BREMERTON — Public health officials and researchers don’t know why West Nile virus failed to hit Washington state hard this 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.
“Since we had horses and we had birds infected, it’s probably just a matter of luck and-or the intensity of exposure to (infected) mosquitoes,” Kimball said. “We had a particularly cold summer, so it’s possible people weren’t out to get bitten.”
At least one species of mosquito known to transmit West Nile is present throughout Washington state, Moyer said.
“We had plenty of mosquitoes. We had plenty of standing water — long enough to breed tons of mosquitoes,” said Terry Whitworth, an adjunct professor of entomology at Washington State University in Puyallup who runs a mosquito-control business.
West Nile was first detected in Washington in 2002, in birds and horses.
“We thought, ‘Here it comes,’ ” Moyer recalled.
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.
In Maine, the virus has been found in animals or mosquitoes in six of the past seven years, but it has never logged a single human case.
Nationwide, 2007 was a relatively mild year for West Nile virus, with the number of human cases dropping for the first time since 2000.
As of Nov. 6, the U.S. Centers for Disease Control and Prevention reported that 3,265 people had been infected this year; 92 died.
Many more people may have been infected, but an estimated four out of five people with West Nile virus don’t know they have it.
Kitsap County Public Health officer Scott Lindquist speculated that Washington may have an environment that impedes West Nile transmission.
“For some reason, something in this mix — between environment, humans, mosquitoes and animals — is not supporting the same level of activity as in other parts of the country,” Lindquist said.
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