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History of Swine Flu

Swine influenza was first projected to be a disease related to human influenza during the 1918 flu pandemic, when pigs become ill at the same time as humans. The first classification of an influenza virus as a reason of disease in pigs occurred about ten years later, in 1930. For the next 60 years, swine influenza strains were almost entirely H1N1. Then, between 1997 and 2002, new strains of three different subtypes and five different genotypes emerge as causes of influenza among pigs in North America. In 1997-1998, H3N2 strains emerged. These strains, which include genes derived by reassortment from human being, swine and avian viruses, have become a main cause of swine influenza in North America. Reassortment between H1N1 and H3N2 produced H1N2. In 1999 in Canada, a strain of H4N6 crossed the variety barrier from birds to pigs, but was contained on a single farm.

The H1N1 appearance 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 germ have also circulated in humans through the 20th century, contributing to the normal cyclic epidemics of influenza. However, direct transmission from pigs to humans is rare, with only 12 cases in the U.S. since 2005. Nevertheless, the maintenance of influenza strains in pigs after these strains have vanished from the human population might make pigs a reservoir where influenza viruses could persevere, later emerging to reinfect humans once human resistance to these strains has waned.

Swine flu has been reported frequent times as a zoonosis in humans, usually with restricted distribution, rarely with a widespread distribution. Outbreaks in swine are common and cause major economic losses in industry, primarily by causing stunting and extended time to market. For example, this disease costs the British meat industry about 65 million every year.