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West Nile Virus is spreading throughout the Western Hemisphere

American Crow, © Mdf, Wikimedia projects

Since its accidental introduction into North America in 1999, West Nile Virus has swept rapidly across the Western Hemisphere and is likely to significantly increase the risk of extinction for many already threatened species.

West Nile Virus (WNV), a disease indigenous to Africa, the Middle East and southern Europe, was accidentally introduced into North America via New York in 1999. Since then it has swept rapidly across the continent and down into the Caribbean and Latin America. Most organisms in the Western Hemisphere have low resistance to the virus owing to their lack of previous exposure. As a result, fatalities in domestic animals and humans have occurred, and many tens of thousands of wild birds from 300 species have died (Male 2003, Townsend Peterson et al. 2004).

Although WNV is a largely mosquito-borne disease, it can be transmitted directly from individual to individual. This poses a particular threat to predatory or carrion-feeding birds such as Corvidae (crows and their allies) that feed on dead or sick animals. Corvidae account for over 70% of all reported dead birds infected with WNV since the virus arrived in 1999, with American Crow Corvus brachyrhynchos showingthe highest levels of mortality (Male 2003, Townsend Peterson et al. 2004, CDC 2007). Individual American Crow populations have been known to decline by over 60% during a single WNV outbreak (Yaremych et al. 2004, Caffrey et al. 2005), and nationwide bird surveys such as the Christmas Bird Count reveal that the population in the US has declined by over 20% (National Audubon Society 2007).

Common species like American Crow (i.e. species with global populations of over 500,000 and with ranges of more than one million square kilometres) are unlikely to be at risk of extinction due to WNV. However, the disease is likely to interact with existing pressures (e.g. habitat loss) to increase the risk of extinction for already threatened species such as the Vulnerable White-necked Crow Corvus leucognaphalus, Endangered Gundlach’s Hawk Accipiter gundlachi, and Critically Endangered Ridgeway’s Hawk Buteo ridgwayi, Cuban Kite Chondrohierax wilsonii and Cozumel Thrasher Toxostoma guttatum (Male 2003, Marra et al. 2004, BirdLife International 2008).

If the virus spread to Hawaii it would be especially lethal to the island’s threatened birds, which have evolved in the absence of biting insects and are relatively immunologically naïve. Now that mosquitoes have been introduced to the island it seems only a matter of time before WNV spreads and becomes established there (e.g. by human travel, the bird trade or via migratory birds). Hawaiian Crow Corvus hawaiiensis may be particularly vulnerable to the effects of WNV, owing to its observed effects on similar species (e.g. American Crow). Hawaiian Crow is now extinct in the wild with only 37 individuals remaining in captivity, all of which could be wiped out by a single WNV outbreak (Van Riper et al. 1986, Kilpatrick et al. 2004, Townsend Peterson et al. 2004).

Related Species


BirdLife International (2008) Threatened birds of the world 2008. Cambridge, UK: BirdLife International.
Caffrey, C., Smith, S. C. R. and Weston, T. J. (2005) West Nile Virus devastates an American Crow population. Condor 107: 128–132.
Centres for Disease Control and Prevention (CDC) (2007) West Nile Virus activity—United States, 2006. Morbidity and Mortality Weekly Report 298: 619–621.
Kilpatrick, A. M., Gluzberg, Y., Burgett, J. and Daszak, P. (2007) Quantitative risk assessment of the pathways by which West Nile Virus could reach Hawaii. EcoHealth 1: 205–209.
Male, T. (2003) Potential impact of West Nile Virus on America avifaunas. Conserv. Biol. 17: 928–930.
Marra, P. P., Griffing, S., Caffrey, C., Kilpatrick, A. M., McLean, R., Brand, C., Saito, E., Dupuis, A. P., Kramer, L. and Novak, R. (2004) West Nile Virus and wildlife. Bioscience 54: 393–402.
National Audubon Society (2007). The Christmas Bird Count Historical Results [Online]. Available
van Riper, C., van Riper, S. G., Goff, M. L. and Laird, M. (1986) The epizootiology and ecological significance of Malaria in Hawaiian land birds. Ecol. Monogr. 56: 327–344.
Townsend Peterson, A., Komar, N., Komar, O., Navarro-Sigüenza, A., Robbins, M. B. and Martínez-Meyer, E. (2004) West Nile virus in the New World: potential impacts on bird species. Bird Conserv. Int. 14: 215–232.
Yaremych, S. A., Warner, R. E., Mankin, P. C., Brawn. J. D., Raim. A. and Novak, R. (2004) West Nile Virus and high death rate in American Crows. Emerg. Infect. Dis. 10: 709–711.

Compiled 2008

Recommended Citation:
BirdLife International (2008) West Nile Virus is spreading throughout the Western Hemisphere. Presented as part of the BirdLife State of the world's birds website. Available from: Checked: 21/10/2016