Swine influenza was first proposed to be a disease related to human influenza during the 1918 flu, when pig became sick at the same time as human. The first identification of influenza viruses as a cause of disease in pigs occurred in 1930, about ten years later. For the following 60 years, swine influenza strains were almost exclusively H1N1. Then, between 1997 and 2002, new strains of three different subtypes and five different genotype emerge as causes of influenza among pig in North American states. In 1997-98, H3N2 strains emerged. These strains, which include genes derived by reassortment from human, swine and avian virus, have become a major cause of swine influenza in North American states. Reassortment between H1N1 and produced an H3N2 H1N2. In 1999 in Canada, a strain of H4N6 crossed the species barrier from birds to pig, but was contained on a single farm.The H1N1 form of swine flu is one of the descendants of the strain that cause the 1918 flu pandemic. As well as persisting in pigs, the descendants of the 1918 viruses have also circulated in humans through the 20th century, contributing to the normal seasonal epidemics of influenza. However, direct transmission from pig to humans is rare, with only 12 cases in the United States since 2005. Nevertheless, the retention of influenza strains in pigs after these strains have disappeared from the human population might make pig a reservoir where influenza viruses could persist, later emerging to reinfect human once human immunity to these strains has waned.
Swine flu has been reported numerous times as a zoonosis in human, usually with limited distribution, rarely with a widespread distribution. Outbreaks in swine are common and cause significant economic losses in industry, primarily by causing stunting and extended time to market. For example, this disease costs the meat Industry of Britain, about £65 million pounds every year.
