West Nile Virus Surveillance

Overview

How to Interpret West Nile Virus Surveillance Data

The District’s West Nile virus surveillance program reports several types of positive West Nile virus results, including dead birds, mosquito samples (“pools”), sentinel chickens, and human cases.

For more information about the District’s West Nile virus and mosquito population surveillance, visit the Mosquito Surveillance page.

For real-time West Nile virus data for California, visit the California Vectorborne Disease Surveillance Maps (CalSurv) page.

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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.  

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West Nile virus update for September 2020

San Mateo County

As of October 2, 2020, there have been 172 dead birds reported in San Mateo County. Of those, 50 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 on September 22, 2020. 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.

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Mosquitoes Collected in South San Francisco Negative for West Nile Virus

Today (Oct. 1st) the District laboratory collected mosquitoes in the area where a dead crow infected with West Nile virus was found earlier this week. These mosquitoes have all tested negative for West Nile virus.

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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.

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West Nile Virus Update for July 2020

San Mateo County

As of August 4, 2020, there have been 120 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). To date, there have been no confirmed West Nile virus detections in mosquitoes or sentinel chickens in San Mateo County in 2020.

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West Nile Virus Updates – June 2020

San Mateo County

As of June 30, 2020, there have been 95 dead birds reported in San Mateo County. Of those, 29 have been suitable for testing and all 29 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.

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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.

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West Nile Virus Update – December 2019

San Mateo County

In the 2019 calendar year, the District closed out one of its lowest seasons for West Nile virus activity since the virus first appeared in San Mateo County in 2004.  No virus was detected in the county this year in any of the standard WNV surveillance indicators.

Surveillance Factor

Previous 10-year average (2009-2018)

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West Nile Virus Update – November 2019

San Mateo County

As of December 1, 2019, there have been 231 dead birds reported in San Mateo County. Of those, 57 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 2019.

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West Nile Virus Update – Ocotober 2019

San Mateo County

As of November 1, 2019, there have been 215 dead birds reported in San Mateo County. Of those, 53 have been suitable for testing and all 53 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 2019.

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West Nile Virus Update – September 2019

San Mateo County

As of October 2, 2019, there have been 191 dead birds reported in San Mateo County. Of those, 44 have been suitable for testing and all 44 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 2019.

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West Nile Virus Update – August 2019

San Mateo County

As of September 3, 2019, there have been 179 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 tested positive for West Nile virus in San Mateo County in 2019.

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West Nile Virus Update – July 2019

San Mateo County

As of August 1, 2019, there have been 155 dead birds reported in San Mateo County. Of those, 31 have been suitable for testing and all 31 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 2019.

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West Nile Virus Update – June 2019

San Mateo County

As of June 28, 2019, there have been 120 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 2019.

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West Nile Virus Update – May 2018

San Mateo County

As of June 3, 2019, there have been 86 dead birds reported in San Mateo County. Of those, sixteen have been suitable for testing and all sixteen 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 2019.

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End of Season West Nile Virus Update
October 2018

San Mateo County

 

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West Nile Virus Update for September 2018

San Mateo County

As of September 28, 2018, there have been 330 dead birds reported in San Mateo County. Of those, 103 have been suitable for testing and five have tested positive (5%) for West Nile virus (WNV). Two mosquito pools have tested positive for WNV, both from East Palo Alto during the month of August. Positive birds and pools for 2018 are summarized in the tables below. No sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.

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West Nile Virus Update
August 2018

San Mateo County

As of August 31, 2018, there have been 291 dead birds reported in San Mateo County. Of those, 91 have been suitable for testing and five have tested positive (5%) for West Nile virus (WNV). Additionally, two mosquito pools have tested positive for WNV, both from East Palo Alto. Positive birds and pools for 2018 are summarized in the tables below. No sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018, although a sentinel chicken in Santa Clara County, near the border with San Mateo County, tested positive in August.

Commands