West Nile Virus - Diagnosis

Diagnosis

Preliminary diagnosis is often based on the patient's clinical symptoms, places and dates of travel (if patient is from a non-endemic country or area), activities, and epidemiologic history of the location where infection occurred. A recent history of mosquito bites and an acute febrile illness associated with neurologic signs and symptoms should cause clinical suspicion of WNV.

Diagnosis of West Nile Virus infections is generally accomplished by serologic testing of blood serum or cerebrospinal fluid (CSF), which is obtained via a lumbar puncture. Typical findings of WNV infection include lymphocytic pleocytosis, elevated protein level, reference glucose and lactic acid levels, and no erythrocytes.

Definitive diagnosis of WNV is obtained through detection of virus-specific antibody Immunoglobulin M, (IgM) and neutralizing antibodies. Cases of West Nile virus meningitis and encephalitis which have been serologically confirmed produce similar degrees of CSF pleocytosis and are often associated with substantial CSF neutrophilia. Specimens collected within 8 days following onset of illness may not test positive for West Nile IgM, and testing should be repeated. A positive test for West Nile IgG in the absence of a positive West Nile IgM is indicative of a previous flavavirus infection and is not by itself evidence of an acute West Nile virus infection.

If cases of suspected West Nile virus infection, sera should be collected on both the acute and convalescent phase of the illness. Convalescent specimens should be collected 2–3 weeks after acute specimens.

It is common in serologic testing for cross-reactions to occur among flaviviruses such as Dengue virus (DENV) and tick-borne encephalitis virus, this necessitates caution when evaluating serologic results of flaviviral infections.

Four FDA-cleared WNV IgM ELISA kits are commercially available from different manufacturers in the U.S., each of these kits is indicated for use on serum to aid in the presumptive laboratory diagnosis of WNV infection in patients with clinical symptoms of meningitis or encephalitis. Positive WNV test kits obtained via use of these kits should be confirmed by additional testing at a state health department laboratory or CDC.

In fatal cases, nucleic acid amplification, histopathology with immunohistochemistry and virus culture of autopsy tissues can also be useful. Only a few state laboratories or other specialized laboratories, including those at CDC, are capable of doing this specialized testing