CDC Confirms Detection of A Different Influenza A Variant Virus

December 28, 2011 — 

An influenza A (H1N1) virus that is known to circulate in U.S. swine but not humans has been detected in an adult in the state of Wisconsin. The patient reported occupational contact with swine prior to illness onset. No human-to-human transmission with this virus has been identified. CDC laboratory testing has confirmed that this is a new reassortant influenza A H1N1 virus as it has acquired the matrix [M] gene from the 2009 H1N1 pandemic virus. Laboratory testing shows that the virus is susceptible to the influenza antiviral medications, oseltamivir and zanamivir. Surveillance for additional human cases in the area has been enhanced. Providers are being asked to collect specimens for influenza virus testing from any patient presenting with influenza-like illness.

The virus identified in Wisconsin has genes from avian, swine and human influenza viruses, making it a so-called “triple reassortant” (tr) virus. Triple reassortant viruses have been circulating in U.S. swine since the 1990s. However the virus detected in Wisconsin is different from earlier triple reassortant influenza A H1N1 viruses in swine (tr-H1N1) in that it has acquired the matrix [M] gene) from the 2009 influenza A (H1N1) virus. A review of publicly posted influenza genome sequence web sites indicates this reassortment (the tr-H1N1 virus common in swine with the 2009 H1N1 M gene) has been found in U.S. swine since 2010. However, this is the first time this genetic sequence has been detected in a human. Gene sequences of the virus detected in Wisconsin have been posted to publicly available web sites.

A recently adopted naming convention for viruses that commonly circulate in swine uses a “v” (for “variant”) when these viruses infect humans, regardless of whether the virus contains the 2009 H1N1 M gene. Following this convention, the Wisconsin virus will be called H1N1v. This genetic change (acquisition of the 2009 H1N1 virus matrix [M] gene) has been seen in triple reassortant H3N2 viruses that have infected 12 people since August 2011. (These cases occurred in West Virginia (2), Indiana (2), Pennsylvania (3), Maine (2), and Iowa (3).) These variant H3N2 viruses are being called “H3N2v.”

The 2009 H1N1 virus was likely transmitted to swine from humans during and after the 2009-2010 pandemic and now is commonly spreading in both humans and in swine. Given that, and the ability of influenza viruses to change, it is not surprising to see influenza viruses that normally circulate in swine acquire gene segments from the 2009 H1N1 virus. In fact, monitoring of influenza viruses circulating in swine has indicated that the 2009 influenza A (H1N1) virus has reassorted (swapped genes) with other swine influenza A viruses as well.

The M gene plays a role in influenza virus infection, assembly and replication, but the significance of the acquisition of the 2009 H1N1 M gene in influenza viruses that normally circulate in swine is unknown at this time. CDC is continuing to investigate the implications of this genetic change; however, limited data from one animal model (in guinea pigs) indicate that acquisition of this M gene may make influenza viruses more readily transmissible among guinea pigs. Whether the same would be true among swine or humans is not known.

Although the majority of human infections with animal influenza viruses do not result in human-to-human transmission, each case should be investigated fully to determine if these viruses are transmitted among humans and to limit further exposure of humans to infected animals, if infected animals are suspected. Such investigations require close collaboration among state, local, and federal public and animal health officials. While human-to-human transmission of the virus in Wisconsin has not been detected, human-to-human transmission of H3N2v has been reported, previously in Iowa and most recently in West Virginia.

CDC recommends an annual seasonal flu vaccine to protect against seasonal influenza viruses; however, a seasonal flu vaccine is unlikely to protect people against variant flu viruses that are very different from circulating human viruses. Two FDA–cleared drugs – oseltamivir and zanamivir - are expected to be effective in treating illness associated with H1N1v, as well as H3N2v viruses.

Influenza has not been shown to be transmissible to people through eating properly handled and prepared pork (pig meat) or other products derived from pigs. For more information about the proper handling and preparation of pork, visit the USDA website fact sheet “Fresh Pork From Farm to Table.”

More information about swine influenza and links to all previous reports related to cases of variant influenza A infections are available on the CDC swine influenza website at http://www.cdc.gov/flu/swineflu/index.htm.

Source: CDC Have You Heard, December 23, 2011