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West
Nile Virus (WNV) Infection: Information for Clinicians
Fact
Sheet:
West Nile Virus (WNV) Infection: Information for Clinicians
Clinical
Features
Mild Infection
Most
WNV infections are mild and often clinically unapparent.
- Approximately
20% of those infected develop a generally mild illness (West
Nile fever).
-
The incubation period is thought to range from 3 to 14 days.
- Symptoms
generally last 3 to 6 days.
Reports
from earlier outbreaks describe the mild form of WNV infection
as a febrile illness of sudden onset often accompanied
by
|
malaise |
|
headache |
|
anorexia |
|
myalgia |
|
nausea |
|
rash |
|
vomiting |
|
lymphadenopathy |
|
eye
pain |
|
|
The
full clinical spectrum of West Nile fever has not been determined
in the United States.
Severe
Infection
Approximately
1 in 150 infections will result in severe neurological disease.
-
The most significant risk factor for developing severe neurological
disease is advanced age.
- Encephalitis
is more commonly reported than meningitis.
In
recent outbreaks, symptoms occurring among patients hospitalized
with severe disease include
|
fever |
|
gastrointestinal
symptoms |
|
weakness |
|
change
in mental status |
-
A minority of patients with severe disease developed a maculopapular
or morbilliform rash involving the neck, trunk, arms, or legs.
- Several
patients experienced severe muscle weakness and flaccid paralysis.
-
Neurological presentations included
|
|
ataxia
and extrapyramidal signs |
|
optic
neuritis |
|
cranial
nerve abnormalities |
|
polyradiculitis |
|
myelitis |
|
seizures |
Although
not observed in recent outbreaks, myocarditis, pancreatitis, and
fulminant hepatitis have been described.
Clinical
Suspicion
Diagnosis
of WNV infection is based on a high index of clinical suspicion
and obtaining specific laboratory tests.
- WNV,
or other arboviral diseases such as St. Louis encephalitis,
should be strongly considered in adults >50 years who develop
unexplained encephalitis or meningitis in summer or early fall.
- The
local presence of WNV enzootic activity or other human cases
should further raise suspicion.
- Obtaining
a recent travel history is also important.
Note:
Severe neurological disease due to WNV infection has occurred
in patients of all ages. Year-round transmission is possible in
some areas. Therefore, WNV should be considered in all persons
with unexplained encephalitis and meningitis.
Diagnosis
and Reporting
Procedures
for submitting diagnostic samples and reporting persons with suspected
WNV infection vary among states and jurisdictions. Links to state
and local websites are available at
http://www.cdc.gov/ncidod/dvbid/westnile/city_states.htm
Diagnostic
Testing
West
Nile virus (WNV) testing for patients with encephalitis, meningitis,
or other serious central nervous system infections can be obtained
through local or state health departments. For WNV diagnosis,
public health laboratories usually perform an IgM antibody capture
enzyme-linked immunosorbent assay (MAC-ELISA). Using this assay,
virus-specific IgM can be detected in nearly all cerebrospinal
fluid (CSF) and serum specimens received from WNV-infected patients
at the time of their clinical presentation. Because serum IgM
antibody may persist for more than a year, physicians must determine
whether the antibody is the result of a WNV infection in the previous
year and unrelated to the current clinical presentation. The following
procedures are recommended:
- The
most conclusive diagnostic method to identify persons with WNV
infection of the central nervous system (CNS) is detecting WNV-specific
IgM antibody in CSF using MAC-ELISA. This can be done with a
CSF specimen obtained during initial clinical presentation.
Because IgM antibody does not readily cross the blood-brain
barrier, IgM antibody in CSF strongly suggests acute CNS infection
- If
CSF is not obtained and serum samples are used to make the diagnosis,
paired acute- and convalescent-phase serum samples should be
acquired. The acute-phase specimen should be obtained during
initial clinical presentation and the convalescent-phase specimen
should be obtained 7-14 days later. Both samples should be tested
with MAC-ELISA.
- If
a convalescent-phase specimen cannot be obtained, the acute-phase
specimen should be tested with MAC-ELISA. If the specimen is
IgM-negative, then the illness is very unlikely to be an acute
WNV infection. If the specimen is IgM-positive and the illness
is clinically compatible, then it may be a recent WNV infection
(presuming the test results for IgM antibody to St. Louis encephalitis
(SLE) virus are significantly lower or negative; see below).
Ideally,
MAC-ELISA testing should be performed, using both WNV and SLE
virus. If the MAC-ELISA results for WNV and SLE are similar, it
is necessary to use the plaque-reduction neutralization test (PRNT)
to confirm either a WNV or SLE virus infection. Note: Patients
who have been recently vaccinated against or recently infected
with related flaviviruses (e.g., yellow fever, Japanese encephalitis,
dengue) may have positive WNV MAC-ELISA results.
Reporting
Suspected WNV Infection
Refer
to local and state health department reporting requirements: http://www.cdc.gov/ncidod/dvbid/westnile/city_states.htm
-
WNV encephalitis is on the list of designated nationally notifiable
arboviral encephalitides.
- Aseptic
meningitis is reportable in some jurisdictions.
The
timely identification of persons with acute WNV or other arboviral
infection may have significant public health implications and
will likely augment the public health response to reduce the risk
of additional human infections.
Laboratory
Findings
Among patients in recent outbreaks
- Total
leukocyte counts in peripheral blood were mostly normal or elevated,
with lymphocytopenia and anemia also occurring.
- Hyponatremia
was sometimes present, particularly among patients with encephalitis.
- Examination
of the cerebrospinal fluid (CSF) showed pleocytosis, usually
with a predominance of lymphocytes.
- Protein
was universally elevated.
- Glucose
was normal.
- Computed
tomographic scans of the brain mostly did not show evidence
of acute disease, but in about one-third of patients, magnetic
resonance imaging showed enhancement of the leptomeninges, the
periventricular areas, or both.
Treatment
Treatment
is supportive, often involving hospitalization, intravenous fluids,
respiratory support, and prevention of secondary infections for
patients with severe disease.
-
Ribavirin in high doses and interferon alpha-2b were found to
have some activity against WNV in vitro, but no controlled studies
have been completed on the use of these or other medications,
including steroids, antiseizure drugs, or osmotic agents, in
the management of WNV encephalitis.
For
additional clinical information, please refer to Petersen LR and
Marfin AA, "West
Nile Virus: A Primer for the Clinician [Review]" Annals
of Internal Medicine (August 6) 2002:137:173-9.
PDF
(287 KB/7 pages)
For clinical and laboratory case definitions, see "Epidemic/Epizootic
West Nile Virus in the United States: Revised Guidelines for Surveillance,
Prevention, and Control, 2001," at http://www.cdc.gov/ncidod/dvbid/westnile/surv&control.htm
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