West Nile Virus Infection in the United States

West Nile virus (WNV), an arthropod-borne flavivirus belonging to the Japanese encephalitis virus antigenic complex, first appeared as a cause of naturally acquired meningitis and encephalitis in the United States in the New York City area in the summer of 1999. 1 During succeeding years, the virus has spread rapidly throughout North America, becoming enzootic in bird and mosquito populations throughout much of the continental United States. In 2003, WNV was responsible for one of the largest arboviral encephalitis epidemics in US history (9858 cases; 2863 with meningoencephalitis and 262 deaths) and one of the largest WNV meningoencephalitis epidemics yet recorded.