Avian Influenza Viruses

Abstract Avian influenza viruses comprise all recognised antigenic subtypes within the genus Influenza A of the Orthomyxoviridae family. They are found prominently in waterfowl, shorebirds and terrestrial birds and are sporadically transmitted to mammalian species. The consequences of such transmissions range from a subclinical infection to a mild infectious and/or lethal respiratory disease, as witnessed with highly pathogenic avian H5N1 viruses, which occasionally infect humans (resulting in cases of fatality rates >50%). Further, avian influenza viruses may cause pandemics by transmitting directly to humans (e.g. possibly the 1918 pandemic) or by reassorting with human and/or other (e.g. swine) influenza viruses (1957, 1968 and 2009 pandemics). Avian influenza viruses thus play a critical role in the ecology of influenza A viruses, and in the generation of novel variants that may cause severe infections or pandemic outbreaks in humans. Key Concepts: Influenza A viruses of all known haemagglutinin (HA) and neuraminidase (NA) subtypes are found in aquatic birds. Avian influenza viruses can be classified as low pathogenic avian influenza (LPAI) or highly pathogenic avian influenza (HPAI), based on their pathogenicity in chickens. All mammalian influenza A viruses originate from avian influenza viruses. Avian influenza viruses play a critical role in the generation of pandemic influenza viruses through direct transmission to humans or reassortment with human and/or swine influenza viruses. Host species‐specific amino acids exist that distinguish avian and mammalian influenza viruses. Host species‐specific amino acids may affect critical functions in the viral life cycle such as receptor recognition and replicative ability; adaptive changes at such key amino acid positions may be critical for avian influenza viruses to cause pandemics in humans.