Arboviral Disease

Cause: Various viruses transmitted by arthropods. Arthropod-borne viral (arboviral) diseases include West Nile virus disease and yellow fever (both discussed separately below), Chikungunya virus disease, Zika virus disease, Colorado tick fever, dengue fever, eastern and western equine encephalitis, St. Louis encephalitis, and Japanese encephalitis.

Illness and treatment: There are 4 main clinical forms: central nervous system (CNS) illnesses; fevers of short duration with or without rash; hemorrhagic fevers; and polyarthritis and rash with or without fevers. Treatment is supportive.

Sources: Transmission is most commonly by the bite of arthropods (e.g., mosquitoes, sandflies, ticks). Rare transmission occurs through blood transfusions, sexual contact, and mother to fetus.

Prevention: Avoid arthropod bites by wearing appropriate clothing and using insect repellents. If traveling to risk areas, consult with a travel clinic or the CDC Travelers' Health website regarding additional measures, including vaccination for Japanese encephalitis or yellow fever. Pregnant women should not travel to areas currently experiencing Zika outbreaks.

Recent Washington trends: One to 23 cases of travel-associated dengue are reported each year. Prior to 2014, 0-3 travel-associated chikungunya cases were reported annually. In late 2013, the first local transmission of chikungunya virus was identified in the Caribbean; widespread local transmission was then identified in many countries in Central and South America. In 2014, 15 cases of chikungunya virus infection were identified in travelers returning from the Caribbean and in 2015, 40 travel-associated cases were reported in Washington. Since 2016, chikungunya cases have decreased to pre-outbreak levels. In 2015, a Zika outbreak began in Brazil and quickly spread to many countries in South and Central America, the Caribbean, and the South Pacific. Increased testing for Zika virus due to concerns about birth defects likely led to increased detections of chikungunya and dengue. During 2016, 68 cases of travel-associated Zika virus disease were reported in Washington residents, decreasing to 16 cases of Zika virus disease in 2017;  since 2017 and no Zika virus disease cases have been reported.

Rare reports of other travel-associated arboviral diseases include Colorado tick fever and Japanese encephalitis in 2008, and St. Louis encephalitis and Toscana virus in 2009.Other than West Nile virus, the last reported human arboviral infection acquired in Washington State was a single case of St. Louis encephalitis in 2023; prior to that, western equine encephalitis was reported in 1988.

Purpose of Reporting and Surveillance

  • To distinguish arboviral infections acquired locally from those related to travel
  • To better understand the epidemiology of these infections in Washington State in order to target education and control measures
  • To identify emerging arboviral infections in Washington

Legal Reporting Requirements

  • Health care providers and Health care facilities: notifiable to local health jurisdiction within 3 business days
  • Laboratories: Arboviral (eastern and western equine encephalitis, dengue, St. Louis encephalitis, La Crosse encephalitis, Japanese encephalitis, Powassan, California serogroup, chikungunya, Zika) any positive result excluding IgG notifiable to local health jurisdiction within 2 business days; submission on request – specimen associated with positive result excluding IgG, within 2 business days.
  • Veterinarians: animal cases notifiable to Washington State Department of Agriculture (see:
  • 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.