Pig Can Serve as a "Mixing Bowl"

Swine are unusual in that they can harbor influenza viruses that are usually confined to birds (bird flu or avian flu) or usually confined to humans, in addition to their own swine strains.

In part, this is because the swine trachea (airway) has receptors for all of these strains. Thus, the pig can serve as a "mixing bowl" in which viruses from different species coexist in close proximity.

This can lead to swapping of genes between the viruses (reassortment), leading to novel strains. The 2009 H1N1 swine influenza strain appears to be a result of genetic reassortment. The 2009 strain has genes that come from avian, swine, and human influenza viruses.

This is referred to as "triple reassortment." In fact, sequencing studies indicate that the swine component of the virus may have components from both the usual North American strain and the Eurasian strain of swine influenza.

This strain has not previously caused infections in humans or pigs. Thus, it is unlikely that humans will have preexisting immunity to this new strain.