Washington State remains the only state in the US that has not detected a case of locally acquired human West Nile virus (WNV) infection. In 2005, two mosquito pools, one bird and one horse in Yakima County tested positive. These were the first positive WNV results since 1992, when four birds and two horses in six separate counties tested positive. It seems inevitable that eventually human cases will occur in Washington, and so we are dutifully reprinting our annual human WNV case identification and reporting guide for clinicians.

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. 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. The incubation period is thought to range from 3 to 14 days, with symptoms lasting 3 to 6 days, or longer.

Laboratory Diagnosis: The Washington State Public Health Laboratory (PHL) will only test King County patients who meet the following criteria, after consultation with Public Health-Seattle & King County:

The PHL tests for WNV-specific IgM antibody on serum or CSF by capture enzyme immunoassay (EIA) and Micro-sphere Immunoassay (MIA). This is the most sensitive test for WNV infection in immunocompetent patients, as more than 90% of those infected will have detectible serum IgM eight days after onset, and CSF antibody may be present even earlier. Positive specimens will be forwarded to the CDC for confirmatory testing. To evaluate immune deficient individuals, the PHL can do polymerase chain reaction (PCR) testing on CSF or blood, though PCR testing is not recommended for routine diagnosis of WNV.

Patients who don’t meet the PHL WNV testing criteria can be tested by a commercial lab. Please note that 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 additional information on WNV, see the Public Health – Seattle & King County WNV website.