WEST NILE VIRUS IN NORTH AMERICAN BIRDS

Nile historical in the this mosquito- 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 six years, ultimately aff ecting 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 aff ected 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 pa(cid:11) ern 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.