West Nile Virus

OVERVIEWMosquito Image

West Nile virus (WNV) is the most commonly spread mosquito-borne disease in the United States. The virus is spread to people and other mammals through the bite of an infected mosquito. Mosquitoes become infected with the virus when they feed on an infected bird.  

WNV has been detected in nearly all counties in Washington state. Though it is most often found in south central Washington, it has been detected in humans, horses, birds, and mosquito samples in Walla Walla County. Cases are often detected during summer and fall months, when mosquito populations are most active. 

West Nile virus can cause serious, and even fatal, illness in humans. People who are over 60 years old or with certain medical conditions such as cancer, diabetes, high blood pressure, or who have received a transplant, are at greater risk for developing serious illness. There are no vaccines or medicines available to treat human West Nile virus infections. Those with severe illness may take weeks or months to recover. Some effects may be permanent and about 1 out of 10 people who develop severe illness die. 

WNV may also cause severe illness in horses and some species of birds such as crows, ravens, magpies, and jays. There is a vaccine available to protect horses, mules, and donkeys from West Nile virus. Contact your veterinarian for more information. 

While most people infected with WNV do not develop symptoms, some infected people develop a fever, often accompanied by flu-like symptoms. In rare cases, severe neurological complications can occur. 

  • About 8 in 10 people who become infected will not experience any symptoms. 
  • About 1 in 5 people infected will develop febrile illness (fever) which can be accompanied by other symptoms like headaches, body aches, joint pains vomiting, diarrhea, and or rash. 
  • About 1 in 150 people infected will develop severe illness, leading to neurological complications. This occurs when the virus disrupts the blood-brain barrier that protects the brain from toxins. Symptoms of severe illness can include high fever, tremors, confusion, and paralysis.
If you think you or a family member may be infected with West Nile virus, contact your healthcare provider.

The best way to prevent West Nile virus is to prevent mosquito bites. Protect yourself and your family by: 

  1. Applying an Environmental Protection Agency (EPA) registered insect repellent with one of the following active ingredients when you go outside: 
    • Deet
    • Picaradin
    • IR3535
    • Oil of Lemon Eucalyptus (OLE)
    • Para-methane-diol (PMD)
    • 2-Undecanone
2. Controlling mosquitoes around your home by:

    • Using screens on windows and doors that are in good condition. 
    • Removing any source of standing water. Mosquitoes require water to grow from eggs to biting adults. If the source cannot be emptied, cover or exchange the water at least twice a week. Common household items that can hold water and become mosquito sources include:

      - Pet water bowls

      -Animal Troughs
      -Potted plant trays
      -Buckets and other miscellaneous items
      - Roofs and gutters
      - Areas on the property where water leaks could occur
3. Avoiding outdoor activities around dawn and dusk when mosquito activity is highest. Wear long sleeved shirts and pants and apply an EPA-registered insect repellent if you must go outside.

4. Keep pools spas, and hot tubs well maintained, including the appropriate level of disinfectants such as chlorine. 


Repellent Resources
Use the EPA search tool to help you choose which propellent is right for you based on which type of arthropod you are trying to repel, and the protection time needed. For additional information, please visit the EPA Repellents webpage. 

Disclaimer per the Environmental Protection Agency:  Any products listed are for informational purposes only. Inclusion of a product listed/referenced is not an endorsement. EPA and the U.S. Government do not endorse any product or service. The information in this database was last updated in June 2019. If you believe a product not on the list should be included, use the contact us link to request that the product be added.

Additional Information
Centers for Disease Control and Prevention: West Nile virus 
Washington Department of Health: West Nile Virus 

Data and Maps
DOH West Nile Virus Activity Map 
CDC West Nile Virus Statistics & Maps
VectorSurv: Mosquito Surveillance Data 
Washington Department of Health: West Nile Virus Data 
Washington Department of Health: Zoonotic Disease Data and Reports 

West Nile in Domesticated Animals
USDA APHIS | West Nile Virus (WNV)

West Nile in Wildlife
West Nile virus | Washington Department of Fish & Wildlife
 Washington Department of Fish & Wildlife: Report Dead Birds