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Swine Flu Surfaces Again

August 10, 2012

Steven M. Jones – Associate Professor – Livestock

Susan Weinstein- State Public Health Veterinarian


Swine influenza (swine flu) is a respiratory disease of pigs caused by type A influenza viruses that regularly cause outbreaks of influenza in pigs. Influenza viruses that commonly circulate in swine are called “swine influenza viruses” or “swine flu viruses.” Like human influenza viruses, there are different subtypes and strains of swine influenza viruses. The main swine influenza viruses circulating in U.S. pigs in recent years are:

  • swine triple reassortant (tr) H1N1 influenza virus
  • trH3N2 virus
  • trH1N2 virus


Swine flu viruses do not normally infect humans. However, sporadic human infections with swine influenza viruses have occurred. When this happens, these viruses are called “variant viruses.” They also can be denoted by adding the letter “v” to the end of the virus subtype designation. Human infections with H1N1v, H3N2v and H1N2v viruses have been detected in the United States.


Between August and December 2011, 12 U.S. residents were found to be infected with an influenza A H3N2 variant virus (H3N2v) with the matrix (M) gene from the 2009 H1N1 pandemic virus. This M gene may cause increased transmissibility to and among humans, compared to other variant influenza viruses. This virus also has been found in swine in many U.S. states. Investigations into the human cases revealed infections with these viruses following contact with swine as well as some limited human-to-human transmission. In 2012, another case of H3N2v in a child was detected in April. Then, beginning in July 2012, multiple H3N2v human infections were identified.  Currently there have been 158 cases in 2012 (the vast majority at agricultural fairs). All cases DO have swine contact for sure. Almost all are children.
Most commonly, human infections with variant viruses occur in people with exposure to infected pigs (e.g. children near pigs at a fair or workers in the swine industry). There have been documented cases of multiple persons becoming sick after exposure to one or more sick pigs and also cases of limited spread of variant influenza viruses from person-to-person. The vast majority of human infections with variant influenza viruses do not result in person-to-person spread. However, each case of human infection with a swine influenza virus should be fully investigated to be sure that such viruses are not spreading in an efficient and ongoing way in humans and to limit further exposure of humans to infected animals if infected animals are identified.


It’s possible that sporadic infections and even localized outbreaks among people with this virus will continue to occur. While there is no evidence at this time that sustained human-to-human transmission is occurring, all influenza viruses have the capacity to change and it’s possible that this virus may become widespread. So far, the severity of illnesses associated with this virus in people has been similar to the severity of illnesses associated with seasonal flu virus infections. Limited serologic studies indicate that adults may have some pre-existing immunity to this virus while children do not. CDC is closely monitoring human infections with all novel influenza viruses, including H3N2v viruses, and will provide more information as it becomes available.

Information for People Attending Fairs


The National Association of State Public Health Veterinarians has developed the ”Compendium of Measures to Prevent Disease Associated with Animals in Public Settings, 2011, available online at,  to provide some preventive actions that are applicable to people raising animals, showing animals at fairs, or attending fairs including swine.

Take Action to Prevent the Spread of Flu Viruses Between People and Pigs

    • Wash your hands frequently with soap and running water before and after exposure to animals.
    • Never eat, drink or put things in your mouth while in animal areas and don’t take food or drink into animal areas.
    • Children younger than 5 years, people 65 years and older, pregnant women, and people with certain chronic medical conditions (like asthma, diabetes, heart disease, weakened immune systems, and neurological or neurodevelopmental conditions) are at high risk from serious complications if they get influenza. These people should consider avoiding exposure to pigs and swine barns this summer, especially if sick pigs have been identified.
    • If you have animals – including swine – watch them for signs of illness and call a veterinarian if you suspect they might be sick.
    • Avoid close contact with animals that look or act ill, when possible.
    • Avoid contact with pigs if you are experiencing flu-like symptoms.
    • If you must come in contact with pigs while you are sick, or if you must come in contact with pigs known or suspected to be infected, or their environment, you should use appropriate protective measures (for example, wear protective clothing, gloves, masks that cover your mouth and nose, and other personal protective equipment) and practice good respiratory and hand hygiene.
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