West Nile virus transmission and ecology in birds.

The ecology of the strain of West Nile virus (WNV) introduced into the United States in 1999 has similarities to the native flavivirus, St. Louis encephalitis (SLE) virus, but has unique features not observed with SLE virus or with WNV in the old world. The primary route of transmission for most of the arboviruses in North America is by mosquito, and infected native birds usually do not suffer morbidity or mortality. An exception to this pattern is eastern equine encephalitis virus, which has an alternate direct route of transmission among nonnative birds, and some mortality of native bird species occurs. The strain of WNV circulating in the northeastern United States is unique in that it causes significant mortality in exotic and native bird species, especially in the American crow (Corvus brachyrhynchos). Because of the lack of information on the susceptibility and pathogenesis of WNV for this species, experimental studies were conducted at the USGS National Wildlife Health Center. In two separate studies, crows were inoculated with a 1999 New York strain of WNV, and all experimentally infected crows died. In one of the studies, control crows in regular contact with experimentally inoculated crows in the same room but not inoculated with WNV succumbed to infection. The direct transmission between crows was most likely by the oral route. Inoculated crows were viremic before death, and high titers of virus were isolated from a variety of tissues. The significance of the experimental direct transmission among captive crows is unknown.