There seems to be many subtypes and strains of influenza A viruses that have and are circulating in animals and humans?
Yes, there are many subtypes and strains of influenza A virus circulating in different animals. Individual strains of influenza A viruses generally only become established in one or a limited number of animal species. These strains are continuously spread within these animal populations, resulting in a limited number of circulating strains. Occasionally an influenza strain can cross over and infect another species of animal. Most of the time, this kind of cross-species infection does not spread well in the new population; however, in certain cases, the influenza virus may become established as new circulating strain in this new population.
