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Disease Name: West Nile Virus

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Please review the Idaho Reportable Disease Rules (IDAPA 16.02.10) for the most up-to-date information.

West Nile Virus: http://www.cdc.gov/westnile/index.html

Overview / Case Definition

West Nile fever (WNV Non-neuroinvasive disease)

Up to 20–30% of the people who become infected will display some symptoms, including fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or eye pain. A minority of patients develop a maculopapular or morbilliform rash involving the neck, trunk, arms, or legs. Symptoms typically last 3 to 6 days and resolve completely but may last 60 days or longer in some individuals. Sequelae, including lingering fatigue, weakness, and depression have been described. The case fatality rate is <1%.

West Nile virus neuroinvasive disease

Approximately one in 150 people (<1%) infected with WNV will develop a neurologic illness such as encephalitis, aseptic meningitis, meningoencephalitis or acute flaccid paralysis (AFP). According to CDC, most cases require hospitalization and 50%–75% need assisted living or rehabilitation. Approximately 75% of neuroinvasive disease is encephalitis or meningoencephalitis. Symptoms can include high fever, headache, neck stiffness, alteration in consciousness (which may be mild to severe including a coma) tremors, convulsions, muscle weakness, vision loss, numbness, and paralysis. In addition, ataxia and extrapyramidal signs, cranial nerve abnormalities, myelitis, severe muscle weakness, optic neuritis, polyradiculitis, and seizures have been reported. A minority of patients develop a maculopapular or morbilliform rash involving the neck, trunk, arms, or legs. Gastrointestinal symptoms are also possible. A rare, serious manifestation is AFP, presenting as weakness of one or more limbs and/or diaphragmatic muscles. Sensation and pain typically remain; however, motor skills are greatly diminished. AFP may occur in the absence of any other sign of infection. Recovery is often prolonged, if at all. All age groups have been affected; however, severe disease is higher in those over 60y of age and those with underlying medical conditions (such as diabetes, hypertension, cancer, renal disease, and chronic alcohol abuse). The case fatality rate approaches 10% for those with neuroinvasive disease.




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Diagnosis / Testing

WNV antibodies can be detected in humans by commercial laboratories. Serum IgM, which is often detectable at the time of illness onset, has been found to persist for over 500 days post-infection in some individuals. Less is known about the longevity of CSF IgM but one study found CSF IgM 199 days post onset of illness. IgG becomes detectable within several days of illness onset. Because commercial testing is widely available, IBL will only test neuroinvasive cases, unless prior approval is gained from BCDP epidemiologists for testing of those with other clinical manifestations.

IgM levels are determined by an IgM capture-ELISA. Because IgM antibody does not readily cross the blood-brain barrier, IgM antibody in CSF strongly suggests an acute CNS infection. IgM may not be detectable if serum is collected <7day post onset of illness. NOTE: Any person testing positive for WNV antibodies, in the absence of ANY clinical illness, will not be reported as a WNV case.


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Additional Information

West Nile Virus: http://www.cdc.gov/westnile/index.html

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