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Current West Nile Virus Information for San Mateo County

2023 Season

Computer generated image of a silhouette of eight birds on their backs (dead)

In addition to the data below, as of September 25, there are 6 additional dead birds that have tested positive for West Nile virus on in-house tests.

This brings the 2023 San Mateo County total to 8 positive dead birds.  Positive birds have been found in Burlingame (1), Redwood City (2 in Redwood Shores area), San Carlos (1), and Menlo Park (4). Additional mosquito trapping and testing done in areas where dead birds were found has not found any positive mosquitoes. Mosquito trapping and testing is ongoing.


San Mateo County

As of September 22, 2023,  there have been 233 dead birds reported in San Mateo County. Of those, 55 have been suitable for testing and 2 have tested positive for West Nile virus (WNV). Additionally, 426 pooled mosquito samples have been tested for West Nile virus in San Mateo County to date this year.  These pools included a total of 6,616 mosquitoes, and all tested negative for virus.

The District continues to work closely with the San Mateo Health Department to prevent the spread of mosquito-borne diseases. District staff are notified when the SMC Health Department investigates any evidence of mosquito-borne diseases, including West Nile virus, in San Mateo County. Our staff trap and test mosquitoes from areas under investigation as well as from routine mosquito monitoring traps set throughout the County.

For tips on how to avoid mosquito bites and select a safe, effective mosquito repellent, click HERE.


County map of California with some counties colored green to signify West Nile virus activity in non-human samples. Shaded counties include San Mateo County (positive dead birds). For additional information, call 650-344-8592.


As of September 22, 2023, WNV has been detected in most counties statewide, including 25 counties with human cases. There have been 153 human cases in California, of which 6 were fatal. Statewide, 5,490 dead birds reported to the dead bird hotline, 1,641 have been tested and 654 have tested positive for WNV (40%). Additionally, 3,983 mosquito samples, 155 sentinel chickens and 21 horses have tested positive for WNV.

West Nile Virus Dead Bird Hotline

Would you like to help us protect against mosquito-borne illnesses? The California Department of Public Health’s West Nile virus call center is now open with live operators. Residents who find a dead bird in good condition (head and eyes intact and not infested with flies or ants) should call 1-877-WNV-BIRD (1-877-968-2473) or file a report online at If suitable for testing, a District staff member will bring the bird back to the lab and the carcass will be tested for the presence of mosquito-borne disease.


Page last reviewed: September 25, 2023

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