The Franklin County Mosquito Control District says West Nile virus has been detected in a mosquito trapped in the northwest part of the county on Wahluke Road.
The district says it's spraying the area from the air and on the ground Wednesday and Thursday to kill mosquitoes.
The state Health Department says the virus has been detected in six mosquito samples this year in Washington. The previous one was earlier this month on the Fairchild Air Force Base.
The department says no people have acquired the disease in Washington, but one resident was infected out of state.
Most people bitten by an infected mosquito will have no symptoms; some may have flu-like symptoms. The virus can be serious in less than 1 percent of the people bitten.