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Swine Flu
(Novel H1N1 Influenza and H3N2v)

What Is Swine Flu (History and Overview)?

Influenza viruses are small RNA viruses that infect many mammals, including humans, birds, and swine. Before 2009, swine influenza predominately affected swine and was not transmitted often or easily to people. Even in the isolated instances in which swine influenza infected people, it had very limited ability to spread from person to person. Most cases were directly linked to contact with swine through farming or at fairs. Since 2009, the interactions and understanding of the role of swine and flu viruses in human infections has been markedly changed.

Swine were first noticed to have influenza-like illnesses in 1918 during the human influenza pandemic. The term pandemic means that an infection has spread to many countries around the world, causing widespread human disease. Swine influenza did not cause the 1918 pandemic. Rather, pigs apparently acquired the infection from humans or from an undiscovered source. For decades, the swine virus remained relatively unchanged. In the 1990s, however, swine influenza viruses became more diverse and new strains appeared. The reason for this change is not clear but may have been related to overcrowding on large swine farms.

Before 2009, there was only one swine influenza outbreak in people that caused public health concerns. This outbreak occurred in 1979, in soldiers at Fort Dix, N.J. One recruit died, and approximately 12 were hospitalized with influenza. Further testing showed that more than 200 recruits had acquired the virus, although most had few or no symptoms. The infecting strain was found to be strongly related to swine influenza virus, raising concerns that a new pandemic might occur. In response, public health officials began a massive public vaccination program. Up to 25% of people in the United States were vaccinated. Unfortunately, the 1979 vaccine was associated with a small increased risk of Guillain-Barré syndrome, a serious neurological condition, with the risk estimated to be one to nine excess cases per million doses. Importantly, the 1979 strain did not spread easily from person to person and there was no epidemic. Human cases outside of Fort Dix were uncommon. Moreover, the 1979 vaccine was made using an old-fashioned process which is no longer utilized.

The lessons learned from the 1979 swine influenza event were applied in dealing with pandemic threats, including the severe acute respiratory syndrome (SARS) outbreak of 2003 and the 2009 influenza outbreak. Key lessons included ensuring adequate communication with the public, producing a rapid but measured response to potential threats, and ensuring that any new strain fulfills criteria to cause a pandemic before large-scale vaccination was undertaken.

Last Reviewed 9/17/2017

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Patient Comments & Reviews

The eMedicineHealth doctors ask about Swine Flu:

Swine Flu - Prevention and Concerns

Are you concerned about contracting swine flu? What prevention measures do you use to avoid getting swine flu?

Swine Flu - Symptoms

What symptoms did you experience with your swine flu infection?

Swine Flu - Treatment

What treatment did you receive for swine flu?

What does a person do if they suspect they have swine flu?

Swine Flu Symptoms...What to Do? One ER Doctor's Perspective

Swine flu is no different that any other influenza-like disease when it comes to symptoms:

  • fever,
  • cough,
  • sore throat,
  • chills,
  • body aches, and
  • fatigue.

Read What Your Physician is Reading on Medscape

Swine Influenza A (H1N1) Virus »

Swine influenza is a highly contagious respiratory disease in pigs caused by one of several swine influenza A viruses.

Read More on Medscape Reference »

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