OLYMPIA, Wash. -- A King County woman may be the first case of West Nile virus infection reported in the state, said the Department of Health.

The woman in her 50s was being screened to donate blood, when tests showed she had West Nile virus, DoH said.

She did not have any symptoms or develop West Nile fever, DoH said. The exact location of her exposure may never be confirmed because she traveled through eastern Washington and into Oregon during the exposure period.

Washington’s first three cases in humans were in 2006, but there were no human cases during 2007, said Tim Church of DoH.

Like this case, most people who are infected with West Nile virus don’t ever get sick or have symptoms, yet it can be a very serious disease for a small number of people, especially those older than 50, said Dr. Marcia Goldoft, Acting State Epidemiologist for Communicable Disease.

Avoiding mosquito bites is the best prevention. West Nile infection can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the spinal cord and brain).

Mosquitoes become infected with West Nile virus by feeding on infected birds, and then pass the virus to uninfected birds, people, horses, or other hosts. Crows, ravens, magpies, and jays are especially susceptible to dying from the virus.

Environmental monitoring has detected West Nile virus in 14 mosquito samples and six horses, all in eastern Washington so far this year.

The Department of Health has a West Nile virus information line 1-866-78-VIRUS (1-866-788-4787) and the agency’s West Nile virus Web site for more information.