West Nile Virus


Cause West Nile virus.

Illness and treatment: About 80% of those infected are asymptomatic, around 20% have WNV fever (fever, headache, rash), and less than 1% develop WNV neuroinvasive disease (meningitis, encephalitis, paralysis). Treatment is supportive.

Sources: Many bird species are reservoirs. Mosquitoes are the vectors, transmitting the virus through bites to humans and other mammals such as horses. WNV can be transfused, so donated blood is screened and asymptomatic presumptive viremic donors are reported.

Prevention: Avoid mosquito bites by wearing appropriate clothing and using insect repellents. Make sure windows and doors are "bug tight." Maintain window screens. Eliminate breeding sites by draining standing water such as in pots or tires.

Recent Washington trends: Infected birds and horses were first detected in 2002. The first locally acquired human infections were reported in 2006. In 2009, Washington had the highest number of cases to date with 38 cases and 2 presumptive viremic donors. Of these cases, 36 were known to be endemically acquired within Washington.

For current or historical information on WNV in Washington State, please see the DOH West Nile Virus hompage.

Arboviral (arthropod-borne viral) Diseases, including West Nile virus (WNV) infections, must be reported to local health jurisdictions in Washington State.

Purpose of Reporting and Surveillance

  • To identify areas in which West Nile virus (WNV) is being transmitted
  • To target public education about reducing mosquito habitats and avoiding mosquito bites
  • To provide information for mosquito control and environmental health initiatives
  • To identify periods of time when WNV poses a significant risk to the blood supply
  • To identify new routes of exposure

Legal Reporting Requirements

  • Health care providers and Health care facilities: notifiable to local health jurisdiction within 3 business days
  • Laboratories: any positive result excluding IgG notifiable to local health jurisdiction within 2 business days; submission on request – specimen associated with any positive result excluding IgG, within 2 business days.
  • Veterinarians: animal cases notifiable to Washington State Department of Agriculture (see: https://app.leg.wa.gov/wac/default.aspx?cite=16-70)
  • Local health jurisdictions: notifiable to Washington State Department of Health (DOH) Communicable Disease Epidemiology (CDE) within 7 days of case investigation completion or summary information required within 21 days