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CDC Confirms the 6th and 7th Cases of Swine-Origin Influenza A H3N2 Virus with 2009 H1N1 M Gene


November 4, 2011 -- CDC has confirmed two additional cases of human infection with a swine–origin influenza A (H3N2) virus that carries the M gene from the 2009 H1N1 virus. The cases were reported by Maine and Indiana. There is no evidence at this time of an epidemiological link between these two patients or any person to person transmission associated with either of these cases. Both patients reported exposure to pigs prior to their illness. Human infections with swine influenza viruses are rare, but do occur. In most cases, these infections are associated with exposure to infected pigs.

The swine-origin influenza A (H3N2) virus with the M gene acquired from the 2009 H1N1 virus that caused the 2009-2010 pandemic was first detected in a child in Indiana in July 2011. Subsequently, three additional cases (cases 2 through 4) of human infection with swine–origin influenza A (H3N2) viruses carrying the same genetic change were detected in Pennsylvania. A fifth case was identified in Maine in October. All of these prior cases had direct exposure to pigs, except for one patient who had a caretaker with swine exposure.

The acquisition of the M gene likely occurred as a result of swine being co–infected with the swine influenza A (H3N2) virus and the human 2009 H1N1 virus. (A visual interpretation of this reassortant process is available at http://www.cdc.gov/media/haveyouheard/stories/lab_testingbigimg.html.) While we know the M gene plays a role in influenza virus infection, assembly and replication, the significance of this change in these swine–origin influenza A (H3N2) viruses is unknown at this time. CDC continues to investigate the implications of this genetic change.

Both of the most recent patients confirmed with swine–origin influenza A (H3N2) infection had been in close contact with live pigs during the week prior to their illness onset. Both patients have recovered from their illness.
Seasonal flu vaccine would not be expected to protect against these swine flu viruses because they are very different from seasonal human influenza A (H3N2) viruses. While there is no vaccine to protect humans against these swine–origin influenza viruses, there are two FDA–cleared drugs that can be used to treat illness with these viruses. The antiviral drugs oseltamivir and zanamivir – which are used to treat infection with human seasonal influenza viruses – also have shown activity against swine–origin influenza viruses. 

These reports bring the total number of confirmed cases of human infection with swine–origin influenza A viruses in the United States to 28 since 2005, with 15 of these having been infected with swine–origin influenza A (H3N2) viruses, and seven of these being the H3N2 virus with the M gene from the 2009 H1N1 virus.
Pigs can become infected with and spread influenza viruses, including swine, human and avian influenza viruses. Ill pigs infected with flu viruses may exhibit signs similar to human influenza infections, such as coughing (barking), lack of appetite, runny nose and lethargy.

CDC is required to report all cases of human infection with novel influenza viruses – including swine influenza viruses -- to the World Health Organization (WHO) as part of the International Health Regulations (IHR). CDC also reports these cases in its weekly influenza surveillance report, FluView. Although the vast majority of human infections with novel influenza viruses do not result in human-to-human transmission, these cases should be fully investigated to be sure that such viruses are not spreading among humans and to limit further exposure of humans to infected animals if infected animals are identified.

More information on swine influenza viruses is available at http://www.cdc.gov/flu/swineflu.

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