

“The one receptor we found in females is involved in the host-seeking behavior, so the other female ones are particularly interesting,” Hallem said. “And the lab will be testing other receptors, and possibly studying some behavior sometime down the road.”

The research was funded by an National Institutes of Health grant to Carlson and an National Science Foundation predoctoral fellowship to Hallem. Other authors on the paper, published in the Jan. 15 issue of Nature, are Laurence J. Zwiebel and A. Nicole Fox of Vanderbilt University.