Crow Deaths as a Sentinel Surveillance System for West Nile Virus in the Northeastern United States, 1999

Emerging Infectious Diseases West Nile VirusWest Nile (WN) virus (family Flaviviridae) causes inapparent infection, mild febrile illness, meningitis, encephalitis, or death in humans and horses in Europe, Africa, Asia, and Australia (1).Wild birds are considered the principal hosts of WN virus, and mosquitoes, particularly Culex species, are the primary vector (1).Bird deaths had not been frequently documented in previous human WN virus outbreaks, although infected carcasses of a variety of bird species were found in Israel in 1998 (1,2), and deaths were observed after experimental infection in crows and sparrows (3).As early as the end of June 1999, an unusual number of dead and dying crows were noted by residents of northern Queens in New York City (NYC).In July, a local veterinarian noted neurologic illness in some birds with unstable gait.Although not then recognized, the earliest cases of human illness due to West Nile virus occurred in this area, beginning in the first week of August (4).After initial evaluation of dead birds by the New York State Department of Environmental Conservation's Wildlife Pathology Unit and the Wildlife Conservation Society, a virus isolated from specimens by the National Wildlife Health Center and the U.S. Department of Agriculture's National Veterinary Services Laboratory was identified as WN virus by the Centers for Disease Control and Prevention (CDC) on September 23 (5).The virus was also recovered by the Connecticut Agricultural Experiment Station in specimens from a Connecticut bird on September 13 (6).A West Nile virus genomic sequence identical to that derived from the bird isolates was then observed in a brain specimen from a human encephalitis case (7).In response to the initial indications of WN virus in bird specimens, surveillance systems for bird deaths and laboratory testing were established and used in the assessment and control of the outbreak.We reviewed data from systems in New York State, New Jersey, and Connecticut to describe how surveillance of bird deaths was used in 1999 to guide public health action, as well as the advantages and disadvantages of using dead birds as sentinels for West Nile virus in a given geographic area.