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Avian influenza (AI) is a notifiable disease. It is part of the [[Orthomyxoviridae]] family, possessing a single negative sense RNA strand. Within the influenza [[viruses|virus]] family there are 3 subtypes; A, B and C, with only A causing disease in birds.  Type A can then be further subdivided based on the haemagglutinin (HA) and neuraminidase (N) envelope glycoproteins present, with subtype antigens H1-17 and N1-9. Each virus possesses one HA and one N antigen. Each isolate can then be further subdivided into viral lineages called clades <ref> Mahy, B.W.J., Meulen, V.T.(2007)''Topley & Wilson’s Microbiology and Microbial Infections Virology Vol. 1.'' Amer. Soc. for Microbiology; 10th  edition </ref>

Avian influenza (AI) is a notifiable disease. It is part of the [[Orthomyxoviridae]] family, possessing a single negative sense RNA strand. Within the influenza [[viruses|virus]] family there are 3 subtypes; A, B and C, with only A causing disease in birds.  Type A can then be further subdivided based on the haemagglutinin (HA) and neuraminidase (N) envelope glycoproteins present, with subtype antigens H1-17 and N1-9. Each virus possesses one HA and one N antigen. Each isolate can then be further subdivided into viral lineages called clades <ref> Mahy, B.W.J., Meulen, V.T.(2007)''Topley & Wilson’s Microbiology and Microbial Infections Virology Vol. 1.'' Amer. Soc. for Microbiology; 10th  edition </ref>

   

  [[File: EM of influenza virus.jpg|right|100px|thumb|EM of influenza virus]]

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Influenza A viruses affecting birds are divided into two groups based of the severity of clinical disease; highly pathogenic avian influenza (HPAI) and low pathogenic avian influenza (LPAI). HPAI viruses are found within (though not all of) H5 and H7 subtypes. HPAI viruses are thought to be a result of mutations within an LPAI strain. HPAI is defined by the ability to infect and kill chickens using a standardized dose given intravenously (''World Organization for Animal Health, 2006''). The mutation occurs after the virus has moved from the wild bird host into the poultry population and may take days to months to occur. The longer the virus persists the more likely it is to adapt and mutate into a highly pathogenic strain, and once in poultry in can then spread to other species.<ref> Suarez, D.L. (2010), ''Avian influenza: our current understanding, Animal Health Research Reviews 11(1); 19–33''</ref>  

Influenza A viruses affecting birds are divided into two groups based of the severity of clinical disease; highly pathogenic avian influenza (HPAI) and low pathogenic avian influenza (LPAI). HPAI viruses are found within (though not all of) H5 and H7 subtypes. HPAI viruses are thought to be a result of mutations within an LPAI strain. HPAI is defined by the ability to infect and kill chickens using a standardized dose given intravenously (''World Organization for Animal Health, 2006''). The mutation occurs after the virus has moved from the wild bird host into the poultry population and may take days to months to occur. The longer the virus persists the more likely it is to adapt and mutate into a highly pathogenic strain, and once in poultry in can then spread to other species.<ref> Suarez, D.L. (2010), ''Avian influenza: our current understanding, Animal Health Research Reviews 11(1); 19–33''</ref>