West Nile virus update for October 2020

San Mateo County

As of October 31, 2020, there have been 204 dead birds reported in San Mateo County. Of those, 62 have been suitable for testing and one has tested positive for West Nile virus (WNV). The WNV positive bird was an American crow collected from South San Francisco in September. This has been the only detection of WNV in San Mateo County in 2020. No mosquito samples or sentinel chickens have been confirmed positive for WNV in San Mateo County this year.  

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


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.

Other Arboviruses

In 2020, three human cases of St. Louis encephalitis virus (SLEV) have been reported in California, from Madera, Fresno, and San Joaquin Counties. The SLEV pathogen has been detected in 2020 in 509 mosquito samples, all from counties in central or southern California.