Submitted to: Medicin Veterinaire Du Quebec
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 13, 2000
Publication Date: December 15, 2000
Interpretive Summary: Influenza virus can infect and cause disease in many different animals, including humans, horses, pigs, chickens, and turkeys. However, the normal host for influenza virus is in wild birds, primarily ducks, gulls, and shorebirds. Influenza viruses once in a new host, like in humans, quickly changes to better fit the new host. These changes are specific for the host that they are presently in, so human viruses usually only infect humans and they don't normally infect pigs or chickens. However, influenza viruses crossing over to new host species do occur, and occasional cause serious disease outbreaks. We still do not know why some viruses cross species and others don't. Influenza also is highly variable in two of its proteins, including the hemagglutinin and neuraminidase genes. This variation is important because vaccines against influenza viruses can only protect for one subtype of the virus. For the hemagglutinin protein there are 15 subtypes of virus. Fortunately for humans and other mammals, the number of subtypes that have infected these species are small, but with pigs and chickens recent introductions of virus with different hemagglutinin subtypes makes protecting them from disease much more difficult.
Technical Abstract: Influenza type A viruses have a wide host range and commonly infects swine, chickens, turkeys, horses, humans and wild birds. The normal host for influenza is in wild birds, primarily ducks, gulls, and shorebirds. The other host species are aberrant hosts, although influenza can occasionally become endemic in these new hosts. For poultry, influenza can occur as either a localized respiratory infection, termed low pathogenic avian influenza (LPAI), or it can be a systemic disease with high mortality, termed highly pathogenic avian influenza (HPAI). Vaccines are protective against disease from influenza, primarily from antibodies. Swine influenza is a respiratory disease pigs typically that occurs commonly in North America swine production areas. The main form of swine influenza in North America for the past 70 years has been H1N1 viruses, but recently several new subtypes of virus have been isolated from North American swine herds, including H1N2, H3N2, and H4N6. These new viruses are reassortants between human, swine and avian influenza viruses or completely avian viruses that have crossed over to swine. Vaccines for swine influenza are also subtype specific, so vaccines for the classic H1N1 viruses will not protect against the new subtypes. Although we generally consider influenza viruses to be species specific, the viruses do occasionally cross species, including swine influenza viruses causing infections in humans and turkeys.