The American Association of Swine Veterinarians has offered a brief explanation of the origin of swine influenza in North America and the Type A H1N1 (2009) influenza strain that's currently circulating among the human population in more than 40 countries.
As AASV points out, classical H1 (cH1N1) swine influenza viruses — or SIV — were first identified in North America in the 1930s. The virus remained genetically stable until the 1998 emergence in swine of a triple-reassortant virus, H3N2, comprised of genes of swine, avian and human origin. The H3N2 virus then combined with the cH1N1 endemic in the swine population to form two reassortant viruses comprised of human, avian and swine genes, rH1N2 and rH1N1. To this point, all of the genetic components were derived from influenza viruses of North American lineage or seasonal human viruses.
In 2009, a new variant, Type A H1N1 (2009), was detected in the human population. This novel H1N1 contains avian genes (PB2 and PA), human genes (PB1) and swine genes (HA, NP and NS) of North American lineage. Notably, however, it also contains neuraminidase and matrix protein genes (NA and MP) of Eurasian swine origin which have never been identified previously in North America.
For more information, visit Pork Magazine's H1N1 Special Section.