West Nile Virus Update for August 2020

San Mateo County

As of August 31, 2020, there have been 140 dead birds reported in San Mateo County. Of those, 38 have been suitable for testing and all 38 have tested negative for West Nile virus (WNV). No mosquito pools or sentinel chickens have been confirmed positive for West Nile virus in San Mateo County in 2020.

The District asks residents to report dead birds. Specimens that appear to have been dead for less than 24 hours and are in good condition will be tested for WNV.  Residents should contact the state WNV hotline at 877-WNV-BIRD (968-2473). Reports can also be made online at


WNV has been detected in 29 counties in 2020, and ten counties have had human cases (see map). There have been 35 human cases of West Nile virus, including two fatalities. Statewide, 3,820 birds have been reported and 1,042 have been tested with 142 positive results (14%). At this time last year, 91 WNV positive dead birds had been reported; the five-year average at this time of year is 516. Mosquito pools are testing positive below average this year, with a total of 1,420 positive pools in 2020 compared to a five-year average of 2,354. Sentinel chickens are also testing positive below average, with 41 positive chickens in 2020 compared to a five-year average of 143.

Other Arboviruses

In 2020, St. Louis Encephalitis virus (SLEV) has been detected in 268 mosquito samples, from eight counties: Riverside (131), Imperial (9), Fresno (101), Kings (5), Madera (2), San Joaquin (1), Tulare (7) and Kern (12). At this time last year, 178 SLEV positive mosquito pools had been reported, from seven counties.