Clinical Testing and Diagnosis for West Nile Virus Disease

Antibody Testing

Laboratory diagnosis is generally accomplished by testing of serum or 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 serum 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. Therefore, in certain situations, positive results obtained with these assays should be confirmed by neutralizing antibody testing at a state public health laboratory or CDC. Plaque-reduction neutralization tests (PRNTs) 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.

Indications for confirmatory testing by PRNT:

• Possible exposure to cross-reactive flaviviruses (e.g., St. Louis encephalitis virus, dengue virus)
• Atypical or unusually severe presentation or death
• Suspected unusual route of transmission (e.g., organ transplant, blood transfusion, laboratory)
• Presentation outside of the typical arboviral season (i.e., April–October)

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.

Other Testing

Viral cultures and tests to detect viral RNA (e.g., 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 in immunocompetent patients. Patients with immunocompromising conditions can have prolonged viremia and delayed or absent antibody response; therefore, molecular testing may be preferred in some patients.

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.

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.

WNV disease is a nationally notifiable condition. All cases should be reported to local public health authorities in a timely manner. Reporting can assist local, state, and national authorities to recognize outbreaks and to implement control measures to reduce additional infections.