West Nile Virus (WNV) Serology


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Subject: West Nile Virus (WNV) Serology

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  • WNV is a mosquito-transmitted Flavivirus of the family Flaviviridae. It is maintained in a cycle between birds and mosquitoes mostly belonging to the Culex genus. Besides horses and humans, several other mammals are dead-end hosts of WNV. About 80% of humans infected with WNV develop no or only very mild symptoms. In about 20% of the cases, patients develop more severe symptoms such as fever, myalgia, and lymphadenopathy. Furthermore, in a small proportion of cases, the infection progresses to life-threatening neuroinvasive forms characterized by meningitis, encephalitis, and/or flaccid paralysis. The risk of developing lethal forms is increased in the elderly or in immunocompromised patients. WNV is most widely spread in temperate areas: isolated in parts of Europe, Middle East, Africa, Asia, America, and Australia. The IgM enzyme immunoassay (EIA) on CSF and/or serum is currently the most sensitive screening test for West Nile virus in humans.


  • As an aid to the diagnosis of West Nile virus encephalitis.


  • Following infection with WNV, IgM antibodies are produced and can be detected within 4–7 days after exposure and may persist for about 1 year, while IgG antibodies can be reliably detected from day 8 after infection.


  • There are several types of serologic tests routinely used for WNV diagnostics: enzyme-linked immunosorbent assay (ELISA), immunofluorescence assay (IFA), neutralization test (NT), and the hemagglutination inhibition assay.

  • Because West Nile IgM may not be positive until up to 8 days following onset of illness, specimens collected <8 days after onset may be negative for IgM, and testing should be repeated.

  • A positive West Nile IgG in the absence of a positive West Nile IgM is consistent with past infection with a Flavivirus and does not by itself suggest acute West Nile virus infection.

  • If acute West Nile virus infection is suspected, it is best to collect both acute and convalescent sera. Convalescent specimens should be collected 2–3 weeks after acute specimens.

  • A major issue in WNV diagnostics is cross-reactivity with antibodies against heterologous flaviviruses, for example, dengue virus (DENV), Japanese encephalitis virus (JEV), tick-borne encephalitis virus (TBEV), or yellow fever virus, which is especially true for IgG antibodies.