CDC Confirms 5th Case of New Swine-Origin H3N2 Reassortant in Maine

October 21, 2011 – – CDC has confirmed the fifth case of human infection with a swine–origin influenza A (H3N2) virus that carries the M gene from the 2009 H1N1 virus. This virus was first detected in a child in Indiana in July. Subsequently three additional cases of human infection with swine–origin influenza A (H3N2) viruses carrying the same genetic change were detected in Pennsylvania. Though rare, human infections with swine–origin influenza viruses can occur, usually after close contact with infected swine. The patient in Maine was reported to have attended an agricultural fair where animals, including pigs, were present. The child was not hospitalized. He received antiviral treatment and is recovering from his illness. Though an investigation is ongoing and surveillance for influenza–like–illness is being enhanced, no human–to–human transmission of this virus has been identified in Maine.

The patient had not received a seasonal influenza vaccination. However it should be noted that seasonal influenza vaccine would NOT be expected to protect against this swine–origin influenza virus because it is so different from the human viruses in the seasonal vaccine. Seasonal influenza vaccination is important to decrease the risk of getting infected with human seasonal influenza viruses, which are beginning to circulate in the United States for the 2011–2012 season.

While there is not a 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 viruses – also have shown activity against swine–origin viruses.

Background
On October 14, 2011, CDC received a report from the state of Maine that an influenza virus specimen had tested positive for a likely swine influenza A (H3N2). CDC laboratory testing confirmed a case of human infection with swine–origin influenza A (H3N2) virus. This case brings the total number of confirmed cases of human infection with swine–origin influenza A viruses in the United States to 26 since 2005, with 13 of these having been infected with swine–origin influenza A (H3N2) viruses and five 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. Swine influenza A (H3N2) viruses normally infect pigs, but on rare occasions, can also infect humans. Human infection with swine–origin virus is mostly likely to occur when people are in close proximity to infected pigs. 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.

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. (Visual Interpretation) 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.

More information about this case is available from the Maine Center for Disease Control & Prevention (Maine CDC) Health Alert Network System (HAN).

More information on swine influenza viruses is available.