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  • West Nile Virus Additional Information

    Background and Epidemiology

    West Nile virus (WNV) infections first emerged as a public health problem in the United States in the late 1990s. It is a mosquito-borne flavivirus in the same family as yellow fever, dengue fever and St. Louis encephalitis. Other routes of transmission in rare situations are blood transfusions, organ transplant, transplacental, breastfeeding, and percutaneous injuries of laboratory workers.

    In the United States, outbreaks occur from late spring through autumn when mosquitoes are active. Usually, WNV outbreaks are associated with bird die-offs and cases in horses and other mammals that may precede or occur simultaneously with human cases.

    In Washington State, WNV activity has historically been very low. Since surveillance began, the highest level of WNV activity occurred in 2009 when there were 38 human cases (36 were acquired in Washington State), 73 cases in horses or other mammals, and 22 dead birds tested positive. In 2016, there were 9 human cases in Washington and 24 human cases in 2015 (none in Pierce County).  Although WNV has historically been detected in dead birds in Pierce County, most WNV activity in Washington State occurs in Eastern Washington.

    Across the U.S., WNV outbreak activity varies from year to year. From 1999 to 2015, the states reporting the most cases (from 2,214 to 5,589, in order) are Illinois, Nebraska, Texas, Colorado and California. During the same time period, Washington State reported 87 cases.

    When to Suspect WNV Infection

    A person who hasn’t traveled outside of Western Washington is unlikely to have WNV. However, local presence of WNV infection in birds/animals/humans or travel to areas with WNV activity should raise suspicion in persons with unexplained meningitis or encephalitis. Recent history of transfusion, transplant or vaccination may be important. Testing for WNV or other arboviral disease such as St. Louis encephalitis should be strongly considered in:

    • Adults > 18 years with unexplained encephalitis or meningitis, particularly in summer or early fall.
    • Children < 18 years hospitalized with encephalitis.
    • Cases of acute flaccid paralysis or presumptive Guillain-Barré syndrome.

    Clinical Features

    • Most patients with WNV infections are asymptomatic.
    • Less than one percent of those infected with WNV develop severe disease.
    • Incubation period ranges from 2 to 14 days.

    Mild Infection

    • Approximately 20 percent of those infected develop a mild, self-limited illness known as West Nile fever.
    • Symptoms from mild infection generally last three to six days.
    • Individuals with this form of WNV infection do not progress to more severe disease.
    • West Nile fever is characterized by sudden onset of fever often accompanied by:
      • Anorexia.
      • Headache.
      • Nausea.
      • Lymphadenopathy.
      • Eye pain.
      • Malaise.
      • Rash.
      • Gastrointestinal symptoms.

    Severe Infection

    • Approximately one in 150 infections will result in severe neurological disease.
    • The most significant risk factor for severe disease is advanced age.
    • Encephalitis and meningitis are the most common severe clinical syndromes.
    • Additional symptoms among patients hospitalized with severe disease include:
      • Gastrointestinal symptoms.
      • Weakness.
      • Fever.
      • Change in mental status.
    • Maculopapular or mobilliform rash involving the neck, trunk, arms or legs is rare.
    • Neurological symptoms include:
      • Myelitis.
      • Acute flaccid paralysis.
      • Ataxia and extrapyramidal signs.
      • Tremor, Parksinson-like syndrome.
      • Cranial nerve abnormalities.
      • Optic neuritis.
      • Polyradiculitis.
      • Seizures.

    Diagnosis and Reporting

    Diagnostic Testing

    WNV testing for patients with encephalitis or meningitis can be obtained commercially. Positive commercial tests should be confirmed at the Washington State Department of Health Public Health Laboratories (PHL).

    • The most efficient diagnostic method is detection of IgM antibody to WNV in serum or CSF collected > eight days after illness onset using the antibody capture enzyme-linked immunosorbent assay (MAC-ELISA). Since IgM antibody does not cross the blood-brain barrier, IgM antibody in CSF strongly suggests central nervous system infection.
    • Patients recently vaccinated against or infected with related flaviviruses (e.g., yellow fever, Japanese encephalitis, dengue) may have positive (cross-reactive) WNV MAC-ELISA results.
    • Based on the clinical presentation, diagnostic testing should be obtained to rule out other conditions such as herpes encephalitis, or meningitis due to fungal, bacterial or parasitic pathogens.
    • Patients who test negative prior to the eighth day after onset should be re-tested eight days after onset of symptoms

    Reporting Suspected WNV Infection

    Please report suspected or confirmed cases of WNV encephalitis to the Tacoma-Pierce County Health Department Communicable Disease Control 24-hour reporting line, (253) 649-1413. For information during business hours, call (253) 649-1412.


    Treatment is supportive, often involving hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections for patients with severe disease.

    Additional Resources


    Contact Communicable Disease Control at (253) 649-1412.