Last April, about a week after the snow finally melted in central Minnesota, Casey, a five-year-old mare near the small town of Brainerd, became a statistic - the first horse to die of West Nile virus in 2003. "That worried us since it was so early in the season," says David Neitzel, epidemiologist with the state's health department. Seven horses died in the area the previous year, but it was not considered a hot spot. "This virus can pick up where it left off," notes Neitzel.

In 2003, West Nile virus (WNV) didn't have to wait to hitch a ride north with migrating birds; the season had a record early start, because the virus never left. Protected within the bodies of infected mosquitoes drowsing away the winter in diapause, it survived. When warm weather returned, so did the mosquitoes and the virus.

WNV spreads fast and sticks. In 1999, the...

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