New research is helping to unravel the machinery that allows a mosquito to sniff out its human quarry, which could lead to more and better ways of foiling the disease-spreading insects. A report published today in the online version of the Proceedings of the National Academy of Sciences describes four genes that appear to produce odor-sensing molecules in Africa's Anopheles gambiae, a carrier of malaria, the number two killer in the developing world. Understanding how such genes operate could enable scientists to develop new compounds that will repel mosquitoes or lure them to poisons. Such chemicals are needed, senior author Laurence J. Zwiebel of Vanderbilt University explains, because "current levels of malaria and other insect-borne diseases suggest that we're not controlling these insects very well."

Zwiebel and colleagues scanned the mosquito genome looking for genes similar to those that generate fruit fly odorant receptors, proteins that project from nerve cells and initiate a biochemical cascade when they encounter certain molecules in the air. The four candidates the team found were all active in the antennae and mouthparts of the mosquito, where its sense of smell resides. Significantly, one of the genes the team isolated was active only in females¿the mosquito gender that bites¿and its activity dropped off sharply 12 hours after a blood meal. Previous studies have found that a female's sense of smell is dulled after feeding on human blood. Zwiebel says he and co-workers have now isolated a total of 30 possible receptors, and he expects to find anywhere from 60 to 100 in the end.

"Understanding the switch in the mosquito nose is just step one," he notes. Individual receptors generally bind to a range of molecules with varying strengths. A longer and more difficult task, he says, will be to figure out how a mosquito's brain processes the signals that various receptors send. Controlling malaria will require an international effort, Zwiebel stresses, and "we hope that by identifying these sorts of genes¿ we'll be able to help."