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
May 2018

San Mateo County

As of May 25, 2018, there have been 115 dead birds reported in San Mateo County. Of those, 25 have been suitable for testing and two have tested positive (8%) for West Nile virus (WNV). No mosquito pools or sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.

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West Nile Virus Surveillance Update for April 27, 2018

San Mateo County

As of April 27, 2018, there have been 46 dead birds reported in San Mateo County. Of those, 13 have been suitable for testing and two have tested positive (15%) for West Nile virus (WNV). San Mateo and Santa Clara counties are the only counties with positive WNV dead birds (4) thus far this year. No mosquito pools or sentinel chickens have tested positive for West Nile Virus in San Mateo County in 2018.

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Mosquitoes Collected in South San Francisco Not Infected with West Nile Virus

On April 2nd, a dead American crow collected in South San Francisco tested positive for West Nile virus. This is the second dead bird positive for West Nile virus found in San Mateo County in 2018. The first was collected in February in Foster City.

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Dead Crow Positive for WNV Collected in South San Francisco

On April 2nd, a dead American crow collected in South San Francisco tested positive for West Nile virus. This is the second dead bird positive for West Nile virus found in San Mateo County in 2018. The first was collected in February in Foster City.

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Dead Crow Positive for WNV Collected in Foster City

UPDATE: Feb. 7th, 2018

Follow-up mosquito trapping in the area where the bird was found did NOT detect any mosquitoes infected with WNV.

On Feb. 5th, a dead American crow collected in Foster City tested positive for West Nile virus. This is the first sign of the virus in San Mateo County in 2018.

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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

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WNV Surveillance
October 5, 2017

San Mateo County

As of September 28, 2017, there have been 320 dead birds reported in San Mateo County. Of those, 56 have been suitable for testing and 1 has tested positive (2%) for West Nile Virus (WNV). Nine dead squirrels and four mosquito pools have been tested for West Nile Virus in San Mateo County thus far this year, and none of them have tested positive.

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WNV Surveillance
September 7, 2017

San Mateo County

As of August 25, 2017, there have been 286 dead birds reported in San Mateo County. Of those, 52 have been suitable for testing and 1 has tested positive (2%) for West Nile Virus (WNV). No dead squirrels or mosquito pools have been tested for West Nile Virus in San Mateo County thus far this year.

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West Nile Virus Surveillance
July 7, 2017

San Mateo County

As of June 630, 2017, there have been 215 dead birds reported in San Mateo County. Of those, 45 have been suitable for testing and 1 has tested positive (2%) for West Nile Virus (WNV). No dead squirrels or mosquito pools have been tested for West Nile Virus in San Mateo County thus far this year.

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West Nile Virus Surveillance Update
June 7, 2017

San Mateo County

As of June 6, 2017, there have been 134 dead birds reported in San Mateo County. Of those, 21 have been suitable for testing and 1 has tested positive (5%) for West Nile Virus (WNV). No dead squirrels or mosquito pools have been tested for West Nile Virus in San Mateo County thus far this year.

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West Nile Virus Surveillance Update
April 28, 2017

San Mateo County

As of April 28, 2017, there have been 56 dead birds reported in San Mateo County. Of those, 15 have been suitable for testing and 1 has tested positive (7%) for West Nile Virus (WNV). No dead squirrels or mosquito pools have been tested for West Nile Virus in San Mateo County thus far this year.

Post Theresa Shelton

First Positive Bird of 2017 in San Mateo County

It is unusual to detect West Nile Virus in dead birds during the winter months.  West Nile Virus season begins mid-April, with most activity in the summer.  However, residents can report some species of dead birds (crows, ravens, scrub-jays, finches, sparrows, hawks and owls) year-round online at westnile.ca.gov and the District laboratory will test fresh specimens without signs of trauma (roadkill or partially decomposed specimens cannot be tested).The first West Nile Virus positive bird of 2017 in San Mateo County was detected in Redwood City.

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West Nile Virus Dead Bird Hotline

The West Nile Virus Dead Bird Hotline, operated by the California Department of Public Health (CDPH), will not be answered by live operators after October 14, 2016 until next spring.  However, residents can still call the phone number 1-877-WNV-BIRD and leave a message or fill out an online form at westnile.ca.gov to report dead birds.  During the winter season, the District will continue to collect and test suitable birds of the following groups:  corvids, finches, sparrows, owls, and hawks.

Risk Assessment provided by California Department of Public Health
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WNV Risk Assessment
San Mateo County

The California Department of Public Health generates a risk assessment level ranging from 1-5 for West Nile Virus (WNV).  The risk level is determined by analyzing a combination of data on mosquitoes and infection rates gathered by the District, weather patterns and the state WNV hotline.  The risk levels are explained as:                

Risk Rating 1.0—2.5 Normal Season, “No Alert Level”

Regular district operations

Commands