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Figure 1. Emergence of H5N1 influenza virus and control options.
A nonpathogenic H5 influenza virus is believed to have spread to domestic
ducks and geese, then to domestic chickens. In chickens, the H5 virus
became highly pathogenic before it was transferred back to domestic ducks
and geese. The highly pathogenic H5 virus reassorted its genome with those
of other influenza viruses in aquatic birds, and the resulting viruses
spread to domestic poultry farms, humans, and occasionally to pigs. These
viruses acquired mutations in their PB2, HA, NA, and NS genes that made
them lethal to domestic and wild waterfowl and humans. Solid lines, transmission
demonstrated; dotted lines, transmission postulated but not demonstrated.
Multiple opportunities exist for control of highly pathogenic avian influenza:
1) prevent contact between wild and domestic poultry by use of screened
poultry houses and treated water; 2) prevent contact between domestic
waterfowl and gallinaceous poultry by use of screened houses and treated
water and by exclusion of waterfowl from "wet markets"; 3) eradicate
H5/H7 influenza viruses from gallinaceous poultry by culling or the use
of vaccines that prevent disease and transmission; 4) prevent contact
between poultry, pigs, and humans and make vaccines and antiviral drugs
available.
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