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

Chart showing cycle of bird to insect, insect to birdWest 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. In 2008 there were three human cases in the state, one in King County and two in Yakima County. There were also 41 horses in eastern Washington that contracted the disease. Dead birds in Thurston, Lewis, Pierce, and King counties tested positive for the virus. It is very likely that Washington will see more humans, horses, and birds with the disease in 2009.

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.

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

Reporting Dead Birds

We are currently seeking information from citizens about the location, species, and numbers of dead birds in Thurston County. Find out more about reporting procedures. ...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
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This page last updated: 04/29/17