Definition: a serious viral disease of many birds, both domestic (especially chickens, ducks, and turkeys) and wild. Wild birds act as reservoirs for influenza A virus (Orthomyxovirus); in birds, these viruses are responsible for clinical pictures ranging from low-grade illness to serious outbreaks with high mortality rates; signs range from reduced egg production to a fulminant peracute clinical course; respiratory signs (sinusitis, blood-stained nasal discharge) often occur in acute infections. Historically, close contact of fowl with humans has been shown epidemiologically to foster cross-species jump (avian to human) of new influenza strains (e.g., Hong Kong 1997 [AH5N1], Hong Kong 1999 [AH9N2]). New neuroaminidase and hemagglutinin subtypes develop as a result of antigenic drift, and less commonly, antigenic shift resulting in serious human influenza epidemics, and give rise to the need for annual reformulation of flu vaccines to protect against newer active strains. Currently 15 hemagglutinin and 9 neuraminidase subtypes have been identified in reservoir species.
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