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Number of Swine Origin H3 Infections in Pennsylvania Rises to Three

The cases of human infection with swine-origin H3N2 influenza resulted from existing influenza viruses exchanging genetic material through a process called “reassortment.” Reassortment typically occurs when a host – animal or human – becomes infected with two or more different influenza viruses at the same time. This allows the influenza viruses to mix and exchange genetic information with each other, which in turn, can result in the emergence of new influenza viruses. Because pigs can be infected with and spread influenza viruses from birds, pigs and humans, they can represent a source for influenza virus reassortment to occur. This is particularly true in environments where humans, pigs and birds come into close contact with one another, such as farms.

The above graphic depicts how the human cases of swine-origin H3N2 influenza virus reported in Indiana and Pennsylvania resulted from the reassortment of two different influenza viruses. The image on this page shows three influenza viruses placed side by side, with eight color-coded RNA segments inside of each virus. Note: All influenza viruses contain 8 RNA segments. These RNA segments are labeled HA (hemagglutinin), NA (neuraminidase), PB1, PB2, PA, NP, M and NS.

The virus in the center of the picture represents the human cases of swine-origin H3N2 influenza in Indiana and Pennsylvania. Seven of these viruses’ eight RNA segments are derived from influenza viruses that commonly circulate in pigs. The virus on the left of the diagram, labeled “1998-2011 swine H3N2 triple reassortant viruses,” represents these swine influenza viruses. These swine viruses are called triple reassortant viruses because they contain genetic material from humans, pigs and birds.

Arrows between the left virus and center virus in the diagram show which seven of the eight RNA segments found in H3N2 triple-reassortant swine influenza viruses (represented by the left virus) are shared with the human cases of swine-origin H3N2 reported in Indiana and Pennsylvania (center virus).

The virus image on the right in the diagram represents the 2009 H1N1 virus that caused the pandemic which began in the spring of 2009. The human cases of swine–origin H3N2 influenza found in Indiana and Pennsylvania share one RNA segment, the M segment, with this 2009 H1N1 virus, as reflected by the red arrow placed next to the M segment between the right and center viruses in the diagram. No other RNA segments are shared between these two viruses, as denoted by the X’s next to each RNA segment between the right and center viruses in the diagram.

A key at the bottom of the diagram explains the color-coding scheme for each of the RNA segments. Purple-coded RNA segments represent human origin HA (hemagglutinin) and NA (neuraminidase), which are genetically and antigenically different from those of current human H3N2 viruses. The center virus and left virus in the diagram contain purple-colored, HA and NA segments. Blue–coded RNA segments represent PB1 segments that came from humans. All three viruses have PB1 segments that came from humans. Green-coded RNA segments represent segments that came from birds in North America. All three viruses have green PB2 and PA segments. Yellow–coded RNA segments represent segments that came from classical North American swine. The NP and NS segments from all three viruses are yellow. The M segment within the left virus in the diagram is also yellow. And lastly, red-coded RNA segments represent segments that came from Eurasian swine. The M segment in both the right virus and the center virus in the diagram are red.