Types
There are many different subtypes of type A influenza viruses. These subtypes differ because of certain proteins on the surface of the virus (hemagglutinin [HA] and neuraminidase [NA] proteins). Each different combination of HA and NA proteins is a different subtype of the virus.
Infection with certain avian influenza A viruses (e.g., some H5 and H7 strains) can cause widespread disease and death among some species of domestic birds. Since 1997, more than 16 outbreaks of H5 and H7 influenza have occurred among poultry in the United States.
In some parts of the world, the H5N1 avian influenza virus has resulted in serious outbreaks of bird flu in humans. In people, avian flu caused by the H5N1 virus is very aggressive and poorly understood. Bird flu has a high death rate in humans, possibly because people have little immune system protection against avian influenza viruses.
Once infected, birds shed (cast off) the influenza virus in saliva, nasal secretions, and feces. Other birds become infected when they come into contact with contaminated excretions (e.g., feces) or surfaces. The viruses spread when infected birds move from place to place, and also through contaminated shoes and clothing, farm equipment, feed, cages, and vehicles.
In people, most cases of bird flu infection have occurred as a result of contact with infected birds. Infection also may result from contact with contaminated surfaces, and from eating undercooked poultry or eggs from infected birds. Avian influenza infection may be possible through contact with a person who has it, but the spread of bird flu from one person to another is rare. Changes (mutations) in the H5N1 virus making person-to-person transmission easier could result in a worldwide outbreak (pandemic) of bird flu.
Physician-developed and -monitored.
Original Date of Publication: 02 Aug 2007
Reviewed by: Stanley J. Swierzewski, III, M.D.
Last Reviewed:
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