Prosser Benton County health officials said Thursday they believe they've identified the first human case of West Nile virus in the county.

A sample taken from a Prosser resident tested presumptive positive in a local laboratory and has been forwarded to the Washington State Department of Health for confirmation.

"Although this initial positive test may not be confirmed as West Nile virus, we are notifying the public and our medical providers about the potential case as a reminder that the virus can cause severe illness," said Dr. Larry Jecha, health officer for Benton and Franklin counties. "This time of year — late summer and early autumn — is when cases in people traditionally appear. There is no cure for West Nile virus, but fighting mosquito bites helps to prevent infection."

The announcement of a possible human case came on the heels of the identification of five more West Nile-positive birds in Benton County earlier Thursday.

Six infected birds have been found so far this year in West Richland, Richland, Kennewick and Prosser.

"This widespread West Nile virus activity indicates a high risk of cases to residents and visitors in our area," said Susan Shelton, environmental health specialist for the Benton-Franklin Health District.

The Benton Franklin Health District now will focus testing on Franklin County and south Benton County in the vicinities of Paterson, Plymouth and Crow Butte.

West Nile has been present in the United States since 1999, and in Washington state since 2005. Benton County saw its first infected birds and horses last year, but no humans.

Birds carry the disease and mosquitoes pick it up when they bite birds. Then mosquitoes pass it on to horses or humans through bites.

Most people who become infected show no symptoms. But up to 20 percent of those infected could suffer from a fever, headache, body aches, nausea, and vomiting and sometimes swollen lymph glands or a skin rash.

About 1 in 150 infected people may develop more severe symptoms that last several weeks and cause permanent neurological damage or death. People over 50 are more likely to develop severe symptoms.

To reduce the risk of exposure to West Nile:

• Eliminate sources of standing water.

• Avoid outdoor activities at dusk and dawn when mosquitoes are active.

• Wear long-sleeved shirts and long pants in mosquito-infested areas.

• Use mosquito repellents containing DEET, oil of lemon eucalyptus and Picardin.

• Make sure screen doors and windows are in good repair and fit tightly.