West Nile Virus Update – May 2020

San Mateo County

As of May 31, 2020, there have been 68 dead birds reported in San Mateo County. Of those, 23 have been suitable for testing and all 23 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 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 six counties in 2020 (see map), compared to a five-year average of ten counties at this date. Statewide, 1,625 birds have been reported and 313 have been tested with six positive results (1.9%), from Napa (1), Sacramento (1), and Santa Clara (4) counties. At this time last year, one WNV positive dead bird had been reported from one county. Four mosquito pools have tested positive for WNV this year, from Los Angeles (1), Stanislaus (2), and Tulare (1) counties. No positive sentinel chickens or human cases have yet been reported in California. 

Other Arboviruses

In 2020, St. Louis Encephalitis virus (SLEV) has been detected in six mosquito samples, all from Riverside County. At this time last year, two SLEV positive mosquito pools had been reported, from one county.