Animal to human flu virus transmission

How are influenza viruses transmited from animals to humans?
Many different animals, including birds such as ducks, geese, gulls and domestic poultry, and mammals such as pigs, whales, horses, dogs and seals, can be infected with influenza viruses. Remarkably, however, certain subtypes of influenza are solely or predominantly found in certain animal species. Birds, of course, are hosts to all known subtypes of influenza A virus, but also in birds a preference of certain influenza viruses for certain species does exist.

Occasionally, an influenza A virus normally seen in one species crosses the species barrier and causes illness in another species. In humans, this may for instance happen after close contact with birds infected with avian influenza viruses. Notoriously, sporadic infections with highly pathogenic avian influenza (HPAI) H5N1 viruses transmitted from birds to humans in Asia and in Europe have caused at least hundreds of serious infections reported in the past few years, the majority of which were associated with a fatal outcome. Significant human to human transmission of this virus was not observed. Obviously, the main concern is that this could happen and a pandemic outbreak of influenza would thus emerge. It cannot be predicted whether an HPAI-H5N1 virus will indeed be at the basis of a future pandemic. Other influenza A virus subtypes such as the H1, H2, H7, and H9 subtypes are also likely candidates. They may spill over directly from birds, but also from other mammals previously infected by birds, such as pigs, horses, dogs or cats. The adaptation to transmission among humans or other mammals may either happen by acquiring sequential mutations, or by a re-assortment event.