Since it first emerged in the U.S. with an outbreak in NYC in 1999, the West Nile virus (WNV) has expanded to at least 27 states and to the other Washington (D.C.). It would not be surprising if WNV arrived on the West Coast this season. WNV is a flavivirus, (as are the Japanese encephalitis and St. Louis encephalitis (SLE) viruses. WNV can affect humans, horses, birds, and other vertebrates. The clinical presentation cannot be reliably distinguished from other viral encephalitides. Most WNV cases have occurred in the late summer and fall.

The incubation period is usually 3 to 15 days. The ratio of infected to symptomatic cases is 150:1 and most infections are mild, with symptoms including fever, headache, and body aches, skin rash and swollen lymph glands. Severe infections cause meningoencephalitis, with a range of neurologic and systemic manifestations, including headache, high fever, neck stiffness, stupor, disorientation, cranial nerve abnormalities, nausea, vomiting, coma, tremors, convulsions, muscle weakness (present in 40% of 1999 New York cases), paralysis (present in 20% of 1999 New York cases with electromyographic findings consistent with an axonal neuropathy) and, rarely, death. The presence of neuromuscular weakness in a patient with meningoencephalitis is suggestive of WNV infection. Case-fatality rates range from 3% to 15% and are highest in the elderly. Long-term sequelae are a significant problem among hospitalized cases.

WNV cases and outbreaks have been described in Africa, Europe, the Middle East, west and central Asia, Oceania (sub-type Kunjin), and, as of August 1999, North America. In 1999, 62 cases of severe disease, including 7 deaths, occurred in New York City. In 2001, there were 149 confirmed human cases of West Nile virus illness in the U.S., including 18 fatalities. WNV has been documented in Alabama, Arkansas, Connecticut, Delaware, the District of Columbia, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Mississippi, Missouri, New Hampshire, New Jersey, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, Tennessee, Virginia and Wisconsin.

WNV is transmitted by the bite of one of a number of mosquito species (primarily Culex species, which are present in Washington) that are infected with the virus after feeding on infected birds. Ticks infected with WNV have been found in Asia and Africa, although their role in the transmission and maintenance of the virus is uncertain. WNV is not transmitted person-to-person, or to humans directly from dead or living animals other than mosquitoes.

There is no vaccine or specific therapy for WNV. In severe cases, intensive supportive therapy is indicated, i.e., hospitalization, intravenous (IV) fluids, airway management, respiratory support, prevention of secondary infections (pneumonia, urinary tract, etc.), and good nursing care.

Human cases of Western equine encephalitis and SLE virus have occurred in Washington in the past. Most cases occurred in eastern Washington locations; the last reported case being a Chelan County resident in 1982. Viral encephalitis, including WNV infection, is reportable by health care providers within 3 working days. A definitive diagnosis is not required for reporting and we encourage reporting once the diagnosis is suspected or presumed.

WNV testing will be done at no charge on specimens that meet the following diagnostic criteria:

^ back to top