The United States harbours mosquitoes that are particularly adept at spreading West Nile virus, researchers have discovered. This may be one reason why the disease it causes has stormed across the country.

The virus, which is transmitted by mosquitoes from birds to humans, is thought to have arrived in the United States from Israel. After hitting New York in 1999, it swept to the west coast and Central America, and killed more than 230 people in the United States last year.

Experts have long wondered why the virus spread so quickly and lethally across North America. In contrast, mild forms of the virus are found in Europe and Africa, and occasional severe outbreaks have fizzled out.

To answer this, Dina Fonseca of the National Museum of Natural History in Washington DC and colleagues examined one of the mosquitoes that transmit the virus, called Culex pipiens. They created and compared genetic fingerprints of the insect's populations from 33 spots around the world.

In northern Europe there are two genetically distinct versions of C. pipiens, the team report1. One population lives above ground, bites birds and hibernates to survive the winter. The other preys on mammals year round by huddling in underground sewers and subways.

In the United States, however, more than 40% of C. pipiens mosquitoes are a genetic hybrid of the two European strains. These hybrids are able to bite both birds and humans and ferry the virus between them, says Fonseca.

The finding helps to explain why the virus spread so widely and viciously in the northern United States, agrees Laura Kramer who studies West Nile virus at New York State Department of Health: "It's exciting to think that this is one of the explanations."

But there are many other reasons why the disease has mushroomed in the United States, experts point out, including the existence of other mosquito species that are prominent in the west and south and that bite both birds and humans.

The circulating viral strain may also be particularly harsh - and American animals have little natural immunity to it. "The birds have never seen West Nile virus at all," says Harry Savage, who studies the disease at the Centers for Disease Control and Prevention in Fort Collins, Colorado.

Insect evolution

Fonseca's analysis also sheds some light on how the world's different C. pipiens populations evolved. Previously, it was thought that the underground dwellers in northern Europe arose when a few above-ground mosquitoes adapted to subterranean life.

Instead, Fonseca suggests that the underground insects are more closely related to populations in southern Europe and northern Africa, and that these migrated northwards as snug, urban environments were built. The North American hybrids appeared when the two populations interbred, perhaps after migrating across the Atlantic.

Either way, the study does not rule out the possibility that West Nile virus could strike again in Europe. An increase in the number of serious outbreaks, such as one that affected more than 800 Russians in 1999, suggests that the disease is already on the rise, says Kramer: "It looks like something has changed even in Europe."

 
References
Fonseca, D. M. et al. Science, 303, 1535 - 1538, (2004). |Homepage|