Image: U.S.G.S.

So many birds of different feathers have tested positive for the West Nile virus this year that scientists from the U.S. Geological Survey are saying the New Yorkbased bug is bound to spread much farther south and west than was expected. "West Nile virus has been isolated from more than 63 species of birds, including 53 free-ranging species from eight states," Robert McClean, director of the U.S.G.S. National Wildlife Health Center in Madison, Wis., said at a press conference last Friday. "That means it's not a matter of if, but when, the virus will move south." As birds migrate down the Atlantic coast for the winter, mosquitoes in new areas will no doubt pick up the virus and pass it on to humans and other animals.

Since January, U.S.G.S. researchers have looked for the viruswhich made its debut in the Western Hemisphere only one year agoin 1,230 bird carcasses and tissues (right), 1,990 bird serums, 277 mammal carcasses and 24 mammal serums. In addition to birds, the animals most often affected, they found three species of bat, a raccoon, eight horses and an eastern chipmunk that were infected. So far the virus has caused 16 cases of illness and one death in humans. And increasingly many varieties of mosquitoes are carrying West Nile: eight speciesincluding dawn, day and dusk bitersare affected. To track the virus's progress, the government has set up a new Web site with various informative maps.