Historical Perspective of Swine Origin Influenza A H1N1
General Facts

Influenza Change






                 A hospital full of patients infected with spanish influenza

'I had a bird named Enza, opened the window and In-flu-enza'
Childrens Rhyme - 1920's

A nurse wearing a mask during the
1918 Spanish influenza epidemic
1918 Spanish Influenza: Simultaneous Appearance in Humans and Swine

Part of the reason that the 2009 outbreak of swine flu is so disconcerting to us now is because of the severity of the outbreak of H1N1 in 1918.  Many consider this pandemic an example of the worst case scenario for the current swine flu outbreak.  Before 1918 influenza viruses had long been described in humans but never in pigs.  It was this year that a particularly virulent strain of influenza spread across the world infecting up to one third of the world's human population and killing 40 to 50 million1, 2 people worldwide.  Symptoms of this Influenza A strain were seen in the United States, Canada (including the far north), Europe, Asia and even the most remote Pacific islands.  In the spring of 1918 the first wave of the virus emerged in humans, beginning in the United States and sweeping through Canada, Europe and Asia.  It is thought that in the summer the virus mutated or reassorted creating an even more virulent strain of the virus.  As a result, a violent wave of disease swept through the world in the fall .2 At the same time, a virus was infecting swine, causing a disease strikingly similar to the disease seen in humans infected with Influenza A H1N1.  After the pandemic disappeared in humans, the viral disease was seen in swine seasonally for many decades2.  

Veterinarian Richard Shope
Studying the 1918 Swine Origin H1N1
A veterinarian named Richard Shope was the first to isolate any influenza virus.  He isolated swine influenza A H1N1 virus in 1931.  Dr. Shope discovered excperimentally that the swine influenza A H1N1 virus of 1931 could be transmitted from sick pigs to healthy animals4.  While experimentally infecting mice, he found that antibody from any person 12 years or older (in 1931) could protect the mouse from infection4.  These individuals had been exposed to the 1918 H1N1 Influenza A virus. Dr. Shope showed that the swine influenza virus and the human influenza H1N1 virus of 1918 were the same or at least very closely related4.   He also showed that the swine Influenza A H1N1 virus of 1918 had changed very little in the swine population.

Though the Influenza A H1N1 virus of 1918 continued to circulate in pigs unchanged, the human Influenza A H1N1 virus underwent antigenic shift quickly.  After 1918 the human H1N1 serotypes drifted until it was no longer related to that of the swine strain.  Different serotypes of human H1N1 continued to seasonally circulate until 1957 when the virus reassorted with another strain producing Influenza A H2N2.  After this, Influenza A H1N1 strains temporarily disappeared from the human population untill the 1970's.  The unchanged swine influenza was referred to as classic swine flu3 (cH1N1).

A trolley officer refusing to allow passenger without masks on, 1918
Cross-Species Transmission
Though it continued to circulate in swine populations after 1957, Influnza A H1N1 infections in humans were rare for several decades.  The swine influenza virus was isolated from humans in 1974, confirming that it is zoonotic3.  In 1976 an outbreak of respiratory disease occured at an army base in Fort Dix, New Jersey.  A swine-origin  Influenza A H1N1 virus was isolated and determined to be the cause of this outbreak4.  

Coincidentally, H1N1 reemerged in the human population in 1977, probably as a result of accidental laboratory release, and continued to circulate through the population.  H1N1 strains alternated with the H3N2 strain, another common swine origin influenza virus.  The H3N2 human influenza virus was introduced into the swine population in the 1990's3.  The H1N1 and H3N2 viruses reassorted.  The mutated virus contained the hemagglutinin and neuraminidase antigens of cH1N1 virus and several proteins of H3N23.  This triple reassortmed Influenza A H1N1 virus was isolated from a 17 year old frequently exposed to swine with severe respiratory disease in 19984.  Between 1999 and 2009, 11 cases of triple reassorted swine origin H1N1 infections were reported4.

2009 Swine Flu Outbreak
In April 2009, a new swine origin Influenza A H1N1 virus causing severe respiratory disease was isolated in California.  This virus was found to be a quadruple reassorted H1N1.  The triple reasorted H1N1 virus reasorted with Eurasian-like swine H1N1 producing the quadruple H1N1 Influenza A virus which is currently circulating.  For more information on 2009 H1N1 influenza click here.

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1.  Gatherer D, 2009.  2009 H1N1 influenza outbreak in its historical context.  Journal of Clinical Virology; 45: 174-178.
2.  Taubenberger J, 2006.  The origin and virulence of the 1918 "Spanish" Influenza Virus.  Preceeding of the American Philosophical Society; 150: 1.
3.  Thacker E, Janke B, 2008.  Swine Influenza Virus: zoonotic potential and vaccination stratagies for the control of avian and swine influenzas.  The Journal of Infectious Diseases; 197: S19-24
4.  Zimmer S, Burk D, 2009.  Historical Perspective - Emergence of Influenza A (H1N1) Viruses.  New England Journal of Medicine; 361: 279-85