Abstract and Introduction
Abstract
Twenty-nine laboratory-confirmed West Nile virus (WNV encephalitis patients were bled serially so that WNV-reactive immunoglobulin (Ig) M activity could be determined. Of those patients bled, 7 (60%) of 12 had anti-WNV IgM at approximately 500 days after onset. Clinicians should be cautious when interpreting serologic results from early season WNV IgM-positive patients.
Introduction
In late summer and early fall of 1999, human West Nile virus (WNV) infections were recognized for the first time in the Western Hemisphere. Since its original introduction into the New York City (NYC) area, WNV has caused disease in humans, horses, and a wide variety of birds and other vertebrates, spreading into the eastern two thirds of the United States and also into Canada and the Caribbean Basin. The apparent ability of WNV to be disseminated by infected birds and to persist from year to year indicates that it will continue to be a public health problem for the foreseeable future.
The detection of human cases of WNVencephalitis early in the transmission season is a valuable tool to identify human risk and seasonal virus activity. Diagnosis of WNV encephalitis is made by using an immunoglobulin (Ig) M antibody capture enzyme-linked immunosorbent assay (MAC-ELISA), which demonstrates virus-reactive IgM in serum or cerebrospinal fluid (CSF) from a person with a clinically compatible illness. However, if WNV-reactive IgM is long lasting (e.g., from one transmission season to the next), an early season positive IgM test could result from either a recent or past infection.
At least four previous studies, three with Japanese encephalitis virus (JEV) and one with WNV, included evaluations of flavivirus-reactive IgM persistence. Only one of the JEV studies evaluated the IgM activity in case-patients >6 months after illness. Three of 41 JEV case-patients demonstrated JEV-reactive IgM in serum >10 months (300, 330, and 350 days) after onset. In the latter study of human WNV infections, 50% of patients were still IgM positive after 2 months. We report the results of a longitudinal study that followed laboratory-confirmed human WNV encephalitis case-patients for up to 18 months to determine the longevity of their serum WNV-reactive IgM.