Dearth of West Nile virus is a state mystery

BREMERTON -- Public health officials and researchers don't know why West Nile virus failed to hit Washington state hard during the summer, defying some predictions.

The virus turned up in only one bird and eight horses, all in Yakima County.

"We're not sure what's up with our state," Donn Moyer, a spokesman for the state Department of Health, told the Kitsap Sun. "We're certainly glad and relieved."

West Nile virus, which is spread by the bite of infected mosquitoes, was first found in the United States in New York in 1999. Since then, it has steadily spread westward. In most areas, once the virus becomes established, people catch it in growing numbers for one or two years before the case count levels off.

Washington saw its first human cases last year: two in Pierce County, one in Clark County. Meanwhile, there were nearly 1,000 human infections in neighboring Idaho. Had Washington followed the general pattern, 2007 would have been its turn for a big year. It wasn't.

"I think it's a mystery, to be honest," said Dr. Ann Kimball, a professor in the University of Washington School of Public Health and director of the international Emerging Infections Network.

West Nile was first detected in Washington in 2002, in birds and horses.

But over the following two years, researchers detected no West Nile virus activity at all. By 2004, Washington was the only state besides Alaska and Hawaii to be West Nile virus-free.