OLYMPIA — West Nile virus has been found in a horse in Yakima County, the state Department of Agriculture announced yesterday.

The discovery marked the first case this year in a horse that was infected in Washington state. Earlier this month, the virus was detected separately in mosquitoes and a magpie, both in Yakima County.

"This is a clear indication that West Nile virus is in our state and it's already amplified enough in mosquitoes and birds to spread the infection to horses," Dr. Leonard Eldridge, state veterinarian, said in a news release. "We have been anticipating this since it's been seen in neighboring states. The public and horse owners should take protective measures and avoid mosquito bites."

The mare from the Prosser area was tested after she became unsteady and developed signs of a central-nervous-system infection. The horse was not vaccinated for the virus. The positive test results were reported yesterday by the Washington Animal Disease Diagnostic Laboratory at Washington State University in Pullman.

West Nile virus has spread across the country since it was first detected in 1999.

Last year, 2,535 human cases and 98 deaths were reported to the Centers for Disease Control and Prevention, with Western states hit the hardest. The worst year for West Nile virus since its appearance six years ago was 2003, with 9,862 reported cases and 264 deaths.

The virus has been confirmed in every state except Alaska and Hawaii over the past six years.

According to the CDC, most people infected by West Nile virus show no symptoms. About two out of every 10 suffer fever, aches, nausea and vomiting. About one in 150 people develop more serious symptoms and some die.

Though people of any age can become severely ill, the risk is highest for people age 50 and over and for organ-transplant recipients.