Influenza A viruses are detectable in many different animals, including, wild fowl, poultry, pigs, sea mammals, horses, companion animals and mink. The zoonotic influenza infections most likely to threaten human health are those in wild fowl and poultry (avian influenza) and pigs (swine influenza). See the avian influenza pages for more information.
Influenza was first isolated from pigs in 1931, though it was subsequently recognised that the H1N1 virus was probably introduced into pigs at the time of the human pandemic in 1918.
'Classical' swine influenza viruses do not normally infect humans and circulate among pigs causing moderately high levels of respiratory illness but low death rates. Sporadic cases of classical swine flu infection in humans, mainly associated with exposure to pigs, have been reported in North America, Europe and Asia (fewer than 50 documented cases worldwide) [Myers et al, 2007].
So far three, main influenza A subtypes have been isolated from pigs: H1N1, H1N2, and H3N2 although periodically others are detected. The most common subtype is H1N1; however, there is considerable variation in these H1N1 viruses depending on where they are circulating and their origins. The H3N2 subtype has been circulating in pig populations since shortly after its appearance in humans in 1968 and has been maintained in pigs ever since, independently of the human population.
The symptoms of swine influenza infection in humans are essentially the same as that for seasonal influenza with a range of clinical outcomes from mild or asymptomatic illness and rarely to more serious illness and the risk of death.
The most notable outbreak of swine influenza affecting humans occurred in 1976 among army recruits in Fort Dix, New Jersey. In this outbreak 230 soldiers were subsequently identified as infected with the virus. The outbreak caused one death and respiratory infections in 12 other soldiers with four pneumonias. At the time the possibility that this outbreak might herald another pandemic was felt to be very real and this led to a mass vaccination campaign across the US.
In general, persons who work with pigs are at an increased risk of exposure to infection with swine influenza viruses and may play a role in bridging transmission between human and swine populations.
Pigs are considered to be a possible route by which a new pandemic strain of influenza may emerge. This is the result of the presence of receptors to both avian and human influenza virus strains in the respiratory tract of pigs which enable pigs to act as 'mixing vessels' for influenza viruses of different sub-types leading to the emergence of 'reassorted' influenza viruses with pandemic potential. Reassorted viruses have been detected in pigs since the late 1970's but H1N1 reassortment viruses in pigs emerged in the 1990's. The recent H1N1(2009) 'swine' influenza pandemic was caused by a triple reassorted North American swine influenza virus acquiring genes from Eurasian strains of influenza (see the pandemic H1N1 (2009) influenza pages for more information).
Myers KP, Olsen CW, Gray GC. Cases of Swine Influenza in Humans: A Review of the Literature. Clin Infect Dis 2007; 44(8) 1084-1088.
Shinde V, Bridges CB, Uyeki TM, Shu B, Balish A, Xu X et al. Triple-Reassortment Swine Influenza A (H1) in Humans in the United States, 2005-2009.The New England Journal of Medicine 2010; 360(25) 2616-2625.
Yu H, Hua RH, Zhang Q, Liu TQ, Liu HL, Li GX et al. Genetic Evolution of Swine Influenza A (H3N2) Viruses in China from 1970 to 2006. Journal of Clinical Microbiology 2008; 46(3) 1067-1075.
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