infectious diseases
West Nile Virus
Diagnosis of West Nile Virus
Serologic testing West Nile virus infection is diagnosed by finding West Nile virus-specific IgM antibodies in the serum or cerebrospinal fluid (CSF). These 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. False-positive results may result from cross-reactive antibodies due to infection with other flaviviruses, recent immunization with flavivirus vaccines (yellow fever or Japanese encephalitis), or from nonspecific reactivity. Plaque-reduction neutralization tests (PRNTs) done in reference laboratories, including some state public health laboratories and the Centers for Disease Control and Prevention (CDC), can help determine the specific infecting flavivirus, including West Nile virus. PRNTs can also confirm acute infection by demonstrating a fourfold or greater change in West Nile virus-specific neutralizing antibody titer between acute- and convalescent-phase serum samples collected 2 to 3 weeks apart. Viral cultures and tests to detect viral RNA (eg, reverse transcriptase-polymerase chain reaction [RT-PCR]) can be done on serum or CSF and used to confirm infection. 