Mendocino County health authorities on Friday reported the county's first case of West Nile virus in a dead bird this year.
Sonoma and Lake counties previously have reported cases of the virus. They include 12 birds and a squirrel in Sonoma County and at least five birds in Lake County. Multiple mosquito samples from Rohnert Park have recently tested positive, officials reported Friday.
No human cases have been reported in any of the three counties this year.
Health officials encourage residents to protect themselves from mosquito bites and make sure their yards don't contain standing water, which provides breeding grounds for mosquitoes.
Avoid being outside at dawn and dusk, when mosquitoes are most active, use insect repellent and wear long sleeves to avoid being bitten, they said.
Most people who are infected don't experience symptoms, but they can range from flu-like symptoms to coma, or even death.