Current West Nile Virus Information for San Mateo County
2025
Mosquito testing
Culex mosquitoes are able to carry and spread West Nile virus. Our staff collect mosquitoes from around the County using traps set overnight and baited with carbon dioxide and scent lures. Mosquitoes are grouped into batches called "pools" for testing. They are tested using a triplex PCR test.
As of June 16, 2025, a total of 284 mosquitoes (15 pools) have been tested for West Nile virus - none have tested positive for West Nile virus.
Dead bird testing
Dead birds can be the first sign of West Nile virus in an area. Birds like crows, ravens, and jays are more likely to die from West Nile virus. You can report dead birds to the Dead Bird Hotline (https://westnile.ca.gov/report). Staff will determine whether the bird is able to be tested for West Nile virus. They are tested using a triplex PCR test. Our District testing protocol does not test dead birds for H5N1 avian influenza ('bird flu').
As of June 16, 2025, a total of 318 dead birds have been reported in San Mateo County. 83 were in testable condition - none of the 83 birds tested positive for West Nile virus.
Additional information
For current information about West Nile virus across the state of California, visit the California Department of Public Health's West Nile virus page.
To see data from previous years, visit our page about historical West Nile virus findings.
Page last reviewed: June 16, 2025
