Avian influenza (AI) is a highly contagious viral infection caused by an orthomyxovirus, primarily in avian species. Clinical manifestations range from inapparent in waterfowl to a rapidly fatal condition characterized by gastrointestinal, respiratory and/or nervous signs in chickens, turkeys and other gallinaceous birds. The disease caused by different viruses varies in severity from high mortality, with sudden deaths preceded by few or no clinical signs, to a very mild form or even inapparent infection. The World Organization for Animal Health (OIE) now lists as notifiable highly pathogenic avian influenza (HPNAI) and low pathogenicity avian influenza caused by H5 and H7 subtypes (LPNAI) and refers to these diseases collectively as notifiable avian influenza (NAI).