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Viral Infections
Challenges in the Diagnosis of West Nile Virus Infection
a report by
Karen C Bloch, MD, MPH
Assistant Professor of Medicine (Infectious Disease) and Preventive Medicine, Vanderbilt University Medical Center
West Nile virus (WNV) is a single-stranded RNA virus of the family must be performed in a biosafety level 3 laboratory. Culture is insensitive
Flaviviridae, belonging to the Japanese encephalitis virus serogroup. It is compared with other techniques and is time-consuming, thus remains
transmitted by mosquito vectors (primarily Culex species), with avian species primarily a research tool.
serving as zoonotic hosts. Humans play a limited role in the transmission
cycle given the relatively low level and short duration of viremia. Nucleic Acid Amplification Tests
Nucleic acid amplification testing (NAAT) plays an important role in
While most human infections are asymptomatic, approximately 20% of avian and mosquito surveillance programs
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and in screening of blood
individuals develop West Nile fever (WNF), an undifferentiated febrile donors for occult infection.
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Techniques that have been studied include
illness often associated with a prolonged convalescence.
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WNV realtime reverse transcription polymerase chain reaction (RT-PCR) and
neuroinvasive disease (WNND), which includes meningitis, encephalitis, nucleic acid sequence-based amplification (NASBA), both of which are
and acute flaccid paralysis, as well as overlap syndromes, accounts for able to detect as few as 50 viral RNA copies/ml.
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NAAT testing has been
<1% of infections, yet is the most serious complication. Risk factors for applied to diagnosis of human WNV disease with varying results.
development of encephalitis include advanced age, hypertension, and
cardiovascular disease.
2
The case-fatality rate of WNND ranges from 4 to Early studies suggested a limited rule for NAAT based on the short
15%, with age >75 years cited as a risk factor for death.
3
WNV was first duration of viremia.
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A more recent study performed during a large
recognised in the US in 1999, following a cluster of encephalitis cases in community outbreak of WNV infection in Canada found that 45% of
New York City. Since this time, both the number and geographical range plasma samples from patents with WNV infection had detectable virus.
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of infections have grown exponentially, with human disease documented In this study, the primary determinant of a positive PCR was duration of
throughout the continental US. Between 1999 and 2007, more than symptoms. Plasma NAAT was positive among 56% of patients presenting
10,000 cases of WNND were reported, making WNV the leading cause within eight days of symptom onset compared with 4% of those
of epidemic encephalitis in the US.
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presenting more than a week into clinical illness. The plasma viral load
ranged from 50 copies to 1.4x10
5
copies/ml (mean 7.5 x10
3
copies/ml),
Diagnostic Testing with no association between the quantity of virus detected and the
The clinical presentation of human WNV infection is non-specific, making duration of symptoms or the presence of CNS involvement.
laboratory confirmation essential. Epidemiological features such as
season of onset, residence in an area of enzootic activity, or outdoor Patients with WNND are much less likely to have detectable virus
recreation may be useful clues. Findings of objective muscle weakness or compared with those with WNF, perhaps reflecting the delayed onset of
acute flaccid paralysis are suggestive of WNV myelitis. Cerebrospinal fluid neurological symptoms. In the Canadian study cited above, 36% of
(CSF) evaluation typically shows a lymphocytic pleocytosis, although patients with WNF had detectable virus in plasma compared with only
neutrophils predominate in 37–45% of cases.
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9.5% of those with WNND.
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PCR is a sensitive technique for identifying
WNV in brain tissue, but is of limited use in detecting virus in spinal fluid.
To evaluate the various diagnostic tests for WNV, it is important to One study evaluating CSF PCR among patients with serologically
understand the pathophysiology of WNV infection. The virus is confirmed WNND identified virus in 57% of samples using realtime PCR,
introduced subcutaneously at the time of bite from an infected mosquito and in 0% of specimens using conventional PCR.
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The low yield of CSF
vector. Over the ensuing one to three days, viral particles multiply and PCR for diagnosis of WNV has been replicated by other investigators.
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spread into the bloodstream. Constitutional symptoms, when present,
occur three to seven days following infection. Central nervous system
(CNS) involvement typically develops seven to nine days after infection.
Karen C Bloch, MD, MPH, is an Assistant Professor of Medicine
The diagnostic yield of the various testing techniques depends on both
(Infectious Diseases) and Preventive Medicine at Vanderbilt
University Medical Center, and Section Chief of the Division of
the stage of infection and host characteristics.
Infectious Diseases at the Nashville VA Hospital. She is the
principal investigator for the Tennessee Unexplained Encephalitis
Viral Isolation
Surveillance (TUES) study. Her research interests focus on the
epidemiology and outcomes of encephalitis.
WNV has rarely been cultured from serum, CSF, and other tissues.
6,7
E: karen.bloch@vanderbilt.edu
Propagation of virus requires use of live cell cultures or suckling mice, and
© TOUCH BRIEFINGS 2008
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