WNV Surveillance
November 2, 2017

The West Nile Virus season has wrapped up for the year, with the final chicken bleeding on October 30. Although one bird tested positive in January, the District did not detect any evidence of West Nile Virus (WNV), Saint Louis encephalitis, or Western Equine encephalitis in San Mateo County during the active summer season. The surveillance efforts are summarized below:

San Mateo County

As of October 30, 2017, there have been 360 dead birds reported in San Mateo County. Of those, 61 have been suitable for testing and 1 has tested positive (2%) for WNV. Ten dead squirrels and four mosquito pools have been tested for WNV in San Mateo County thus far this year, and none of them have tested positive.

The District will continue limited testing of dead birds during the fall and winter seasons. During these months, residents can report dead birds through the California Department of Public Health West Nile Virus website online at Those who do not have web access can call the District directly at (650) 344-8592. Specimens that appear to have been dead for less than 24 hours and are in good condition may be tested for WNV. 


WNV has been detected in 47 counties in 2017 (see map), compared to a five year average of 40 counties at this date. 403 human case of West Nile Virus has been reported in California this year, from 23 counties, and there have been 24 fatalities. Statewide, 7,212 birds have been reported and 1,884 have been tested with 481 positive results (26%).  3,325 mosquito pools have tested positive for West Nile Virus, compared to a five-year average of 3,055 positive mosquito pools at this time. 297 positive sentinel chickens has been reported from eighteen counties.