Transmission of swine influenza virus from pigs to humans is not common and properly cooked pork poses no risk of infection. When transmitted, the virus does not always cause human influenza and often the only sign of infection is the presence of antibodies in the blood, detectable only by laboratory tests. When transmission results in influenza in a human, it is called zoonotic swine flu. People who work with pigs, especially people with intense exposures, are at risk of catching swine flu. However, only about fifty such transmissions have been recorded since the mid-20th Century, when identification of influenza subtypes became possible. Rarely, these strains of swine flu can pass from human to human. In humans, the symptoms of swine flu are similar to those of influenza and of influenza-like illness in general, namely chills, fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort.
The 2009 flu outbreak in humans, known as "swine flu", is due to a new strain of influenza A virus subtype H1N1 that contained genes most closely related to swine influenza.[2] The origin of this new strain is unknown, however, the World Organization for Animal Health (OIE) reports that this strain has not been isolated in pigs.[3] This strain can be transmitted from human to human,[4] and causes the normal symptoms of influenza.[5]
Classification
Of the three genera of influenza viruses that cause human flu, two also cause influenza in pigs, with Influenzavirus A being common in pigs and Influenzavirus C being rare.[6] Influenzavirus 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
Influenza C viruses infect both humans and pigs, but do not infect birds.[7] Transmission between pigs and humans have occurred in the past.[8] For example, influenza C caused a small outbreaks of a mild form of influenza amongst children in Japan,[9] and California.[9] 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.[10]
Influenza A
Swine influenza is known to be caused by influenza A subtypes H1N1,[11] H1N2,[11] H3N1,[12] H3N2,[11] and H2N3.[13] In pigs, three influenza A virus subtypes (H1N1, H3N2, and H1N2) are the most common strains worldwide.[14] 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.[15]