Avian influenza is caused by a type A influenza virus belonging to the orthomyxoviridae family. The agar gel diffusion test identifies antibody to type A antigen.
Influenza viruses have two important surface antigens, hemagglutinin and neuraminidase, that give rise to subtype names for specific viruses (eg. H4N6). There are 16 hemagglutinins and 9 neuraminidases making for a large number of possible virus subtypes. Influenza viruses are subtyped by hemagglutination inhibition and neuraminidase inhibition tests. Cross-protection does not occur between subtypes.
Influenza viruses vary widely in pathogenicity and ability to spread among birds. Two pathotypes are recognized: LPAI and HPAI. These pathotype designations are derived from laboratory inoculation of 8 susceptible chickens; LPAI isolates cause death in 0 to 5 of 8 chickens and HPAI isolates cause death in 6 or more. Although most H5 and H7 isolates are low path viruses, so far all HPAI outbreaks have been due to H5 or H7 viruses.
All influenza viruses hemagglutinate chicken red blood cells. Most grow readily in embryonating chicken eggs and tissue culture. They are susceptible to detergents, disinfectants and heat.
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