The year's first human case of West Nile virus has been reported in Yakima County.

Few details are available on the case, but earlier this year, an 80-year-old man from Benton County died of the virus. Three horses in Yakima County are also being treated for the illness, which is carried by mosquitoes.

The Yakima County case is the 18th exposure reported in Washington this year --12 of those reports were in Benton County. Regional reports indicate that West Nile virus is also prevalent in Grant and Franklin counties.

Last year the state Department of Health documented 12 cases statewide.

The virus is spread by bites from mosquitoes that have bitten infected birds (crows, ravens, magpies, hawks, eagles, robins and some others). About 80 percent of West Nile virus infections result in no symptoms. In the other 20 percent, however, it causes an illness that begins 2-14 days after the bite.

Symptoms can include fever, tiredness, headache, body aches and rash. People experiencing such symptoms should seek health care and avoid mosquitoes to prevent spreading the infection.

About 1 in 20 cases develops into a more severe illness with brain involvement. Symptoms of this form of West Nile virus include severe headache, decreased level of consciousness, confusion and other nervous system problems. This type of infection appears to be more common in people over 60 years of age or with weak immune systems.

While the virus is active in south central Washington now, it will subside around the time of first freeze in the October or November.

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