The West Nile virus (WNV) is a single-stranded RNA virus of the Flaviviridae family. Like other arboviruses (eg, St Louis encephalitis, dengue fever, and yellow fever), its main route of transmission to humans is through mosquitoes (primarily Culex species) that have acquired the virus from infected birds. Direct human-to-human transmission is not common, although people have become infected through blood transfusions, solid organ transplants, and percutaneous exposure. Transmission during pregnancy and through breast-feeding has also been reported.

The first cases of human WNV infection in the United States were reported in New York City in 1999.¹ The virus has since spread rapidly. In 2011, infected birds or insects were found in 44 states; there were 712 cases of human WNV infection resulting in 43 deaths.²

Although WNV infection is usually asymptomatic, about 20% of those infected develop West Nile fever, which lasts 3 to 6 days. The incubation period from infection to the onset of symptoms, when present, is typically 2 to 14 days. Symptoms are generally mild and may include malaise, anorexia, nausea and vomiting, eye pain, headache, muscle pain, rash, and lymphadenopathy. Only about 1 in 140 people infected with WNV develop severe neurologic West Nile disease,³ which may include meningitis, encephalitis, flaccid paralysis similar to that found in polio, and other neurologic manifestations. Neurologic West Nile disease is about 20 times as common in infected individuals >50 years of age as in younger people; other potential risk factors for neurologic involvement include compromised immunity and coexisting illness. Hospitalized individuals have a case fatality rate of 4% to 18%,⁴ with advanced age being the greatest risk factor.

The IgM antibody capture enzyme-linked immunosorbent assay (MAC–ELISA) is the most conclusive laboratory method for diagnosis of WNV infection of the CNS.⁵ Quest Diagnostics currently offers an IgM MAC–ELISA, an IgG ELISA, and a real-time polymerase chain reaction (PCR) assay for WNV. The methods can be performed using CSF or blood (ie, serum/plasma) samples (Table). A nucleic acid amplification test is also available for donor testing.

West Nile Virus Antibodies (IgG, IgM)

These tests detect the host immune response to WNV infection. Although viremia is detectable earlier than the immune response, serologic (IgG and IgM) assays are typically more sensitive for detecting active and convalescent WNV infection. IgM is typically detectable at the time of initial presentation; IgG can be detected as early as 7 days after illness onset and within 3 weeks of exposure in most infected individuals.⁶ Both assays (IgG and IgM) must be performed on the same specimen to help establish whether or not the infection is recent. For suspected neurologic WNV disease, CSF specimens should be collected at initial presentation. If only serum samples are to be used for diagnosis, paired specimens should be collected during acute illness and again 7 to 14 days later.⁵

West Nile Virus RNA, Qualitative Real-time PCR

This test directly detects WNV RNA in serum and CSF. WNV viremia peaks at about the time of symptom onset and rapidly fades to undetectable levels. Thus, PCR assays can detect WNV RNA in clinical samples as early as several days before symptom onset, prior to seroconversion. Although these assays have excellent analytical sensitivity, they lack the clinical sensitivity of antibody tests and are typically not used alone for screening or diagnosis. However, such tests can be used for confirmation of suspected WNV infection. They also may be useful in patients with delayed or absent antibody response due to immunosuppression or other factors.

West Nile Virus Antibodies (IgG, IgM)

Unlike the relatively brief IgM response to many viral infections, WNV IgM in serum generally remains detectable for at least 1 to 2 months after infection and may persist beyond 12 months.⁷ Thus, serum IgM results must be interpreted in light of the patient’s clinical condition (eg, presence of encephalitis or meningitis) and travel history, as well as regional WNV activity.⁷

A negative IgM result on an acute-phase specimen strongly suggests absence of WNV infection. A positive IgM result on an acute-phase specimen, accompanied by clinical symptoms consistent with WNV infection, strongly suggests recent infection. In cases of WNV central nervous system (CNS) infection, IgM is almost always detectable on the first day of clinical illness. Detection of WNV IgM in CSF strongly suggests acute CNS infection, as IgM does not easily cross the blood–brain barrier.⁸ Because the MAC–ELISA may be cross-reactive with St. Louis encephalitis (SLE) virus, SLE infection should be ruled out if local epidemiology suggests that it is a possibility.⁵

WNV IgG is often detectable as early as 7 days after illness onset and persists indefinitely. Thus, a positive IgG result with a negative IGM result is consistent with past infection. A negative IgG result combined with a positive IgM result in acute-phase specimens suggests recent infection, as does seroconversion from IgG-negative to -positive status from the acute- to convalescent-phase sample.

False-positive WNV antibody results may occur in individuals infected with or recently vaccinated against flaviviruses such as yellow fever, dengue fever, and Japanese encephalitis, as well as those with previous WNV infection or current SLE infection.

West Nile Virus RNA, Qualitative Real-time PCR

Although a positive real-time PCR result is diagnostic of WNV infection, a negative result does not rule out infection.