Dead crow tests positive for West Nile virus in San Mateo County

in Community

A case of West Nile virus was detected in San Mateo County this week, the first indication of the presence of virus since 2018, according to the San Mateo County Mosquito and Vector Control District (SMCMVCD).

A dead American crow collected from South San Francisco tested positive for West Nile virus, which is primarily a disease of birds but which can also infect humans, horses and other animals.

The West Nile virus is most commonly spread to people by an infected mosquito bite, and while most people infected do not feel seek, about 1 in 5 develop a fever and other symptoms and 1 in 150 develop serious, sometimes fatal, illness, according to the Centers for Disease and Control Prevention (CDC).

SMCMVCD is encouraging residents to report fresh carcasses of birds to the California West Nile Virus Hotline at or by phone at 877-WNV-BIRD (877-968-2473). The agency also advises residents to take steps to prevent mosquito breeding on their property and to avoid mosquito bites by reducing standing water and to wear proper clothing and mosquito repellent while outside at dawn or dusk, when mosquitoes typically bite.

SMCMVCD is deploying mosquito traps in neighborhoods where the dead bird was found to collect and test adult mosquitoes, and the District “continues to control mosquitoes in their immature stages through an integrated pest management (IPM) program.”

“The District’s vector control technicians routinely inspect standing water where mosquitoes develop, and treat mosquito breeding sources throughout the county,” the agency said.

For assistance with a mosquito problem in San Mateo County, contact the District at (650) 344-8592. For more information on West Nile Virus and other District services, visit