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Avian Influenza A Viruses

This website is archived and is no longer being maintained or updated. For updated information on avian influenza, see the CDC Avian Influenza website and the World Health Organization (WHO) Avian Influenza Situation Update page.

Influenza viruses that infect birds are called avian influenza viruses. Only influenza A viruses infect birds, and all known subtypes of influenza A viruses can infect birds. However, there are substantial genetic differences between the subtypes that typically infect both people and birds. Within subtypes of avian influenza A viruses there also are different strains (described in Influenza Viruses – Types, Subtypes, and Strains).

Avian influenza A H5 and H7 viruses can be distinguished as “low pathogenic” and “high pathogenic” forms on the basis of genetic features of the virus and the severity of the illness they cause in poultry; influenza H9 virus has been identified only in a “low pathogenicity” form. Each of these three avian influenza A viruses (H5, H7, and H9) theoretically can be partnered with any one of nine neuraminidase surface proteins; thus, there are potentially nine different forms of each subtype (e.g., H5N1, H5N2, H5N3, H5N9).

Summary information follows about these three prominent subtypes of avian influenza A viruses:

Influenza A H5

Influenza A H7

Influenza A H9

Page last modified October 14, 2005

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