West Nile Virus


West Nile Virus (WNV) is a mosquito-borne disease that was first detected in the West Nile District of Uganda. Transmitted by mosquito bites, WNV affects humans, horses, some birds, and squirrels. 

In humans, it causes mild to severe flu-like symptoms such as fever, headache, body ache, and in severe cases, significant neurological symptoms or even death. Most people who contract the virus will have few or no symptoms and will recover completely. About 1 in 150 people who contract the disease may develop serious complications.  The elderly and those with compromised immune systems  are more at risk of such complications. There is currently no vaccine or specific treatment available. 

West Nile Virus in the United States

In the United States, the first WNV outbreak was reported during 1999 in New York City. The virus spread quickly across the nation during the subsequent 5 years. It was first reported in California during 2002 and arrived in Santa Clara County in 2004.

​​What You Should Know 

There is no cure for WNV. Vaccines for humans are not available. People can protect themselves from WNV by taking precautions to avoid mosquito bites, and reducing or eliminating standing water on their property. Please visit our mosquito page for more information on mosquito prevention.

To help us detect WNV activity, report all dead birds directly to the DEAD BIRD HOTLINE at 877-WNV-BIRD (877-968-2473) or online at westnile.ca.gov.

Our District scientist continuously evaluate their procedures and materials to ensure that we are using the most effective, environmentally friendly, and safest treatment methods.  This is achieved by maintaining active memberships in State, National, and Global associations and applying relevant changes to our program.  A few examples include the Mosquito and Vector Control Association of California, the American Mosquito Association, and the Society of Vector Ecology.  These associations produce journals, provide presentations at conferences, and deliver peer reviewed scientific research that showcase the most modern and innovative methods, often focusing on environmentally friendly alternatives.  Information from these sources help the District select the safest and best methods for mosquito control.

In addition, we conduct our own internal research with District staff. Recent efforts include:

  1. Conducting laboratory and field trials to look at two naturally derived treatments, Essentria and Merus™ 3.0, to possibly reduce the use of Zenivex® E4, the District's primary adulticide treatment to kill adult mosqui​toes.
  2. Evaluating the build-up of pesticide resistance by conducting laboratory and field tests for adulticides, Merus™ 3.0 and Zenivex® E4.
  3. Evaluating pesticide resistance levels in various locations throughout the County in an effort to determine the most effective alternative adult mosquito control material.

Every time a new adulticide enters the market, that has been approved by the Federal and State Environmental Protection Agencies, we do a thorough evaluation to determine if it is an appropriate or valuable treatment for use in Santa Clara County.

Additional Information



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