West Nile IgG antibodies are often not detectable until day 4 or 5 of illness.
In a person who is IgM positive but IgG negative, a convalescent phase specimen obtained 7-14 days after the initial specimen should be tested to document IgG seroconversion.
West Nile virus (WNV) IgM is usually detectable in serum specimens from WNV-infected people at the time of clinical presentation. Because serum IgM antibody may persist for more than a year in some patients, its presence may indicate WNV infection in the previous year and be unrelated to the current clinical presentation.
Antibodies induced by other flavivirus infections (e.g. Dengue virus, St. Louis encephalitis virus) may show cross-reactivity with WNV.
West Nile virus (WNV) is a mosquito-borne flavivirus (single-stranded RNA) that primarily infects birds but can also infect humans and horses.
WNV was first isolated in 1937 from an infected person in the West Nile district of Uganda. Until the viral infection was recognized in 1999 in birds in New York City, WNV was found only in the Eastern Hemisphere, with wide distribution in Africa, Asia, the Middle East, and Europe.
Most recently, in 2012, a total of 5,674 cases of WNV were reported to the Centers for Disease Control and Prevention (CDC), among which 2,873 (51%) were classified as neuroinvasive disease (eg, meningitis or encephalitis) and 286 (5%) cases resulted in death.
Clinical progression of disease and symptoms:
Most people who are infected with WNV will not develop clinical signs of illness. It is estimated that about 20% of those who become infected will develop West Nile fever with mild symptoms, including fever, headache, myalgia, and occasionally a skin rash on the trunk of the body. Case fatality rates among patients hospitalized during recent outbreaks have ranged from 4% to 14%.
Advanced age is the most important risk factor for death, and patients older than 70 years of age are at particularly high risk.
The presence of IgG-class antibodies to West Nile virus (WNV) in serum indicates infection with WNV at some time in the past. By 3 weeks postinfection, virtually all infected persons should have developed IgG antibodies to WNV. If acute-phase infection is suspected, serum specimens drawn within approximately 7 days postinfection should be compared with a specimen drawn approximately 14 to 21 days postinfection to demonstrate rising IgG antibody levels between the 2 serum specimens.
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