Health advisory: West Nile Virus (WNV) -- Laboratory Testing & Case Reporting Guidelines, May 31, 2005
West Nile virus (WNV) is a flavivirus related to Japanese encephalitis and St. Louis encephalitis (SLE) viruses. WNV can affect humans, horses, birds, and other vertebrates. The clinical presentation cannot reliably be distinguished from other causes of viral encephalitis. In fall 2002, WNV was detected in a crow, a raven, and two horses in Washington State, although human cases have not been reported to-date. The pattern of geographic spread of the virus in the US suggests that human cases of WNV will likely occur in Washington, and neighboring states have previously reported human cases. The following information summarizes clinical manifestations, diagnosis and laboratory testing, and reporting for WNV infections in King County.
WNV is transmitted by the bite of one of a number of mosquito species (primarily Culex species in Washington) that become infected after feeding on birds carrying WNV. WNV is not transmitted directly from person-to-person or to humans directly from dead or living animals other than mosquitoes. In 2002, transfusion of blood products and organ transplantation were identified as potential routes of infection with WNV, and single cases of transplacental and breast-milk transmission infection were reported.
Clinical presentations: WNV infection should be considered in persons of all ages (particularly May – November) with unexplained encephalitis, aseptic meningitis, acute flaccid paralysis or presumed Guillain-Barré Syndrome, or other neurological presentations described below. Because WNV transmission can occur year-round in some areas, obtaining a recent travel history is always important.
Most WNV infections are mild or asymptomatic. Approximately 20% of infected persons develop West Nile fever, a less severe form of infection. The incubation period is thought to range from 3 to 14 days and symptoms last 3-6 days or longer. Symptoms of
West Nile
fever may include fever, malaise, anorexia, nausea, vomiting, eye pain, headache, body aches, maculopapular or morbilliform rash, and swollen lymph glands.
Approximately 1 in 150 infections cause the more severe neurological forms of disease including encephalitis and meningitis. Neuroinvasive disease is associated with a range of neurologic and systemic manifestations including headache, high fever, gastrointestinal symptoms, neck stiffness, stupor, disorientation, cranial nerve abnormalities, ataxia, coma, tremors, convulsions, muscle weakness, paralysis, and, rarely, death. Case-fatality rates for hospitalized patients range from 3% to 15% and are highest in the elderly. Neuromuscular weakness in persons with a viral meningoencephalitis syndrome is suggestive of WNV infection. Other neurological presentations include acute flaccid paralysis (which may present without meningitis or encephalitis), ataxia and extrapyramidal signs, tremor and Parkinson-like syndrome, cranial nerve abnormalities, myelitis, optic neuritis, polyradiculitis, and seizures.
There is no vaccine or specific therapy for WNV in humans. In severe cases, intensive supportive therapy is indicated including hospitalization, intravenous (IV) fluids, airway management, respiratory support, prevention of secondary infections and good nursing care.
Report WNV cases to Public Health at 206-296-4774 within 3 workdays. Case report forms are available online and by calling 206-296-4774.
Healthcare providers & facilities should report patients with any of the following:
- Viral encephalitis, a clinical diagnosis characterized by:
- Fever >38°C or 100°F and
- CNS signs may include altered mental status (altered level of consciousness, confusion, agitation, or lethargy), coma, or other cortical signs (cranial nerve palsies; paresis or paralysis, or seizures), and
- Abnormal CSF profile suggestive of viral etiology: a negative bacterial stain and culture, CSF pleocytosis and/or moderately elevated protein
- Aseptic meningitis occurring May through November in any patient >18 years of age, characterized by:
- fever >38°C or 100°F and
- signs of meningeal inflammation (stiff neck, headache, photophobia) and
- abnormal CSF profile suggestive of viral etiology: a negative bacterial stain and culture, CSF pleocytosis, and/or moderately elevated protein
- Acute flaccid paralysis or presumed Guillain-Barré syndrome even in the absence of fever and other neurologic symptoms.
- Suspected West Nile virus infection in patients with potential recent blood donation or transfusion histories, organ transplant recipients, laboratory or occupational exposures, pregnant women, and transplacental or breast-feeding associated exposures. When taking a history from a suspected WNV patient, determine if the patient received blood transfusions or organs within the 4 weeks preceding symptom onset (if so, serum or tissue samples should be retained for testing). In addition, please ask about and report any history of blood or organ donation within 2 weeks of symptom onset for persons with suspected WNV infection. Prompt reporting of these cases will facilitate follow-up including withdrawal of potentially infected blood components.
- West Nile fever patients with positive commercial laboratory test results (WA PHL will confirm such results during the initial stages of an outbreak. Subsequent reports will be accepted without confirmation by the WA PHL).
Additional information on WNV is available at:
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