Currently, Washington State is the only state in the US that has still not detected a case of locally acquired human West Nile virus (WNV) infection. In addition, Washington State has not detected an animal infected with WNV since 2002, when four birds, and two horses tested positive, despite intensive bird, horse, and mosquito pool surveillance.

Despite Washington’s current status as a “WNV-Free Zone,” (a designation also shared by British Columbia,) it is likely that we will see our first WNV cases in 2005 because in 2004, 3 human cases each were identified in both Oregon and Idaho, and 771 human cases were identified in California. Already in 2005, California has detected WNV in birds and/or mosquitos in 21 counties.

Clinical Presentation: The majority of WNV infections are mild or clinically inapparent. Approximately 20% of infected persons develop West Nile fever, which may include fever, malaise, anorexia, nausea, vomiting, eye pain, head-ache, body aches, skin rash, and swollen lymph glands. Approximately 1 in 150 infected persons develops severe neurological forms of disease, including encephalitis and meningitis. Neuro-invasive 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. The incubation period is thought to range from 3 to 14 days, with symptoms lasting 3 to 6 days, or longer.

Laboratory Diagnosis: The most efficient method for diagnosis of WNV is through detection of IgM antibody to WNV in serum collected 8-14 days after illness onset, or in CSF collected within 8 days of illness onset, using the IgM antibody capture enzyme-linked immunosorbent assay (MAC-ELISA). Because WNV cannot be distinguished from other causes of meningoencephalitis on clinical grounds, concurrent testing for other common causes of aseptic meningitis/encephalitis syndrome, (including cultures and/or PCR testing for enteroviruses and herpes viruses) is encouraged. 

Test Interpretation: IgM antibody develops by day 8 and IgG antibody within 3 weeks after illness onset.  When indicated, convalescent serum specimens should be drawn about 3-4 weeks after acute specimens. Negative results on any specimen obtained <8 days after onset of illness should be considered inconclusive and a convalescent serum specimen obtained at least 2 weeks after the first specimen, is needed to make a final determination. Cross-reactions may occur among patients who have had yellow fever, Japanese encephalitis vaccination, a previous history of arboviral encephalitis, or dengue fever.

For complete information on WNV, see the Public Health – Seattle & King County WNV web site: www.metrokc.gov/health/westnile