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August 2021 Newsletter

Dead Bird Surveillance

A black bird sits on a stick facing the camera with its mouth open like it is squawking at the camera

Did you know that dead birds can indicate local West Nile virus activity?

Birds become infected with West Nile virus when they are bitten by a mosquito carrying the virus. Mosquitoes that bite birds infected with West Nile virus become infected and can pass the infection to humans, birds, and other animals. Birds from the Corvidae family, like crows, ravens, and jays, are susceptible to West Nile virus and may die.  Other birds, like chickens, may be infected but do not become sick or die from West Nile virus.

Please help us by reporting dead birds online at or by calling 1-877-WNV-BIRD


2021 West Nile virus data

Map of California with county outlines. Counties are colored blue (indicates WNV activity without human cases), green (WNV activity with human cases), or white (no WNV activity). San Mateo County is white (no WNV activity as of 8/5/21).
Image from the California Department of Public Health (

As of August 5, 2021, there have been 205 dead birds reported in San Mateo County. Of those, 61 have been suitable for testing and all have tested negative for West Nile virus (WNV). To date, there have been no West Nile virus detections in mosquitoes or sentinel chickens in San Mateo County in 2021.

Statewide, there have been 8 human cases of WNV in 2021 compared to a five-year average of 8.4 cases at this date. Throughout California, 3,538 dead birds have been reported and 1,069 have been tested with 109 (10%) dead birds positive for WNV. Additionally, 770 mosquito samples have tested positive for WNV compared to a five-year average of 1,166 at this date. Ten sentinel chickens have tested positive, which is also below the five year average of 30.6.

2023 Mosquito Surveillance

Culex erythrothorax, a mosquito that breeds in lakes and ponds with tules, was the most frequently collected mosquito in July. The high number of tule mosquitoes this month reflects a large, seasonal emergence of this species in Pacifica. Adults of this species are usually present from around April through October. Seasonal helicopter treatments for this mosquito began in July, which modestly reduced total abundance since June, but counts remain high. Culex pipiens, the northern house mosquito and typically the most abundant mosquito in San Mateo County, increased in numbers during July, but abundance remains below average for this time of year, likely because of dry weather and cool nighttime temperatures in San Mateo County.

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May contain: chart and pie chart

2023 Service Requests

The total number of service requests is close to average for July (527 compared to an average of 540). Yellowjacket and other wasp requests increased dramatically for the second month in a row, from 34 in May to 174 in June to 367 in July. Yellowjacket and wasp requests are common in the summer and will likely remain numerous until the weather cools in autumn. The number of mosquito service requests is lower than average, likely because of a low abundance of Culex pipiens, which usually prompts the most mosquito requests. Dead bird requests have increased as well, from nine in June to sixteen in July, as West Nile virus has become more seasonally active in California. Fortunately, West Nile virus has not been detected in San Mateo County or any neighboring counties this year through the month of July. The category of “other” included a variety of request types, such as bumblebees, fleas, cockroaches, flies, mites, and ticks.

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