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First Mosquito Sample Tests Positive for West Nile Virus in 2023

Cotati, CA.  September 12, 2023The Marin/Sonoma Mosquito and Vector Control District confirmed today that a sample of adult mosquitoes collected in a trap has tested positive for West Nile virus (WNV). This is the first mosquito sample to test positive for WNV within Marin and Sonoma counties in 2023. The mosquito sample was collected in the vicinity of Asti Road and Citrus Fair Drive in the City of Cloverdale.

In addition, two birds tested positive for West Nile virus in Marin County. The birds were collected from the following locations within the City of San Rafael: one (1) American crow near Blackstone Drive and Heatherstone Drive, and one (1) American crow near Racquet Club Drive and Longwood Drive.

 "The presence of West Nile virus in our local mosquito populations is a stark reminder that this virus is a part of our ecosystem in Marin and Sonoma counties," emphasized Phil Smith, District Manager. "To minimize the risk of West Nile virus transmission, it's essential to adopt personal protection measures while enjoying outdoor activities."

Vector Control Technicians are looking for new sources of mosquito production in the areas, inspecting known sources, and conducting mosquito control as necessary. District staff will continue to trap, test, and monitor the distribution and abundance of adult mosquitoes in the areas of concern.

Residents are urged to continue to report mosquito problems in their area and to reduce mosquito production sites in their yards by removing standing water. Most importantly, residents should use personal protection measures against mosquito bites. Wearing long sleeved shirts, long pants and applying an effective repellent to exposed skin will help reduce the risk of WNV transmission from an infected mosquito.

Simple ways to help reduce exposure to mosquitoes and lessen the risk of WNV:

  • Contact the Marin/Sonoma Mosquito & Vector Control District (707-285-2200) if you have a mosquito-related problem that you cannot resolve on your own. 
  • Check and maintain all window and door screens.
  • Continue to report dead birds to the West Nile virus hotline at 1-877-WNV-BIRD or online.
  • Wear mosquito repellent when outdoors at dusk and dawn. Use a repellent containing one of the following active ingredients: DEET, Picaridin, oil of lemon eucalyptus, or IR3535. 
  • Avoid being outdoors during peak mosquito activity (dawn and dusk).

With West Nile virus being active in other parts of California, District officials recommend residents keep abreast of virus activity when traveling to other parts of the state. More information about West Nile virus in California can be found at


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