West Nile virus: a reemerging global pathogen.

[Volgograd], 942 cases; United States, 62 cases in 1999 and 21 in 2000; Israel, 2 cases in 1999 and 417 in 2000); and 3) high avian death rates accompanying the human outbreaks, in outbreaks in Israel and the United States. Recent outbreaks of WN virus have been accompanied by an apparent evolution of a new WN virus variant. WN virus can be divided genetically into two lineages. Only members of lineage 1 WN viruses have been associated with clinical human encephalitis. Lineage 1 WN viruses have been isolated from Africa, India, Europe, Asia, and North America. Lineage 2 WN viruses are maintained in enzootic foci in Africa and have not been associated with clinical human encephalitis. The close genetic relationship between WN virus isolates from Israel and New York suggests that the virus was imported into North America from the Middle East. The means of its introduction (infected bird, mosquito, human, or another vertebrate host) will likely remain Rev Biomed 2001; 12:208-216.