West Nile Virus
Information for Health Professionals

All EEE and WNV disease cases should be reported to the Vermont Department of Health.

History of West Nile Virus in the US

West Nile virus was introduced into the United States in 1999 in New York City. Since then the virus has spread across the country and has caused illness in the 48 contiguous states. Reports of illness peaked in 2003 when almost 10,000 cases were reported nationwide. In 2010, just over 1000 cases were reported.

West Nile virus was first detected in Vermont in 2000, and activity levels peaked in 2002. The last reported human case was in 2003 and the last horse case in 2005. However, the virus has been detected in a small number of mosquitoes or birds every year since.

The virus is maintained in a bird-mosquito cycle that occasionally spills over into mammals, including humans. Mammals are dead-end hosts and do not contribute to the transmission cycle of the virus.

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Symptoms of West Nile Virus

WNV infection is subclinical in 80% about people. Approximately 20% of infections result in West Nile fever which is characterized by non-specific symptoms such as fever, headache and fatigue. Some people will develop a skin rash on the trunk, swollen lymph nodes or eye pain. Recovery is usually complete.

In about 1% of infections, neuroinvasive disease develops, and clinical syndromes ranging from febrile headache to aseptic meningitis to encephalitis may occur. This is most common in older patients. Symptoms may include fever, gastrointestinal symptoms, ataxia and extrapyramidal signs, optic neuritis, seizures, weakness, change in mental status, myelitis, polyradiculitis. A minority of patients with severe disease develop a maculopapular or morbilliform rash involving the neck, trunk, arms, or legs. A few patients develop flaccid paralysis.

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Testing and Diagnosis

Criteria for Arboviral Testing

In the summer and fall, patients with asceptic encephalitis or meningitis should be tested for both West Nile virus and Eastern equine encephalitis virus. Patients with Guillain-Barré syndrome should also be tested for West Nile virus. Testing is not recommended for persons with mild illness, such as fever or headache, because levels of WNV and EEEV activity in the community would have to be very high for such symptoms to likely be due these viral infections.

Diagnostic testing

Serologic testing remains the primary method for diagnosing WNV and EEEV infection. Combined with a consistent clinical presentation in an endemic area, a rapid and accurate diagnosis of acute arboviral disease can be made by the detection of virus-specific IgM antibody in serum or CSF. However, samples taken early in the course of illness may be negative, and a convalescent sample may be necessary for accurate diagnosis.

Ideal timing of specimens for serology:

Specimen

Timing

Acute

3 to 10 days after onset of symptoms

Convalescent

2-3 weeks after acute sample

At least 1.0 of serum and/or 1.0 mL of CSF is required for serology testing

WNV and EEE virus antibody tests are available commercially. However, a positive IgM test result should be confirmed by neutralizing antibody testing at a state public health laboratory or CDC.

Samples can be sent to the Vermont Department of Health Laboratory. Serum should be refrigerated and shipped with an ice pack, and CSF samples should be submitted frozen. All specimens should be accompanied by a completed VDHL "Clinical Test Request Form.

Mark "WNV and or EEE virus antibody" under the heading "Serology Tests (Misc.)." Date of onset must be included.

Forms and serology mailers can be obtained by contacting the Vermont Department of Health Laboratory at 800-660-9997, extension 7560.

All EEE and WNV disease cases should be reported to the Vermont Department of Health.

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More Information

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