How is West Nile virus (WNV) infection and encephalitis (WNE) diagnosed?

Updated: Apr 05, 2021
  • Author: David J Cennimo, MD, FAAP, FACP, AAHIVS; Chief Editor: Michael Stuart Bronze, MD  more...
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A laboratory diagnosis of WNV infection is based on isolation of the virus antigen or RNA in tissue, blood, CSF, or other body fluid. Diagnosis can also be made based on four-fold antibody titer increase or detection of viral-specific IgM antibody in CSF or serum. [6] The presence of WNV IgM almost always indicates recent WNV infection; however, because of cross-reactivity with IgM, this sometimes indicates recent infection with another flavivirus. The IgM antibodies are usually short-lived, lasting weeks to months; however, in cases of reinfection, it is important to measure titers approximately 2-3 weeks apart to establish a diagnosis. [22] Detection of IgM in CSF is presumptive of recent neuroinvasive disease.

Nucleic acid testing, if positive, is also diagnostic of active disease. However, negative results may reflect viremia that is inadequate for detection, especially in immunocompetent patients. [22] Therefore, RNA PCR is not recommended for diagnosis in immunocompetent patients. [22]

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