West Nile encephalitis: An emerging disease in the United States

In 1999, an epidemic of West Nile virus (WNV) encephalitis occurred in New York City (NYC) and 2 surrounding New York counties. Simultaneously, an epi zootic among American crows and other bird species occurred in 4 states. In digenous transmission of WNV had never been documented in the western hemis phere until this epidemic. In 2000, the epizootic expanded to 12 states and the District of Columbia, and the epidemic continued in NYC, 5 New Jersey counties, and 1 Connecticut county. In addition to these outbreaks, several large epidemics of WNV have occurred in other regions of the world where t his disease was absent or rare 1 5 years ago. Many of the WNV strains isola ted during recent outbreaks demonstrate an extremely high degree of homolog y that strongly suggests widespread circulation of potentially epidemic str ains of WNV. The high rates of severe neurologic illness and death among hu mans, horses, and birds in these outbreaks are unprecedented and unexplaine d. We review the current status of WNV in the United States.