Scientists Discover Mosquito's 'Nose'

Scientists have identified some of the genes that enable a mosquito to smell and seek out its human victim, opening the door to a new generation of chemicals specifically designed to reduce the effectiveness of this disease-carrying killer.

The genes, which are strikingly similar to the genes that give humans a sense of smell, were identified in a particularly deadly mosquito, Anopheles gambiae, an African species that spreads malaria and feeds primarily on humans.

"Many people, including myself, believe it is the most dangerous animal on the planet," says Laurence J. Zwiebel, , assistant professor of biological sciences at Vanderbilt University.

The discovery, made in Zwiebel's lab in collaboration with scientists from the University of Illinois at Urbana-Champaign and Yale University, was revealed in the Nov. 27 issue of the online version of the Proceedings of the National Academy of Sciences.

It is presumed that the same genes are present in all species of mosquitoes, but the researchers zeroed in first on Anopheles gambiae because of its deadly legacy.

"More people die of malaria every year, according to the World Health Organization, than anything else but tuberculous," Zwiebel says. "Literally millions of people die every year from malaria," especially in sub-Saharan Africa where this mosquito is the primary carrier, or vector, of the disease.

That grim fact is why the WHO, the National Science Foundation and the National Institutes of Health are supporting a broad-based research program aimed at reducing the threat from mosquitoes. The identification of the specific genes that allow mosquitoes to detect the odor of humans at a considerable distance doesn't mean the battle has been won, but victory has moved significantly closer.

At the very least, "rather than looking for a needle in a humongous haystack, we have made the haystack a little smaller," Zwiebel says.

The "needle," in this case, is some sort of chemical that would be far more effective than those currently available to repel mosquitoes. And conversely, scientists hope to find an attractant, as well as a repellant, so they can lure mosquitoes into traps loaded with insecticide.

Such a trap, Zwiebel says, "would be a mosquito motel where they check in and never check out."

Similar Sniffing Systems

In addition to Zwiebel, the team included A. Nicole Fox and R. Jason Pitts of Vanderbilt, Hugh M. Robertson of the University of Illinois, and John Carlson of Yale. The genes they have identified are believed to be responsible for coding proteins, called odorant receptors, which in turn produce the sense of smell. They are extremely similar to the odorant receptors in the much-studied fruit fly Drosophila, which serves as the scientific model for insects.

Although insects have been around hundreds of millions of years longer than humans, nature apparently took an "if it ain't broke, don't fix it" attitude toward the genes that give animals the ability to smell. Evolution apparently found no reason to modify a system that has worked so well for mosquitoes and other insects for so long.

"It's astonishing that evolution has conserved the mechanisms between incredibly diverged organisms such as the insects and the vertebrates," Zwiebel says.

It might seem surprising that a mosquito can smell at all, since it has no nose. But it uses its antennae to smell, and the genes discovered by the researchers are expressed only in the antennae, further evidence of their role.

Opening the Door

Only the female mosquito bites humans, because it needs blood for its reproductive process, and it has developed an uncanny ability to find us. In addition to the body odors and carbon dioxide that we exhale, it also uses sight to detect movement, and it can sense infrared radiation, which suggests that a warm body is present.

But of those various skills, the sense of smell is apparently the most important. Studies have shown, for example, that simply taking a shower will reduce a person's vulnerability to mosquitoes. What they relish is the smell of humans, the "cloud of chemicals" that follows us around, as Zwiebel puts it.

Identifying the genes that make it possible for mosquitoes to discriminate among various odors has "opened the door to designing specific chemicals that would throw a very good monkey wrench into their machinery," he says.

Ideally, that chemical should make any mosquito that sniffs a human gag on the stench and look for dinner elsewhere.

It will take awhile, probably at least several years, to come up with such a chemical, but the playing field may have tipped in the favor of humans.

If so, what Zwiebel and his colleagues are calling the "ultimate bioterrorist" is in for a heap of trouble.

Lee Dye’s column appears weekly on A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.