An ELISA is a confirmatory test to detect IgM antibodies to WNV in the serum. Because IgM seroconversion typically occurs between days 4 and 10 of symptom onset, there is a high probability of initial false-negative testing within the first 8 days after symptom onset.19,20 Clinical understanding of this fact is imperative, as an initial negative ELISA does not rule out WNV, and a retest is warranted if clinical suspicion is high. In addition to a high initial false-negative rate with ELISA, there are several other limitations to note. IgM antibodies remain elevated for 1 to 3 months or possibly up to a year in immunocompromised patients.1 Due to this, false positives may be present if there was a recent prior infection. Enzyme-linked immunosorbent assay may not distinguish from different flaviviruses, including the yellow fever, dengue, Zika, Japanese encephalitis, and Saint Louis encephalitis viruses. Seropositivity has been estimated in some states, including 1999 data from New York (2.6%), 2003 data from Nebraska (9.5%), and 2012-2014 data from Connecticut (8.5%).21-23 Regional variance may be expected, as there also were significant differences in WNV seropositivity between different regions in Nebraska (P<.001).23
Because ELISA testing for WNV has readily apparent flaws, other tests have been utilized in its diagnosis. The PNRT is the most specific test, and it works by measuring neutralizing antibody titers for different flaviviruses. It has the ability to determine cross-reactivity with other flaviviruses; however, it does not discriminate between a current infection and a prior infection or prior flavivirus vaccine (ie, yellow fever vaccine). Despite this, a positive PNRT can lend credibility to a positive ELISA test and determine specificity for WNV for those with no prior flavivirus exposure.24 According to the Centers for Disease Control and Prevention (CDC), this test can be performed by the CDC or in reference laboratories designated by the CDC.3 Additionally, some state health laboratories may perform PRNTs.
Viral detection with PCR currently is used to screen blood donations and may be beneficial for immunocompromised patients that lack the ability to form a robust antibody response or if a patient presents early, as PCR works best within the first week of symptom onset.1 Tilley et al25 showed that a combination of PCR and ELISA were able to accurately predict 94.2% of patients (180/191) with documented WNV on a first blood sample compared to 45% and 58.1% for only viral detection or ELISA, respectively. Based on costs from a Midwest academic center, antibody detection tests are around $100 while PCR may range from $500 to $1000 and is only performed in reference laboratories. Although these tests remain in the repertoire for WNV diagnosis, financial stewardship is important.
If there are symptoms of photophobia, phonophobia, nuchal rigidity, loss of consciousness, or marked personality changes, a lumbar puncture for WNV IgM in the cerebrospinal fluid can be performed. As with most viral infections, cerebrospinal fluid findings normally include an elevated protein and lymphocyte count, but neutrophils may be predominantly elevated if the infection is early in its course.26
What are the management options?
To date, there is no curative treatment for WNV, and management is largely supportive. For WNF, over-the-counter pain medications may be helpful to reduce fever and pain. If more severe disease develops, hospitalization for further supportive care may be needed.27 If meningitis or encephalitis is suspected, broad-spectrum antibiotics may need to be started until other common etiologies are ruled out.28
How can you prevent WNV infection?
Disease prevention largely consists of educating the public to avoid heavily wooded areas, especially in areas of high prevalence and during peak months, and to use protective clothing and insect repellant that has been approved by the Environmental Protection Agency.3 Insect repellants approved by the Environmental Protection Agency contain ingredients such as DEET (N, N-diethyl-meta-toluamide), picaridin, IR3535 (ethyl butylacetylaminopropionate), and oil of lemon eucalyptus, which have been proven safe and effective.29 Patients also can protect their homes by using window screens and promptly repairing screens with holes.3
What is the differential diagnosis for WNV?
The differential diagnosis for fever with generalized maculopapular rash broadly ranges from viral etiologies (eg, WNV, Zika, measles), to tick bites (eg, Rocky Mountain spotted fever, ehrlichiosis), to drug-induced rashes. A detailed patient history inquiring on recent sick contacts, travel (WNV in the Midwest, ehrlichiosis in the Southeast), environmental exposures (ticks, mosquitoes), and new medications (typically 7–10 days after starting) is imperative to narrow the differential.30 In addition, the distribution, timing, and clinical characteristics of the rash may aid in diagnosis, along with an appropriately correlated clinical picture. West Nile virus likely will present in the summer in mid central geographic locations and often develops on the trunk and extremities as a blanching, generalized, maculopapular rash around 5 days after symptom onset or with defervescence.1