Background and epidemiology
West Nile virus (WNV) infections first emerged as a public health problem in the United States in the late 1990s. WNV is a mosquito-borne flavivirus in the same family as yellow fever, dengue fever, and St. Louis encephalitis. Other routes of transmission in rare situations include blood transfusion, organ transplant, transplacental, breastfeeding, and percutaneous injuries of lab workers.
In the United States, outbreaks occur from late spring through autumn when mosquitoes are active. WNV outbreaks are usually associated with bird die-offs. Cases in horses and other mammals may precede or occur simultaneously with human cases.
In Washington, WNV activity has historically been very low. Since surveillance began, the highest level of WNV activity occurred in 2009 when there were 38 human cases (36 acquired in Washington), 73 cases in horses or other mammals, and 22 dead birds tested positive. Washington saw 9 human cases in 2016 and 24 human cases in 2015 (none in Pierce County). Although WNV has historically been detected in dead birds in Pierce County, most WNV activity in the state occurs in Eastern Washington.
Across the United States, WNV outbreak activity varies from year to year. From 1999 to 2015, the states reporting the most cases (from 2,214 to 5,589, in order) are Illinois, Nebraska, Texas, Colorado and California. During the same time period, Washington reported 87 cases.
When to suspect WNV infection
A person who hasn’t traveled outside of Western Washington is unlikely to have WNV. However, local presence of WNV infection in birds, animals, or humans or travel to areas with WNV activity should raise suspicion in persons with unexplained meningitis or encephalitis. Recent history of transfusion, transplant, or vaccination may be important. Testing for WNV or other arboviral disease (such as St. Louis encephalitis) should be strongly considered in:
- Adults 18 years or older with unexplained encephalitis or meningitis, particularly in summer or early fall.
- Children under 18 years old hospitalized with encephalitis.
- Cases of acute flaccid paralysis or presumptive Guillain-Barré syndrome.
Clinical features
- Most patients with WNV infection are asymptomatic.
- Less than 1% of those infected with WNV develop severe disease.
- Incubation period ranges from 2 to 14 days.
Mild infection
- Approximately 20% of those infected develop a mild, self-limited illness known as West Nile fever.
- Symptoms from mild infection generally last 3 to 6 six days.
- Those with this form of WNV infection do not progress to more severe disease.
- West Nile fever is characterized by sudden onset of fever often accompanied by:
- Anorexia.
- Headache.
- Nausea.
- Lymphadenopathy.
- Eye pain.
- Malaise.
- Rash.
- Gastrointestinal symptoms.
Severe infection
- Approximately 1 in 150 infections will result in severe neurological disease.
- The most significant risk factor for severe disease is advanced age.
- Encephalitis and meningitis are the most common severe clinical syndromes.
- Additional symptoms among patients hospitalized with severe disease include:
- Gastrointestinal symptoms.
- Weakness.
- Fever.
- Change in mental status.
- Maculopapular or mobilliform rash involving the neck, trunk, arms, or legs is rare.
- Neurological symptoms include:
- Myelitis.
- Acute flaccid paralysis.
- Ataxia and extrapyramidal signs.
- Tremor, Parksinson-like syndrome.
- Cranial nerve abnormalities.
- Optic neuritis.
- Polyradiculitis.
- Seizures.
Diagnosis and reporting
Diagnostic testing
WNV testing for patients with encephalitis or meningitis can be obtained commercially. Positive commercial tests should be confirmed at the Washington State Department of Health Public Health Lab (PHL).
- The most efficient diagnostic method is detection of IgM antibody to WNV in serum or CSF. IgM antibodies are usually detectable 3 to 8 day safter illness onset.
- RT-PCR should be considered in immunocompromised patients.
- Patients recently vaccinated against or infected with related flaviviruses (e.g., yellow fever, Japanese encephalitis, dengue) may have positive (cross-reactive) WNV MAC-ELISA results.
- Based on the clinical presentation, diagnostic testing should be obtained to rule out other conditions such as herpes encephalitis or meningitis due to fungal, bacterial, or parasitic pathogens.
- Patients who test negative prior to the eighth day after onset should be re-tested 8 days after onset of symptoms
Report suspected WNV infection
Report suspected or confirmed cases of WNV encephalitis to our 24-hour reporting line: (253) 649-1413. For information during business hours, call (253) 649-1412.
Treatment
Treatment is supportive, often involving hospitalization, intravenous fluids, respiratory support, and prevention of secondary infections for patients with severe disease.
Additional resources
- WNV—Washington State Department of Health.
- Clinical signs and symptoms of WNV disease—Centers for Disease Control and Prevention (CDC).
- Clinical testing and diagnosis for WNV disease—CDC.
- Petersen LR and Marfin AA, “West Nile Virus: A Primer for the Clinician [Review]” Ann Intern Medicine (August) 2002 Aug. 6; 137(3):173-9.
Questions?
Call (253) 649-1412.