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Positive West Nile Virus Mosquitoes Found in Portions of Palo Alto and Stanford

Targeted adult mosquito control treatment scheduled for Thursday, July 27, in a limited area within Palo Alto and Stanford

SANTA CLARA COUNTY, CALIF. – The County of Santa Clara Vector Control District has confirmed the presence of West Nile virus-positive mosquitoes in a small area that includes Palo Alto and Stanford (ZIP codes 94301, 94304, 94305, and 94306). Weather permitting, this area will be treated to reduce adult mosquito populations with the use of truck-mounted equipment on Thursday, July 27, starting around 10 p.m. and concluding a few hours later.

The District’s mosquito management program largely focuses on preventing mosquitoes from reaching the adult biting stage by proactively targeting immature stages of mosquitoes found in standing water. When a mosquito with West Nile Virus (WNV) is detected, however, the District takes the added step of conducting adult mosquito control treatment to reduce the mosquito population in the area, which decreases the risk of a WNV-human infection.

It is normal to see an increase in West Nile virus during the summer and early fall because mosquitoes thrive in warm weather. The District has a dedicated surveillance program to detect the presence of diseases like West Nile virus, St. Louis encephalitis and western equine encephalitis, all of which are transmitted through the bite of infected mosquitoes. The data collected through surveillance is used to predict locations that are more likely to have these disease-transmitting mosquitoes.

The District has been conducting truck-mounted treatments regularly since 2003 to successfully reduce WNV-transmitting mosquito populations.

Notice is being sent directly to the public in the treatment ZIP codes through AlertSCC and to those who subscribe to Nextdoor neighborhood networks. General notice is being provided on various social media platforms – including Facebook, Instagram and Twitter (@SCCVCD) – and to those subscribed to the District’s mosquito treatment notifications.

Vector Control staff will be available to answer any questions from the public, Monday–Friday, on the dedicated West Nile Virus Hotline at (408) 282-3114, from 7:30 a.m. to 4 p.m. Questions can also be submitted by email to [email protected]

The specific details of the operational areas are:

Treatment date: Thursday, July 27, around 10 p.m., for approximately four hours

Supervisorial Districts: District 5

Cities: Palo Alto and Stanford

Centered at: Page Mill Road and Hanover Street

Bordered by:

  • North – Alma Street, Emerson Street, Ramona Street, S. Ramona Street, and Bryant Street
  • East – Wilton Avenue, Military Way, La Para Way, Ilima Way, and Matadero Creek
  • South – Foothill Expressway and Junipero Serra Boulevard
  • West – Santa Maria Avenue, Dolores Street, Santa Ynez Street, Campus Drive, Arguello Way, Bonair Siding Road, and Churchill Avenue

ZIP Codes affected: 94301, 94304, 94305, and 94306

Interactive map:

There is no need to relocate during the treatment. Mosquito treatments pose minimal risk to people, pets, animals and the environment when applied by a licensed vector control professional following label instructions. Those who would like to take extra precautions can keep family and pets inside during the approximately four-hour treatment, with windows and doors shut. By sunrise, the insecticide will quickly break down with the sunlight. Since the District applies insecticides at ultra-low volume (ULV), individuals aren’t likely to breathe or touch anything that has enough insecticide on it to be harmful. All control materials utilized in our mosquito control program are approved by the Environmental Protection Agency for use in public areas and are widely used by vector control agencies throughout California.

For more information on the products used for this adult mosquito control treatment, including the safety data sheet, insecticide label and a list of our most frequently asked questions, visit our website at For additional information on adulticides, visit the Centers for Disease Control and Prevention’s website at For information on West Nile virus activity in California, go to

Health Effects of West Nile Virus

Since West Nile virus arrived in California in 2003, more than 7,000 people across the state have contracted the disease; nearly 400 of those cases were fatal. In 2022, there were 15 human WNV-related deaths; 2015 was a record year for fatalities in the state with 55 deaths.

WNV infection does not cause symptoms in most people; however, for some individuals it can cause fever, headache, body aches and, in severe cases, significant neurological damage or death. Individuals with certain chronic medical conditions (such as diabetes, high blood pressure, cancer and kidney disease) and the elderly are most at risk for serious complications. 

The public can assist in preventing the spread of West Nile virus by taking the following prevention measures.

On your 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 once a week 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. For more information on our mosquitofish program, visit

Outdoor Activities:

  • Limit outdoor activities during dusk and dawn – these are the times when the mosquitoes that transmit WNV are most active. 
  • If spending time outdoors, 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, always following label instructions.

Contact the County of Santa Clara 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 or submit an online service request.


 July 27, 2023

Mosquito Treatment map of Palo Alto and Stanford Area

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The County of Santa Clara government serves a diverse, multicultural population of 1.9 million residents in Santa Clara County, Calif., making it more populous than 14 states in the U.S. The County provides essential services to its residents, including public health protection, environmental stewardship, medical services through the County of Santa Clara Health System, child and adult protection services, homelessness prevention and solutions, roads, park services, libraries, emergency response to disasters, protection of minority communities and those under threat, access to a fair criminal justice system, and many other public benefits.

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