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The H5N1 influenza virus is often called avian influenza, or bird flu. It is one of a class of viruses designated influenza A. Birds, both wild and domestic, are recognized as natural reservoirs of these viruses. Kurt J. Vandegrift and Peter Daszak of the Wildlife Trust, New York, and colleagues Susanne H. Sokolow and A. Marm Kilpatrick at the University of California provided a review.
The H5N1 designation comes from the proteins hemagglutinin (H) and neuroaminidase (N) in the lipid coating of the virus. There are 16 different H types and nine different N types currently recognized. Influenza A viruses generally cause only mild illness in birds, although some strains can be fatal. These viruses infect both digestive and respiratory cells in birds, so they are shed in feces and respiratory secretions. The viruses evolve quickly because many errors are made in replication, and segments of RNA can be interchanged when two or more virus strains infect the same cell. Most viruses that infect birds do not infect humans, but occasionally a new strain emerges that is able to jump to our species. New virus strains present a threat of epidemics and pandemics, since we have no immunity to them.
Human consumption of chicken has more than tripled since 1960, Vandegrift reported. The increased production of poultry farms has increased the risk of emergence of new influenza strains that can infect humans. Small family farms with free range chickens or ducks are vulnerable to infection by wild birds. Large commercial farms are more protected, but they have high densities of birds that can spread viruses quickly if one becomes infected.
Highly pathogenic H5N1 influenza viruses were first isolated from sick geese in China in 1996, according to a review by Dr. Gabriele Neumann of the University of Wisconsin and colleagues in China and Japan. In 1997, these viruses caused outbreaks of illness in chickens, and were transmitted to humans, resulting in six deaths. No further human cases were reported until 2003.
So far, the H5N1 avian influenza virus has not demonstrated efficient transmission from human to human.