This is the state’s highest yearly confirmed human case total since West Nile virus was first detected in Washington early this decade. A count of three cases in 2008 had been the previous top annual count.

Washington’s West Nile virus monitoring program shows the virus was active this year in 15 counties, mostly in Eastern Washington — Yakima, Benton, Franklin and Grant counties have had the most detections of the virus. The Department of Health tracks West Nile virus in the environment in mosquitoes, birds, and horses or other mammals. This year, there have been 341 positive mosquito samples, 22 positive birds, and 67 positive horses. Only four infected birds have been found in Western Washington — in King, Lewis, Grays Harbor, and Mason counties.

West Nile virus is primarily a bird disease that can be spread to people who are bitten by an infected mosquito. Most infected people do not become ill or have only mild symptoms, but West Nile virus can cause serious infections in the brain (encephalitis) or in the spinal cord (meningitis) and can be fatal. People over age 50 are at highest risk for these serious infections, though younger people can also develop severe illness.

There is no vaccine to prevent West Nile virus infection in people. The best way to prevent infection is to avoid mosquito bites. Now that cooler fall weather has arrived, mosquito activity is winding down. However, some mosquitoes may still be out. People should protect themselves against West Nile virus and other mosquito-borne illnesses by avoiding mosquito bites.

Information about West Nile virus is on the Department of Health’s Web site.