Last year, West Nile virus (WNV) infected more than 9,000 people in 46 states across the U.S. and claimed the lives of 240. The spread of the virus is alarming; it also poses an epidemiological mystery. WNV is a bird disease that spreads to humans through mosquito bites. Yet countries in northern Europe, which has all of the same components for a similar WNV epidemic--including virus-infected birds and plenty of mosquitoes--have seen few human infections. Research published in the current issue of the journal Science suggests that the reason is genetic: whereas northern European mosquito strains preferentially bite either humans or birds, some U.S. mosquitoes are hybrids, carrying genetic material from both "bird-biters" and "human-biters."

The prime mosquito suspect for bird-to-human transmission of WNV is Culex pipiens, a complex of species that live throughout the world. In northern Europe, one strain of Cx. pipiens lives aboveground and another swarms in sheltered underground environments, such as subway tunnels. The aboveground strain gets its blood meals from birds; the underground strain prefers to feed on humans. Geneticist Dina M. Fonseca of the Smithsonian Museum of Natural History and her colleagues compared the genomes of these underground and aboveground populations to Cx. pipiens from other parts of the world, including the U.S. The northern European insects turned out to be genetically distinct from each other, with underground human-biters more closely resembling mosquitoes from warmer southern locales. But more than 40 percent of the U.S. mosquitoes tested genetically resembled both of the northern European groups¿that is, they appeared to be hybrids of the two.

Why the hybrids are so common in the U.S. but virtually absent in northern Europe "is a big mystery to us," says study co-author Robert C. Fleisher, also at the Smithsonian. No one knows whether they arrived from warmer climates in southern Europe, or emerged when northern European aboveground and underground individuals arrived here in the U.S. and mingled, but "there¿s some reason why the reproductive barrier broke down," Fleisher remarks. Observation of the behavior of the mosquitoes themselves may bring answers. Specifically, determining why open-air and subterranean Cx. pipiens do not reproduce in northern Europe and finding out how often individual mosquitoes bite a bird and then a human could illuminate how the virus is transmitted.

Meanwhile, although northern Europeans remain largely untouched by WNV, the new findings highlight how flux in mosquito genetics can affect the spread of the disease. Indeed, the authors conclude that "the arrival of hybrid American forms in northern Europe has the potential to radically change the dynamics of WNV infection in Europe." --Alla Katsnelson