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Avian influenza (AI) continues to be recognised as a transboundary disease of major global importance that presents threats to animal and public health (including concerns regarding pandemic potential), food security and other socioeconomic impacts. It is caused by segmented, negative sense, single-strand RNA viruses of the Influenzavirus A genus of the Orthomyxoviridae family (Alexander 2007a). Whilest the epidemiology of AI is complex, AI viruses (AIV) can be described as two distinct pathotypes, based upon phenotypic and/or genotypic criteria defining the virulence of the virus infecting chickens. Highly pathogenic AI (HPAI) can cause devastating disease in poultry, and of the 16 identified haemagglutinin subtypes (H1–H16) in birds, HPAI viruses have, to date, been confined to subtypes H5 and H7 (OIE 2012). It is also generally accepted that wild and migratory birds, particularly Anseriformes and Charadriiformes, act as AIV reservoirs (Hinshaw and others 1980, Kawaoka and others 1988), harbouring low pathogenicity AI (LPAI) viruses of all 16 haemagglutinin subtypes. Hence, the conventional epidemiological dogma of AI is typically described by the subclinical perpetuation of LPAI viruses (LPAIV) in wild birds, with periodic infection of poultry. Sometimes infection of poultry with H5 or H7 subtypes of LPAIV may result in the mutation of these viruses to a fully virulent HPAI phenotype and genotype (Rohm and others 1995, Banks and others 2000).
Since the mid-1990s, the epidemiology of AI has become more complicated. In 1996 the progenitor of the so-called Eurasian lineage H5N1 HPAI viruses was reported in geese in the Guangdong Province of China (Xu and others 1999). This HPAI virus and its descendants have been detected from wild birds and poultry throughout Asia, Europe and Africa, as a result of movements of infected poultry, poultry products and infected wild birds (Alexander 2007a, b).
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