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Arboviruses cause systemic illness, often with central nervous system involvement. They are transmitted to humans by arthropods, including certain species of ticks and mosquitoes that acquire the virus while feeding on infected wild birds and small mammals. Western equine encephalitis (WEE), St. Louis encephalitis (SLE), and West Nile virus (WNV) are examples of mosquito-borne arboviral diseases that have occurred in Washington state.

International travelers to certain countries (particularly tropical areas) are at risk of Japanese encephalitis, yellow fever, dengue fever, chikungunya, and Zika virus, which emerged in many Central and South American countries in 2015.

Arboviruses are typically not spread from person to person, but in rare cases WNV has been spread through blood transfusions, organ transplants, breastfeeding, and perinatally. Zika virus can be transmitted through sexual contact, and during pregnancy and delivery from mother to baby.


Resources for the general public

Resources for health care professionals


Purpose of surveillance:

  • To identify outbreaks and monitor trends in illness attributed to arboviruses, particularly arboviral encephalitis
  • To detect and characterize the emergence and features of West Nile Virus (WNV) in King County
  • To guide disease investigation and control activities to prevent human infections
  • To facilitate appropriate diagnostic testing

Yersiniosis case data

Local epidemiology:

No confirmed cases of Zika virus were reported among King County residents in 2015.

Chikungunya acquired by travelers was the most commonly reported human arboviral infection in King County in 2015, surpassing dengue fever. Thirteen cases of chikungunya were reported in persons who had traveled to Central America (Costa Rica, El Salvador, Nicaragua), Mexico, Asia (India), Polynesia, and South Pacific. By comparison, seven cases of chikungunya were reported in 2014.
Seven cases of dengue fever were reported in 2015, with travel to the Caribbean (Dominican Republic), the Middle East (Israel), Asia (India), and Southeast Asia (Thailand).

The last reported case of WNV occurred in 2014 with exposure likely outside Washington state.

Since arboviral infections became reportable in Washington state in 2006, each year between zero and 14 dengue virus infections have been reported, a few travel-associated Chikungunya fever cases, and rare reports of other arboviral diseases. The first cases of human WNV infection acquired in Washington state were reported in 2006. Since that time only seven King County cases of human WNV infection have been reported, none of which were acquired in King County.