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.
The influenza virus was first isolated (laboratory confirmed) in pigs in 1931. It was subsequently recognised, however, that the H1N1 virus was probably introduced into the pig population during the human pandemic in 1918.
'Classical' swine influenza viruses do not normally infect humans and usually circulate among pigs causing moderately high levels of respiratory illness but low death rates. Human infections with the influenza viruses currently circulating among swine are rare. Sporadic cases of classical swine flu infection in humans, mainly associated with exposure to pigs, have been reported in North America, Europe and Asia (until recently fewer than 50 documented cases worldwide) [Myers et al, 2007]. The frequency with which they have been detected increased in 2011.
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 where they originated. The H3N2 subtype has been circulating in pig populations, since its emergence in humans in 1968 and has been maintained in pigs ever since, independently of the human population. Influenza viruses that circulate in swine are called swine influenza viruses when isolated from swine, but are called variant viruses when isolated from humans. The vast majority of human infections with animal influenza viruses do not result in human-to-human transmission.
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 risk of death.
A notable outbreak of ‘classical’ 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. Human cases have also been linked with visits to agricultural fairs where the exposure of humans to pigs is a common occurrence.
Pigs are considered to be a possible route by which new pandemic strains 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) influenza pandemic was caused by a triple reassorted (containing genes derived from human, swine, and avian influenza A viruses) North American swine influenza virus acquiring genes from Eurasian strains of influenza (see the pandemic H1N1 (2009) influenza pages for more information).
In the ongoing outbreak (see ‘whats new’ below) currently reported in the USA of swine influenza H3N2v (also triple reassorted ) infection in humans, the virus contains the matrix (M) gene acquired from the influenza A (H1N1) pdm09 virus. Despite the acquisition of the M gene of the pandemic A(H1N1)pdm09 virus, there is no evidence at present that the new virus is any more transmissible to , or among, humans compared with other variant influenza viruses.
Surveillance in swine in the USA has significantly increased since the 2009 influenza pandemic. The increase in reported cases could in part be due to this increased surveillance as well as improvements in laboratory diagnostics. However, the increased frequency of detection could also represent a true increase in the number of such cases.
In Europe, influenza viruses known to circulate in pigs differ to those in the USA. The triple reassortment A(H3N2) viruses, whch are now considered to be endemic in pigs in the USA, have not been detected to date in Europe.
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 [external link].
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 [external link].
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 [external link].
See the external links for other useful sources of information.