Classification

Of the three genera of influenza viruses that cause human flu, two also cause influenza in pigs, with influenza A being common in pigs and influenza C being rare.[3] Influenza B has not been reported in pigs. Within influenza A and influenza 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

Influenza C viruses infect both humans and pigs, but do not infect birds.[4] Transmission between pigs and humans have occurred in the past.[5] For example, influenza C caused small outbreaks of a mild form of influenza amongst children in Japan[6] and California.[6] 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.[7]
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]
Surveillance

Although there is no formal national surveillance system in the United States to determine what viruses are circulating in pigs,[13] there is an informal surveillance network in the United States that is part of a world surveillance network.

Veterinary medical pathologist, Tracey McNamara, set up a national disease surveillance system in zoos because the zoos do active disease surveillance and many of the exotic animals housed there have broad susceptibilities. Many species fall below the radar of any federal agencies (including dogs, cats, pet prairie dogs, zoo animals, and urban wildlife), even though they may be important in the early detection of human disease outbreaks.[14] [15]