Influenza in Animals
Not all hemagglutinin and neuraminidase subtypes have been detected in humans, poultry, swine, and other animals. Some hemagglutinin and neuraminidase subtypes have been detected only in certain species.
Interspecies Infection
Influenza viruses do not just affect humans. Influenza A viruses are found in many different animals, including ducks, chickens, pigs, whales, horses, seals, and cats. All known subtypes of influenza A viruses can be found in wild birds, with the exception of two subtypes which have only been found in bats.
Birds and pigs can experience outbreaks of flu illness just as people do. Occasionally, animal influenza viruses jump the species barrier and cause outbreaks in humans.
As the frequency of interactions between humans and animals grows, the opportunities for influenza viruses to reassort and potentially cause a pandemic also grow. “One Health” recognizes that human health, animal health, and the environment are interconnected. A One Health approach requires human, animal, and environmental health professionals to work together at the local, state, tribal, federal, and global levels to improve the health of people, animals, and their shared environments.
Bats
Until the discovery of influenza in bats by CDC and the Universidad del Valle in Guatemala, it was believed that there were only 16 hemagglutinin types. These influenza viruses are so unique that CDC classified them as two new subtypes.
Pigs
Pigs can be infected by avian influenza and human influenza viruses, as well as swine influenza viruses. When influenza viruses from different species co-infect a host, the viruses can reassort, and new viruses that are a mix of swine, human and/or avian influenza viruses can emerge. For example, the 2009 H1N1 pandemic virus was a mix of genes from swine, human and avian influenza viruses.
Wild Birds
Avian influenza A viruses have been isolated from more than 100 different species of wild birds. The majority of these birds are gulls, terns, and shorebirds or waterfowl such as ducks, geese, and swans. These wild birds are often viewed as reservoirs (hosts) for avian influenza A viruses. All flu pandemics in the past 100 years have been caused by influenza viruses of avian origin.