The PHL tests for WNV-specific IgM antibody on serum or CSF by capture enzyme immunoassay (EIA) and Micro-sphere Immunoassay (MIA). This is the most sensitive test for WNV infection in immunocompetent patients, as more than 90% of those infected will have detectible serum IgM eight days after onset, and CSF antibody may be present even earlier. Positive specimens will be forwarded to the CDC for confirmatory testing.

Because PCR lacks sensitivity, and immunocompetent patients typically clear the virus shortly after symptom onset, PCR testing is not recommended for 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 PHL on either CSF or blood.

Patients who do not meet the Washington State PHL WNV testing criteria can be tested by the Public Health - Seattle & King County Laboratory on a fee for service basis, or by a commercial laboratory. The laboratory offers WNV IgM testing but not PCR.

Because WNV cannot be distinguished clinically from other causes of meningoencephalitis concurrent testing for common causes of aseptic meningitis and encephalitis (including cultures and/or PCR testing for enteroviruses and herpes viruses) is encouraged. Also consider additional testing if a patient’s travel history is suggestive of other arboviral exposure. Though viral encephalitis or meningitis is not a notifiable illness, suspected arboviral encephalitis including WNV is reportable, as is any illness due to arboviral infection.

Test Interpretation: IgM antibody develops by day 8, and IgG antibody within 3 weeks after illness onset. Because IgM does not cross the blood-brain barrier, its presence in CSF indicates neuroinvasive disease. When indicated, a convalescent serum specimen should be drawn about 3-4 weeks after the acute specimen. 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.

For interpretation of positive serum WNV IgM results, there are two caveats: First, antibody may cross react with other causes of arboviral encephalitis including yellow fever, Japanese encephalitis (disease or vaccination), and dengue fever. Second, IgM antibody can persist for more than a year, so the presence of IgM could be indicative of past arboviral infection rather than acute disease.