WNV Antibody Testing
Laboratory diagnosis is generally accomplished by testing of serum or cerebrospinal fluid (CSF) to detect WNV-specific IgM antibodies. Immunoassays for WNV-specific IgM are available commercially and through state public health laboratories.
WNV-specific IgM antibodies are usually detectable 3 to 8 days after onset of illness and persist for 30 to 90 days, but longer persistence has been documented. Therefore, positive IgM antibodies occasionally may reflect a past infection. If serum is collected within 8 days of illness onset, the absence of detectable virus-specific IgM does not rule out the diagnosis of WNV infection, and the test may need to be repeated on a later sample.
The presence of WNV-specific IgM in blood or CSF provides good evidence of recent infection but may also result from cross-reactive antibodies after infection with other flaviviruses or from non-specific reactivity. According to product inserts for commercially available WNV IgM assays, all positive results obtained with these assays should be confirmed by neutralizing antibody testing of acute- and convalescent-phase serum specimens at a state public health laboratory or CDC.
WNV IgG antibodies generally are detected shortly after IgM antibodies and persist for many years following a symptomatic or asymptomatic infection. Therefore, the presence of IgG antibodies alone is only evidence of previous infection and clinically compatible cases with the presence of IgG, but not IgM, should be evaluated for other etiologic agents.
Plaque-reduction neutralization tests (PRNTs) performed in reference laboratories, including some state public health laboratories and CDC, can help determine the specific infecting flavivirus. PRNTs can also confirm acute infection by demonstrating a fourfold or greater change in WNV-specific neutralizing antibody titer between acute- and convalescent-phase serum samples collected 2 to 3 weeks apart.
Other testing for WNV disease
Viral cultures and tests to detect viral RNA (e.g., reverse transcriptase-polymerase chain reaction [RT-PCR]) can be performed on serum, CSF, and tissue specimens that are collected early in the course of illness and, if results are positive, can confirm an infection. However, the likelihood of detecting a WNV infection through molecular testing is fairly low. Immunohistochemistry (IHC) can detect WNV antigen in formalin-fixed tissue. Negative results of these tests do not rule out WNV infection. Viral culture, RT-PCR, and IHC can be requested through state public health laboratories or CDC.
Additional Information about Laboratory Testing
Contact your state or local health department for assistance with diagnostic testing. They can assist you with determining if samples should be sent to the CDC Arbovirus Diagnostic Laboratory for further testing.
- Instructions for Submitting Diagnostic Specimens to the DVBD Arbovirus Diagnostic Laboratory
- CDC Data and Specimen Handling (DASH) Section Form 50.34 for Submission of Laboratory Specimens
- 2013 West Nile Virus in the United States: Guidelines for Surveillance, Prevention, and Control pdf icon[PDF – 69 pages]
- Recommendations to Protect Laboratory and Field Workers from West Nile Virus Exposure