Influenza A

Swine influenza is known to be caused by influenza A subtypes H1N1,[8] H1N2,[8] H2N3,[9] H3N1,[10] and H3N2.[8] In pigs, three influenza A virus subtypes (H1N1, H1N2, and H3N2) are the most common strains worldwide.[11] 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.[12]