DISPERSAL OF WEST NILE VIRUS: FROM THE CRADLE OF EVOLUTION IN AFRICA TO THE NEW WORLD

Fiona J May, Ph.D., and ADT Barrett, Ph.D.

Department of Pathology and Sealy Center for Vaccine Development, UTMB

Background: West Nile virus (WNV) is the most widely distributed of all the flaviviruses and is a major cause of encephalitis, with isolates obtained from all continents, apart from Antarctica. Here, we elucidate the dispersal and evolutionary history of WNV. Methods: BEAST analysis of complete genomes was used to determine the approximate dates of divergence of WNV. In combination with amino acid sequence comparison of closely related isolates, we determined the spread and evolution of WNV. Results: WNV diverged in approximately 437BC (95% HPD 1792BC-1381AD), presumably in Africa. Subsequently, WNV has diverged into individual lineages that correspond with geographic distribution. Comparison of the amino acid sequences of isolates from Europe and North America reveals a pattern of spread of the virus, providing evidence of multiple reintroductions from a source(s) in North Africa. Conclusions: Contrary to previous suggestions, an isolate from the epidemic in Israel in 1998 is not the direct progenitor of the US epidemic; rather, both epidemics originated from the same but unknown location, probably in North Africa. Similarly, epidemics in Europe, the Middle East, Russia, and Central Asia, are caused by isolates from a source in North Africa. This is the first time the evolution of WNV has been examined in a world-wide context, and shows the first evidence of an African, rather than Israeli, source of the North American epidemic. Supported by the NIH.