Classification of Swine Flu
Of the three genera of influenza viruses that cause human flu, two also cause influenza in pigs, with Influenza virus A being common in pigs and Influenza virus C being rare. Influenza virus B has not been reported in pigs. Within Influenzavirus A and Influenzavirus C, the strains found in pigs and humans are largely distinct, although due to reassortment there have been transfers of genes among strains crossing swine, avian, and human species boundaries.
Influenza C viruses infect both humans and pigs, but do not infect birds. Transmission between pigs and humans have occurred in the past. For example, influenza C caused a small outbreaks of a mild form of influenza amongst children in Japan, and California. Due to its limited host range and the lack of genetic diversity in influenza C, this form of influenza does not cause pandemics in humans.
Swine influenza is known to be caused by influenza A subtypes H1N1, H1N2, H3N1, H3N2 and H2N3. In pigs, three influenza A virus subtypes (H1N1, H3N2, and H1N2) are the most common strains worldwide. In the United States, the H1N1 subtype was exclusively prevalent among swine populations before 1998; however, since late August 1998, H3N2 subtypes have been isolated from pigs. As of 2004, H3N2 virus isolates in US swine and turkey stocks were triple reassortants, containing genes from human (HA, NA, and PB1), swine (NS, NP, and M), and avian (PB2 and PA) lineages.
A/Veracruz/2009 (H1N1), the new strain of swine influenza A (H1N1) involved in the 2009 flu outbreak in humans, is a reassortment of several strains of influenza A virus subtype H1N1 that are usually found separately, in humans, birds, and pigs. Preliminary data suggest that the hemagglutinin (HA) gene was similar to that of swine flu viruses present in United States pigs since 1999, but the neuraminidase (NA) and matrix protein (M) genes resembled viruses present in European pigs. Viruses with this genetic makeup had not previously been found to be circulating in humans or pigs, although there is no formal national surveillance system to determine what viruses are circulating in pigs in the United States.