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Influenza in Swine

Influenza is a respiratory disease caused by type A influenza viruses that regularly cause outbreaks in pigs. Influenza is present at low levels in pigs throughout the world, and is monitored by the voluntary USDA Swine Influenza Surveillance Program, although it is not a reportable or regulated disease.

Like human influenza viruses, there are different subtypes and strains of influenza viruses in pigs. The main influenza viruses circulating in U.S. pigs in recent years are H1N1, H1N2 and H3N2. While H1N1 viruses have been known to circulate among pig populations since at least 1930, H3N2 and H1N2 influenza A viruses did not begin circulating among pigs in the United States until about 1998.

While influenza viruses almost always remain infectious only within their host species, at times infections may spread to other species. Influenza viruses in pigs can occasionally infect people, and human influenza viruses can infect swine. Health organizations use the term "variant" to refer to viruses that are genetically different from what is usually isolated from humans. The description is written as a small "v" after the virus subtype, in this case, H3N2v. More information on these variant viruses is available from the U.S. Centers for Disease Control at

In 2011, a new variant virus was detected that was an influenza A (H3N2) virus with genes from avian, swine and human viruses. This virus has acquired the M gene from the 2009 H1N1 influenza virus. This 2009 H1N1 M gene may allow these H3N2 viruses in swine to be more transmissible from pigs to people and possibly from person to person. Most cases of H3N2v have happened after contact with pigs at fairs.

2009 Pandemic H1N1 Influenza

As an every day, every way department, USDA's missions are very broad. Given that reality and USDA'S goal to fulfill its mission during a pandemic, USDA works to ensure that all of the services USDA provides to the public, including protecting the U.S. swine herd, continue in light of the pandemic and possibility of increased absenteeism.

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