June 12, 2000

Migrating Birds as Well as Mosquitoes May Have Role in Spread of New Virus

West Nile virus, a virus new to the United States, could be spread not only by mosquitoes but also by migrating birds.

ATLANTA—In Europe, Asia, and Africa, West Nile virus appears to be spread mainly by migrating birds; if this pattern holds true in North America, West Nile could cause outbreaks throughout temperate and tropical regions of the Western Hemisphere, according to an article in an upcoming issue of Emerging Infectious Diseases, CDC’s peer-reviewed journal, which tracks new and reemerging infectious diseases worldwide.

Last summer in New York City, there were 62 confirmed cases of the disease and 7 persons died during the first outbreak of West Nile virus infection in this country. Humans become infected when they are bitten by mosquitoes that have bitten infected birds. Recent testing has shown traces of West Nile virus in mosquitoes that survived the winter in the New York area.

Birds can travel across the Atlantic by normal migration, storms, or legal and illegal importation. A few birds of some species regularly migrate from breeding grounds in Europe to wintering grounds along the eastern coast of North America. These birds could be infected by mosquitoes before they migrate. Each summer a few birds are swept across the Atlantic by tropical storms; some of these birds belong to species that can be infected by West Nile virus. During 1999, 17,178 birds were legally imported into New York. Even though a quarantine period is required, no tests are done for West Nile virus, and birds can be infected without showing signs of illness.

In North America, spread of the new virus may depend on its survival in birds that spend the winter in tropical, subtropical, and southern temperate regions. According to the author, if migratory birds do play a role in virus spread, the next outbreak may not be in the New York area, but rather in southern wetlands, farmlands, or cities. These areas could provide a permanent base for the virus, from which it could be spread north by migrating birds.

Illness in humans can be prevented by intensive monitoring of birds for abnormal die-offs during fall and winter; collecting and testing dead birds if die-offs occur; and stringent mosquito control measures at die-off sites. Any birds taken from one part of the country to another should be tested for West Nile virus during quarantine.

For more information, contact Dr. John Rappole at 540-635-6537. Access the full article at http://www.cdc.gov/ncidod/EID/vol6no4/rappole.htm. All material in Emerging Infectious Diseases is in the public domain and may be used without special permission; proper citation, however, is appreciated.

For more information on this or related topics, see—

• West Nile Fever—a Reemerging Mosquito-Borne Viral Disease in Europe
• Notice to Readers: Update: West Nile Virus Isolated from Mosquitoes—New York, 2000
• Update: Surveillance for West Nile Virus in Overwintering Mosquitoes—New York, 2000
• Update: West Nile Virus Encephalitis—New York, 1999

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This page last reviewed June 12, 2000

Emerging Infectious Diseases Journal
National Center for Infectious Diseases
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