Skip to main content
King County logo
  • West Nile Virus (WNV) is a flavivirus related to the Japanese encephalitis and St. Louis encephalitis viruses.
  • WNV can infect humans, horses, birds, and other vertebrates.
  • WNV is transmitted by the bite of an infected mosquito.
  • It is not transmitted by person-to-person contact.
  • Blood transfusions and organ transplantation have been identified as routes of transmission to humans; single cases of transplacental and breast milk transmission have also been reported.
  • Most people with WNV have mild symptoms or are asymptomatic.
  • Approximately 20% of infected persons develop a mild form of the disease, called West Nile Fever. Symptoms of West Nile Fever may include fever, malaise, anorexia, nausea, vomiting, myalgia, eye pain, rash, and swollen lymph glands.
  • One in 150 infections results in the more severe neuroinvasive forms of the disease, including meningitis and encephalitis. Symptoms of neuroinvasive disease may include: fever, headache, neck stiffness, gastrointestinal symptoms, disorientation, cranial nerve abnormalities, ataxia, coma, tremors, convulsions, muscle weakness, and paralysis.
  • Approximately 3-15% of patients hospitalized with West Nile neuroinvasive disease die from the disease. The risk of mortality is highest in elderly patients.
  • Consider WNV in the differential diagnosis of all patients with meningitis and/or encephalitis of unknown etiology during mosquito season, particularly in elderly patients presenting with weakness or acute flaccid paralysis or presumed Guillain-Barré syndrome.
  • The best way to diagnose WNV is to test for IgM antibody in serum or CSF.
    • Serum should be collected 8-10 days after illness onset. Negative results on any specimen obtained less than 8 days after onset of illness are inconclusive and require follow up with a convalescent serum specimen obtained at least 2 weeks after the first specimen.
    • CSF should be collected within 8 days of illness onset. IgM may appear in CSF earlier than in serum. Because IgM does not cross the blood brain barrier, its presence in CSF indicates neuroinvasive disease.
    • IgM antibody can persist for more than several months and may be indicative of a past infection.
  • Public Health Seattle-King County will facilitate testing at the Washington Public Health Laboratory (PHL) for patients who fill the following criteria:
    • Patients with suspected WNV neuroinvasive disease (fever and change in mental status, cerebrospinal fluid [CSF] pleocytosis, or other acute central or peripheral neurologic dysfunction) when there is no other likely diagnosis
    • Pregnant or breastfeeding women symptomatic with suspected WNV infection and their neonates or breastfeeding infants
    • Recent blood, tissue, or organ donors or recipients suspected to have WNV infection
    • Early in an outbreak, persons with commercial laboratory evidence of WNV infection to confirm the diagnosis (until WNV disease is established in Washington State)
  • For those patients who do not meet state lab testing criteria, testing is available at commercial laboratories or the Public Health Seattle-King County Laboratory on a fee-for-service basis. The PHSKC laboratory offers inexpensive serum IgM testing; providers may call 206-731-8950 to arrange testing.
  • Both serum and CSF specimens should be refrigerated and transported cold.
  • Because PCR lacks sensitivity, and immunocompetent patients typically clear the virus shortly after symptom onset, PCR testing is not recommended for the routine diagnosis of WNV. PCR may be more useful for immune deficient individuals who clear the virus more slowly, and can be done by the Washington PHL on either CSF or blood.
  • It is difficult to distinguish WNV from other causes of meningoencephalitis, so testing for other potential causes of aseptic meningitis (enteroviruses and herpes viruses) is suggested. Also consider additional testing if a patient's travel history is suggestive of other arboviral exposure.
  • There is no vaccine or specific therapy for WNV in humans.
  • In severe cases, intensive supportive therapy may be indicated, such as hospitalization, intravenous fluids, airway management, respiratory support, and the prevention of secondary infections.

Please report patients with any of the following:

  1. Suspected or confirmed WNV neuroinvasive disease:
    Fever (in the absence of a more likely diagnosis) in a patient with at least one of the following:
    • Acute change in mental status (e.g., disorientation, obtundation, stupor, or coma), or
    • Other acute central or peripheral neurological dysfunction (e.g., paresis or paralysis, nerve palsies, sensory deficits, abnormal reflexes, seizures, or movement disorders), or
    • Cerebrospinal fluid (CSF) pleocytosis associated with an illness compatible with meningitis
  1. WNV non-neuroinvasive disease:
    Documented fever (>38.0º C or 100.4º F) (in the absence of a more likely diagnosis) in a patient who has laboratory evidence of WNV disease:
    • WNV-specific IgM antibodies in serum or CSF measured by any serologic testing method, or
    • Isolation of WNV from, or detection of viral nucleic acid in, blood or CSF
  1. Acute flaccid paralysis or presumed Guillain-Barré syndrome even in the absence of fever and other neurologic symptoms.

  2. Suspected West Nile virus infection in patients with potential recent blood donation or transfusion histories, organ transplant recipients, laboratory or occupational exposures, pregnant women, and transplacental or breast-feeding associated exposures.

    When taking a history from a suspected WNV patient, determine if the patient received blood transfusions or organs within the 4 weeks preceding symptom onset (if so, serum or tissue samples should be retained for testing). In addition, please ask about and report any history of blood or organ donation within 2 weeks of symptom onset for persons with suspected WNV infection. Prompt reporting of these cases will facilitate follow-up including withdrawal of potentially infected blood components.

  3. Asymptomatic WNV disease with laboratory evidence of WNV infection in: even in the absence of fever and other neurologic symptoms.
    • A pregnant woman
    • A neonate or breastfeeding infant of a WNV infected mother
    • Someone who donated or received blood products in the previous month
    • Someone who donated or received a tissue or organ transplant in the previous month
    • Someone who has had occupational exposure to WNV (in a laboratory or through contact with infected animals)

Suspected or confirmed cases of WNV should be reported to Public Health Seattle-King County within three business days, by calling 206-296-4774.

  • Counsel patients to use insect repellents containing DEET, picaridin, or lemon eucalyptus oil. People should take special care when outdoors at dusk and dawn, when mosquitoes are most active. Also ask families to check screen doors and windows for openings that might allow mosquitoes indoors.

  • Advise patients to decrease standing water to the greatest extent possible, particularly around the home. Mosquitoes can breed in water that is as shallow as an inch. Ways to reduce mosquito breeding include changing the water in bird baths and animal feeding troughs weekly, emptying out children's wading pools when not in use, and eliminating other sources of standing water. It is also a good idea to tip over outdoor wheelbarrows, buckets, and children's toys when not in use so that they do not collect water.