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Arboviral Encephalitis (arthropod-borne)


Arthropod-borne viruses, i.e., arboviruses, are viruses that are maintained in nature through biological transmission between susceptible vertebrate hosts by blood feeding arthropods (mosquitoes, psychodids, ceratopogonids, and ticks). There are four main virus agents of encephalitis in the United States: eastern equine encephalitis (EEE), western equine encephalitis (WEE), St. Louis encephalitis (SLE) and La Crosse (LAC) encephalitis, all of which are transmitted by mosquitoes. Another virus, Powassan, is a minor cause of encephalitis in the northern United States, and is transmitted by ticks. Most cases of arboviral encephalitis occur from June through September, when arthropods are most active.

The majority of human infections are asymptomatic or may result in a nonspecific flu-like syndrome. Onset may be insidious or sudden with fever, headache, myalgias, malaise and occasionally prostration. Infection may, however, lead to encephalitis, with a fatal outcome or permanent neurologic sequelae. Fortunately, only a small proportion of infected persons progress to frank encephalitis.



Disease Reporting

What is required?

Health care providers and clinical laboratories are required by law to report cases and suspect cases of arboviral encephalitis to local health departments within one working day.

Disease reporting form (pdf) for health-care practitioners

See disease reporting page for information on how to report and for telephone numbers of local health departments.

For county health departments:
CDC Rickettsial-Tickborne Disease form (pdf)

For reportable diseases lacking Oregon-specific investigative guidelines or case report forms, please contact the epidemiologist on call for assistance at 971-673-1111.


See Also
The CDC fact sheet answers some common questions about these viruses.