Based on the pattern of the disease's spread, Oregon may see a sharp increase in the humans who get West Nile virus in 2007, health officials say.
When the virus crosses state boundaries, officials said, it typically results in a few cases the first year, followed by a surge a year or two later.
Four of Oregon's first six human cases were in Malheur County in 2004. In 2006, the number of human cases in the county ballooned to 55 of the state's 73 cases.
And last year, the virus arrived in populous Multnomah County.
"The movement into Oregon has been rather slow," said Emilio DeBess, epidemiologist and public health veterinarian with the state Public Health Division. "But as it moves into the more populated areas, there is a greater chance of it finding and infecting a human being."
So far, he said, two people have died in Oregon, one in 2006 and another this year from an infection acquired last year.
The illness causes no symptoms in 80 percent of those infected. The remaining 20 percent develop symptoms including fever, muscle aches, headache and vomiting. About one in 150 develops severe illness, including coma, convulsions, paralysis and vision loss. The most severe cases, as well as most deaths, occur in the elderly or those with chronic illness.
Humans get the virus from mosquitoes, which along with birds form a reservoir of the disease.
David Turner, vector control specialist with Multnomah County, said he, too, anticipates an increase in West Nile infections. Four dead birds were found to be infected in 2006 &
three just east of the Willamette River and one in Gresham.
He said helicopters began applying larvicide in April to low areas near the Columbia River, about six weeks earlier than normal, to control one kind of mosquito. And the number of acres treated that way may double this year. He said the county also plans to treat more small basins to control a second kind of mosquito.
He urged residents to drain and refill things that contain standing water, such as children's wading pools and decorative water features, at least once a week.
"Everybody's got something in their yard that's going to be breeding mosquitoes this summer," he said.
Oregon braces for surge in West Nile virus cases