U.S. Department of Agriculture: Animal and Plant Health Inspection Service


Date of this Version

February 2004


Published by Proc. 21th Vertebr. Pest Conf. (R M. Timm and W. P. Gorenzel, Eds.) Published at Univ. of Calif, Davis. 2004. Pp. 180-184.


Since the introduction of the mosquito-borne West Nile Virus (WNV) into New York City (NYC) in 1999, it has expanded westward across the North American continent in an unprecedented fashion, taking in its wake hundreds of thousands and possibly millions of native and exotic birds. Corvid species, particularly the American crow, are particularly susceptible to this virulent strain of virus and have died dramatically during the summer virus transmission season. Experimental studies have shown that the fatality rate from WNV infection in American crows is nearly 100%. This mortality in crows and other corvids was used as a sensitive sentinel system to detect the presence and movement of the virus through a public reporting and laboratory testing national surveillance program. Crows were also the earliest indicator of virus activity in the majority of locations and were a useful predictor of human cases. Bud mortality from WNV peaks during August-September at the height of the mosquito-transmission period but extends from April to November each year in some states. An impact of WNV on local populations of crows was observed in some localities such as the NYC area, but no significant declines have been detected yet by the regional population trend data. The geographical distribution of WNV activity is not continuous across local landscapes and unexposed crows can then serve as a source to repopulate affected areas when overall populations are high. If WNV transmission continues for years with regular mortality, the resiliency of the regional crow populations to sustain this high mortality rate will diminish.