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West Nile virus detected in Redwood Shores, Palo Alto and Stanford

This is the first recorded instance of West Nile Virus (WNV) activity for 2023 in San Mateo County.
Adult female Cx. quinquefasciatus mosquito before a blood meal. Courtesy CDC

West Nile virus has been detected in Redwood Shores, following the discovery of an infected American Crow, according to a report from the San Mateo County Mosquito and Vector Control District.

This is the first recorded instance of West Nile Virus (WNV) activity for 2023 in San Mateo County.

The American Crow, found dead in the Redwood Shores vicinity, tested positive for WNV. This bird is one of several species, including common ravens and American robins, that can carry WNV. Transmission of the virus occurs when a mosquito bites an infected bird and subsequently bites a human or another animal.

Importantly, WNV does not spread via coughing, sneezing, or person-to-person contact. Reports of dead birds often mark the first detection of WNV in an area. The District urges residents to report any dead birds via the phone number 877-WNV-BIRD (877-968-2473) or online at

In Santa Clara County, the Santa Clara County Vector Control District will spray parts of Palo Alto and Stanford with mosquito treatment to suppress West Nile virus activity after some mosquitoes tested positive, according to a district advisory.

Mosquitoes infected with West Nile virus have been detected in a small area within these cities. Spraying to reduce adult mosquito populations will occur on Thursday, July 27, starting at about 10 p.m. and will last approximately four hours, according to the district.

The treatment will use truck-mounted equipment to spray in ZIP codes 94301, 94304, 94305, and 94306. An interactive map of the streets and neighborhoods can be found here.

The treatment area is bordered by:

• Northeast – Alma Street, Emerson Street, Ramona Street

• Southeast – Wilton Avenue, Military Way, La Para Avenue, Ilima Way, Matadero Creek

• Southwest – Foothill Expressway, Junipero Serra Boulevard

• Northwest – Santa Ynez Street, Campus Drive, Arguello Way, Bonair Siding Road, Churchill Avenue.

Residents don't need to relocate during the treatment, but anyone who wants to minimize exposure for themselves, their families or their pets should close windows and remain inside during the spraying period, according to the district.

All of the treatment materials are approved by the Environmental Protection Agency for use in public areas and are widely used by vector control agencies throughout California, the district stated. The product, Merus 2.0, has pyrethrins, a botanical insecticide, as the active ingredient and contains petroleum distillate, which can also cause vomiting if swallowed.

Merus 2.0 is known to be toxic to aquatic organisms, including fish and aquatic invertebrates, and they can be affected if droplets enter ponds or runoff. It is also highly toxic to bees exposed to treatment on blooming crops or weeds, according to the Environmental Protection Agency.

More information on the products used for this mosquito control treatment can be found at Additional information is available at the Centers for Disease Control and Prevention’s website

The district’s mosquito management program usually focuses on preventing mosquitoes from reaching the adult biting stage by targeting immature stages of mosquitoes that are found in standing water.

But adult mosquitoes are treated to reduce the mosquito population in areas where West Nile virus is detected. The county surveillance program detects the presence of diseases like West Nile virus, St. Louis encephalitis, and western equine encephalitis, which are transmitted through infected mosquito bites.

Vector control staff will be available to answer questions from the public, Monday-Friday, 7:30 a.m.-3:30 p.m. on the dedicated West Nile Virus Hotline at 408-282-3114. Questions can also be submitted by email to

What to do about mosquitoes

Here is a list of ways to reduce exposure to mosquitoes and diseases:

On property:

• Inspect for standing water on a weekly basis.

• Drain or turn over anything that can hold water, such as flowerpots, planter bases, pet dishes, buckets, and old tires.

• Clean items like bird baths and pet bowls weekly to remove mosquito eggs.

• Clear debris from rain gutters on a regular basis to allow water to flow.

• Properly screen rain barrels, cisterns and irrigation drains to prevent mosquito access.

• Fix leaky water faucets and broken sprinkler heads and avoid overwatering lawns and plants.

• Ensure window and door screens are in good condition with no holes or tears and are tight-fitting.

• Ensure swimming pool water level is adequate for proper circulation and filtration.

• Free mosquitofish can be requested online at for placement in neglected pools/spas, ornamental ponds, water troughs and other artificial bodies of water. Information on the mosquitofish program can be found at

Outdoor activities:

• Limit outdoor activities during dusk and dawn.

• Dress in long-sleeve shirts and long pants, preferably in light colors; mosquitoes are mostly attracted to dark colors.

• Apply insect repellent that contains DEET, IR3535 or oil of lemon eucalyptus.

• Contact the Vector Control District if you are being bothered by mosquitoes or know of a potential mosquito-breeding source.

• For free assistance with mosquito control or other vectors, residents can contact the district office at 408-918-4770, email, or submit an online service request.

Redwood City Pulse Editor Michelle Iracheta contributed to this report. 


About the Author: Sue Dremann

Sue Dremann is a veteran journalist who joined the Palo Alto Weekly in 2001. She is a breaking news and general assignment reporter who also covers the regional environmental, health and crime beats.
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