Submitted to: Journal of General Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/12/2010
Publication Date: 9/1/2010
Publication URL: vir.sgmjournals.org/cgi/rapidpdf/vir.0.021402-0v1.pdf
Citation: Ma, W., Lager, K.M., Lekcharoensuk, P., Ulery, E.S., Janke, B.H., Solorzano, A., Webby, R.J., Garcia-Sastre, A., Richt, J.A. 2010. Viral reassortment and transmission after coinfection of pigs with classical H1N1 and triple reassortant H3N2 swine influenza viruses. Journal of General Virology. 91(Pt 9):2314-2321. Interpretive Summary: Swine influenza is one of the top three disease concerns for the US pork industry. It is caused by swine influenza virus (SIV), a virus that has the ability to swap genes between different viruses if a pig is concurrently infected with two or more SIV isolates. This process is called reassortment. A pig experiment evaluating influenza virus reassortment was conducted by simultaneously infecting a group of pigs with two different SIV isolates, one an H3N2 virus, and the other, an H1N1 virus. Reassortment did occur within the lungs producing a number of distinct SIVs. However, only one virus was transmitted from the infected pigs to contact pigs that were comingled in the same pen. The contact pigs were moved to a separate isolation room and the same virus was transmitted from them to a second group of contact pigs. The transmitted virus was identical to the H3N2 virus used to infect the pigs. This finding supports the assumption that despite the formation of unique SIVs through a reassortment process within the lung, the most fit virus, or the virus that transmits most efficiently among pigs, will become the dominant virus. As might be expected, in this study the most fit virus was the H3N2 virus since it can induce a more severe disease than the H1N1 virus. Moreover, the H3N2 virus was more fit than any of the reassortant viruses that were produced in the experimentally infected pigs. Results of this study demonstrate reassortants can easily be produced and helps explain why there are many different SIV isolates in the field.
Technical Abstract: Triple reassortant swine influenza viruses circulating in North American pigs contain the internal genes derived from swine (NP, M, NS), human (PB1) and avian (PA and PB2) influenza viruses forming a constellation of genes that is well conserved and called the triple reassortant internal gene (TRIG) cassette. In contrast, the external genes (HA and NA) are less conserved reflecting multiple reassortant events that have produced viruses with different combinations of HA and NA genes. We hypothesize that the maintenance of the TRIG cassette confers a selective advantage to the virus. To test this hypothesis, pigs were co-infected with the triple reassortant H3N2 A/Swine/Texas/4199-2/98 (Tx/98) and the classical H1N1 A/Swine/IA/15/1930 viruses and co-housed with a group of sentinel animals. This direct contact group was subsequently moved into contact with a second group of naive animals. Four different subtypes (H1N1, H1N2, H3N1 and H3N2) of influenza virus were identified in bronchioalveolar lavage fluid collected from lungs of experimentally-infected pigs with most of the viruses containing the TRIG from the Tx/98 virus. Interestingly, only the intact Tx/98 H3N2 virus was transmitted from the infected pigs to the direct contact group, and from them to the second contact group. These results demonstrate that multiple reassortments can occur within a host; however, only specific gene constellations are readily transmissible. It is concluded that certain HA and NA gene pairs, in conjunction with the TRIG cassette, may have a competitive advantage over other combinations for transmission and maintenance in swine.