WEST NILE VIRUS IN NORTH AMERICAN BIRDS

Abstract

Following the introduction of West Nile virus (WNV) into the United States, in New York City in 1999, from its historical range in the eastern hemisphere, this mosquito-borne virus caused an intense outbreak in local bird populations and a small epidemic in the associated human population. West Nile virus became established in this focal area, and in 2000 it spread north and south from there during the summer transmission season. The virus continued to expand during the next six years, ultimately affecting all the continental states and most of North America. The strain of WNV introduced was uncharacteristically virulent as a disease agent in native avian species in North America. Corvid species, particularly the American Crow (Corvus brachyrhynchos), were affected the most, and mortality in American Crows and other corvid species was used as a sensitive sentinel system to detect the presence and movement of the virus through a public-health-reporting and laboratory-testing national surveillance program. American Crows were also the earliest indicator of virus activity in most locations and a useful predictor of human cases. The temporal and spatial pattern and rapidity of the continental spread of WNV, as detected by the national surveillance system, matched the semiannual migratory movements of hundreds of millions of North American birds. Subsequent dissemination of the virus to Canada, the Caribbean, Mexico, and Central America fit this method of spread as well. Nationwid…