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Current West Nile Virus Information for San Mateo County

San Mateo County

As of June 30, 2021, there have been 165 dead birds reported in San Mateo County. Of those, 51 have been suitable for testing and all have tested negative for West Nile virus (WNV). No mosquito pools or sentinel chickens have tested positive for West Nile virus in San Mateo County in 2021.

The District asks residents to report dead birds. Specimens that appear to have been dead for less than 24 hours and are in suitable condition (good body mass, head and eyes intact and not heavily infested with ants or maggots) will be tested for WNV.  Residents can report a dead bird on the State WNV Hotline website at


Map of California with counties outlined. Several counties are colored green, indicating West Nile virus activity in 2021
Calfornia WNV map as of 6/25/21

In California, there have been 155 human cases of WNV from 21 counties (see map), including seven fatalities. At this time last year 178 human WNV cases had been reported from 20 counties. Statewide 1,499 dead birds have been tested, with 336 positive results (22%). At this point last year, 210 WNV positive dead birds had been detected. Mosquito samples are also testing positive below last year’s levels, with a total of 2,617 positive samples in 2020 compared to a 2019’s YTD total of 3,269. Sentinel chickens are also testing positive at a rate comparable to 2019, with 138 positive chickens to date in 2020 compared to a last year’s 135 positives.Statewide, 2,732 dead birds have been reported and 739 have been tested with 27 positive results (4%). At this time last year, ten dead birds had tested positive. This is low compared to a five-year average of 69 positive dead birds at this date. So far in 2021, 54 mosquito samples have tested positive for West Nile virus from nine counties. This is slightly lower than last year at this time, when 65 positive mosquito samples had been reported from nine counties. No human cases or positive sentinel chickens have yet been reported in California.



Date last reviewed: July 9, 2021

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