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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.