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Unit Code 88680:
West Nile Virus (WNV) Antibody, IgG and IgM, Spinal Fluid

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Useful For

Preferred test for the laboratory diagnosis of WNV meningitis or

encephalitis

Clinical Information

WNV is a mosquito-borne flavivirus (single-stranded

RNA) that primarily infects birds but occasionally infects horses

and humans. WNV was first isolated in 1937 from an infected

person in the West Nile district of Uganda. Until the viral infection

was recognized in 1999 in birds in New York City, WNV was found

only in the Eastern hemisphere, with wide distribution in Africa,

Asia, the Middle East, and Europe.(1-3) In 2002, a total of 3,389

human cases of WNV infection were reported from 37 states

(794 cases in Illinois); 2,354 (69%) presented with meningoencephalitis,

704 (21%) had West Nile fever, and 331 (10%) had an unspecified

illness.(2) Overall, the WNV epidemic in the United States was

the largest arboviral meningoencephalitis outbreak documented

in the Western hemisphere. In addition, 33 cases of probable

WNV infection occurred among persons who had received blood

components in the month before illness onset.(3)

 

Most people who are infected with WNV will not have any type of

illness. It is estimated that about 20% of those who become infected

will develop West Nile fever with mild symptoms, including fever,

headache, myalgia, and occasionally a skin rash on the trunk of

the body. About 1 of 150 WNV infections (<1%) result in meningitis

or encephalitis. Case fatality rates among patients hospitalized

during recent outbreaks have ranged from 4% to 14%. Advanced

age is the most important risk factor for death, and patients older

than 70 years of age are at particularly high risk.(1)

 

Laboratory diagnosis of WNV is best achieved by demonstration

of specific IgG and IgM class antibodies in serum specimens from

patients. By the 8th day of illness, most infected persons will have

detectable serum IgM antibody to WNV; in most cases it will be

detectable for at least 1 to 2 months after onset of illness, in some

cases it will be detectable for 12 months or longer. By 3 weeks

postinfection, virtually all infected persons should have developed

serum IgG antibodies to WNV.

 

The specific identification of WNV by detection of IgM

in cerebrospinal fluid (CSF) is the recommended test to document

central nervous system disease, but this test may be falsely

negative in CSF collected <8 days after the onset of symptoms.

PCR (#86197 "West Nile Virus [WNV] RNA Detection by Rapid PCR")

can detect WNV RNA in specimens from patients with WNV infection

when specific antibodies to the virus are not present. However, the

likelihood of detection is relatively low as PCR sensitivity in CSF is

approximately 55%, and in blood, about 10% in patients with known

WNV infection.

Reference Values

IgG: negative

IgM: negative

 

Reference values apply to all ages

Interpretation

IgM:

A positive result is consistent with the acute phase of WNV

meningitis or encephalitis. In the very early stages of acute WNV

infection, IgM may be detectable in CSF before it becomes

detectable in serum.

 

A negative result may indicate the absence of disease. However,

specimens drawn too early in the acute phase may be negative

for IgM-specific antibodies to WNV. If WNV central nervous system

(CNS) infection is suspected, a second specimen should be

collected in 1 to 2 weeks and tested.

 

IgG:

A positive result is consistent with CNS infection with WNV some

time in the past.

Cautions

Test results should be used in conjunction with a clinical evaluation

and other available diagnostic procedures.

 

The significance of negative test results in immunosuppressed

patients is uncertain.

 

False-negative results due to competition by high levels of IgG,

while theoretically possible, have not been observed.

  

WNV antibody results for CSF should be interpreted with caution.

Complicating factors include low antibody levels found in CSF,

passive transfer of antibody from blood, and contamination via

bloody taps.

 

Cross-reactivity has been noted with some specimens containing

IgM antibody to enteroviruses. 

Clinical Reference

1.   Petersen LR, Marafin AA:  West Nile virus: a primer for the clinician.

      Ann Intern Med 2002;137:173-179

 

2.   Petersen LR, Roehrig JT:  West Nile virus: a reemerging global

      pathogen. Emerg Infect Dis 2001;7(4):611-614

 

3.   Brinton MA: The molecular biology of West Nile virus: a new

      invader of the western hemisphere. Ann Rev Microbiol 2002;56:371-402

 

4.   Centers for Disease Control and Prevention (CDC). Provisional

      surveillance summary of the West Nile virus epidemic. United States,

      January-November 2002. MMWR Morb Mortal Wkly Rep 2002;51(50):

      1129-1133

 

5.   Centers for Disease Control and Prevention (CDC). Investigations

      of West Nile virus infections in recipients of blood transfusions.

      MMWR Morb Mortal Wkly Rep 2002;51(43):973-974

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