Environmental Health
Rodents, Bats, Insects & Other Vectors
West Nile Virus (WNV)
  west nile virus (WNV)  

Chart showing cycle of bird to insect, insect to bird

West Nile virus (WNV) is a serious illness that can affect people, horses, certain types of birds, and other animals. West Nile virus is transmitted by mosquitoes, however, less than 1% of mosquitoes are infected with the virus. Of the people who get infected, less than 1% develop a severe illness, which can sometimes be fatal.

In 1999, WNV first appeared in the U.S. in New York. Since that time, it has spread rapidly throughout the country. In 2002, the virus was found for the first time in birds and horses in Washington. Since 2009, almost all of the West Nile Virus detections have been in Eastern Washington, particularly Benton, Franklin, Grant, and Yakima Counties. Pierce County mosquito samples in 2018 were positive for WNV and the area is being monitored to see how wide-spread WNV-positive mosquitoes are. This effort includes mosquito monitoring in Thurston County.

West Nile virus is transmitted to humans through mosquito bites. Mosquitoes become infected when they feed on infected birds that have high levels of WNV in their blood. Infected mosquitoes can then transmit WNV when they feed on humans or other animals. There have been about a dozen human cases in Washington each year since 2009, most acquired in Eastern Washington.

Preventing WNV Infection

How to avoid mosquito bites, what are the most effective mosquito repellents, and how to reduce places where mosquitoes live and breed around your home. ...more

Dead Birds and West Nile Virus

We no longer track dead birds. ...more


Some of the most frequently asked questions about West Nile virus. ...more

Mosquito Repellent Fact Sheet

Information on choosing repellents, their effectiveness, and using them safely. ...more

Info for Veterinarians & Health Care Providers

Clinical and reporting information for veterinarians and health care providers from the WA Dept of Health.

Helpful Links

  • Washington Department of Health - Includes the latest WNV information in Washington, including basics about the disease, surveillance efforts in Washington State, related health issues, and educational materials.
  • Centers for Disease Control and Prevention - Includes West Nile virus basics, maps of WNV activity in the U.S., preventative measures, and other resources. NOTE: This information is offered in several languages.
  • National Wildlife Health Center - West Nile virus information from the U.S. Geological Survey with a focus on U.S. wildlife.
Hot Topics
Popular Pages & Links
  • Dead Bird Reports
    (no pickups)
  • West Nile Virus Questions
This page last updated: