After its favorite birds have migrated away, the mosquito that transmits the West Nile virus starts snacking on people. Now researchers think they know why. Humans and birds produce a common smell that the mosquitoes find irresistible.
To find their hosts, mosquitoes sense heat, CO2, moisture, and certain volatile chemicals; which cues they use depends on the species. Entomologists Walter Leal and Zainulabeuddin Syed of the University of California, Davis, wanted to know what lures Culex pipiens quinquefasciatus, better known as the southern house mosquito. The species is an important--although not the only--carrier of the West Nile fever, the viral disease introduced in New York City from the Middle East in 1999 that has become widespread in North America. West Nile virus has wreaked havoc mostly on bird populations, but humans and a variety of other animals can become infected as well when bitten by a mosquito that previously dined on an infected bird. Symptoms can range from fever and headaches to coma and paralysis; last year 44 patients died from the virus.
Leal and Syed wondered whether the insects followed the same scent to find people and birds. So they analyzed the chemical composition of the odor mixture wafting from the forearms of 16 people of various ethnic backgrounds; they did the same for samples from pigeons and chickens, two species that Culex mosquitoes like. Of the four ingredients that dominated the human odors, the team found one, called nonanal, that for unknown reasons was also present at high levels in the bird samples.
Next, the researchers tested whether neurons in the insects' olfactory receptors, or sensilla, responded to nonanal. They found that the compound triggered a strong reaction in more than half of the insects' 1300 sensilla, suggesting that the molecule drives the insect's behavior. Finally, the duo tested whether mosquitoes are actually drawn to nonanal. Disease surveillance programs often use traps baited with CO2 (in the form of dry ice) to catch mosquitoes, which are then tested for the presence of viruses. When the researchers added nonanal to these traps, their catch increased by about 50%, they report online this week in the Proceedings of the National Academy of Sciences.
The findings help explain why West Nile fever crosses from birds to humans so easily, says Leal. There are practical upshots as well. Coupled with certain design improvements, nonanal might make mosquito traps so efficient that they can become a new weapon in the fight against mosquitoes. That would be welcome, Leal says, because spraying insecticides is controversial.
"It's beautiful work," says Bart Knols, a Netherlands-based medical entomologist who has studied chemical attraction in malaria mosquitoes. "They've gone all the way from the chemical analyses to the field. ... You don't often see that in one paper." Knols says that the study could lead to major improvements in disease surveillance. But he's skeptical that traps can ever catch enough mosquitoes to make a serious dent in populations.