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A Closer Look: MSMVCD’s Mosquito-Borne Virus Surveillance Program

The Marin/Sonoma Mosquito and Vector Control District (the District) maintains a multifaceted surveillance program for viruses transmitted by mosquitoes, including West Nile virus, St. Louis encephalitis virus, and western equine encephalitis virus. Both active and passive surveillance techniques are used to detect and measure the density of mosquito populations and estimate the intensity of virus transmission in the region. This information is used to predict areas of higher disease risk and direct critical vector control interventions to protect human health effectively and efficiently.

Since 2014, the District has expanded its surveillance efforts to look for invasive Aedes mosquito species, which have been moving closer to the Bay Area. Fortunately, as of July 2021, no invasive Aedes mosquitoes have been found in Marin or Sonoma counties.

When travel-related cases of chikungunya, dengue or Zika viruses are found locally, the District sets mosquito traps around these cases to check for the adult Aedes mosquitoes that could transmit these diseases.

West Nile Virus Activity 2021

As of July 26, 2021, mosquitoes infected with West Nile virus have been collected from Petaluma, and a dead bird tested positive for the virus from Santa Rosa.

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When a dead bird or mosquito sample tests positive for West Nile virus, District staff focuses extra effort in the immediate area. Staff trap, test, and monitor the number of adult mosquitoes in the area, performing control operations as necessary. Additionally, vector control technicians inspect all known mosquito breeding locations and search for new sources.

Help us detect West Nile virus by reporting dead birds.

Did you know that dead birds can indicate the presence of West Nile virus in a given area?

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Birds become infected with West Nile virus when they are bitten by a mosquito carrying the virus. Uninfected mosquitoes that bite birds infected with West Nile virus subsequently become infected and can then pass the virus to humans and other animals. Birds from the family Corvidae such as American crows, common ravens, California scrub-jays, and Steller’s jays are more susceptible to West Nile virus than other bird species.

The detection of West Nile virus in dead birds can be a useful surveillance tool for monitoring the virus, may be helpful in locating problematic sources of mosquito production, and can minimize the potential for public health issues.

Please help us detect West Nile virus activity by reporting dead birds to the California Department of Public Health’s West Nile Virus Call Center. You can report dead birds at or by calling 1-877-WNV-BIRD.

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