The avian and mammalian host range of highly pathogenic avian H5N1 influenza

• The ecology of H5N1 influenza including avian species composing the reservoir and those identified as carriers for increased geographic spread.
• Susceptible domestic species infected, including domestic poultry, and discussion of virological and pathological data.
• Molecular changes associated with adaptation to domestic poultry.
• Detailing of mammalian species naturally infected with H5N1 influenza.
• Discussion of molecular changes associated with infection of mammals and known molecular factors required for sustained transmission of H5N1.

Highly pathogenic H5N1 influenza viruses have been isolated from a number of avian and mammalian species. Despite intensive control measures the number of human and animal cases continues to increase. A more complete understanding of susceptible species and of contributing environmental and molecular factors is crucial if we are to slow the rate of new cases. H5N1 is currently endemic in domestic poultry in only a handful of countries with sporadic and unpredictable spread to other countries. Close contact of terrestrial bird or mammalian species with infected poultry/waterfowl or their biological products is the major route for interspecies transmission. Intra-species transmission of H5N1 in mammals, including humans, has taken place on a limited scale though it remains to be seen if this will change; recent laboratory studies suggest that it is indeed possible. Here we review the avian and mammalian species that are naturally susceptible to H5N1 infection and the molecular factors associated with its expanded host range.