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First Mosquito Sample Tests Positive for West Nile Virus in 2021

Cotati, CA. July 22, 2021 - The Marin/Sonoma Mosquito and Vector Control District confirmed today that a sample of adult mosquitoes collected in traps, have tested positive for West Nile virus (WNV).  This is the first mosquito sample to test positive for WNV in Marin County or Sonoma County in 2021. The mosquito sample was collected from the vicinity of the Ellis Creek Water Recycling Facility in the City of Petaluma.

 “The detection of WNV in the local mosquito populations serves as a reminder that the virus is endemic to Marin and Sonoma counties,” stated Erik Hawk, Assistant Manager. “Taking personal protection measures when engaging in outdoor activities is imperative to reducing the risk of West Nile virus transmission.”

 District staff continues to trap, test, and monitor the distribution and abundance of adult mosquitoes in the vicinity of the Ellis Creek Water Recycling Facility. Vector Control Technicians are looking for new sources of mosquito production in the area, inspecting known sources, and conducting mosquito control as necessary.

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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