Submitted to: International Conference on Emerging Zoonoses
Publication Type: Abstract Only
Publication Acceptance Date: 9/18/2003
Publication Date: 9/18/2003
Citation: Webby, R.J., Lager, K.M., Janke, B.H., Richt, J. 2003. Swine influenza viruses in the United States: role of avian influenza virus gene products in emergence of new swine genotypes. 4th International Conference on Emerging Zoonoses. p. 45.
Technical Abstract: Swine are recognized as important intermediate hosts in the emergence of influenza in humans. They are considered "mixing vessels" for reassortment and amplification of viruses from different species. Distinct viral lineages are found in swine populations around the world. In the United States, swine influenza has historically been an endemic disease caused by classical swine H1N1 viruses. This changed during the winter months of 1998 when H3N2 viruses were isolated from pigs with severe cases of respiratory disease. Two genotypes of reassortant viruses were isolated that contained segments from human and avian influenza viruses: one double reassortant (DR) containing human (HA, NA, PB1) and swine (PB2, PA, NP, M, NS) virus gene segments, and a triple reassortant (TR) containing genes derived from human (HA, NA), swine (NP, M, NS), and avian (PB2, PA) viruses. By the end of 1999 the TR viruses had spread throughout the United States whereas the DR viruses had not. Genetic characterization of swine viruses isolated in 2000 and 2001 has shown that these TR H3N2 viruses are still present and that they have undergone further reassortment with the classical H1N1 viruses. The success of the TR viruses, the disappearance of the DR viruses, and the appearance of H1 reassortant viruses containing the avian PA and PB2 genes suggest some biological advantage is provided by the avian influenza polymerase genes. In order to determine the possible selective advantage conferred by these genes we recreated TR and DR H3N2 viruses and reassortants between the two using a plasmid-based reverse genetic approach. In MDCK cells the TR virus displayed faster growth and a superior yield to the DR virus. The substitution of both the avian PA and PB2 genes together, or the PB2 gene alone, for the DR virus homologues increased the kinetics and yield of growth of this virus to TR-like levels. These in-vitro data suggest that the avian gene components may confer a growth advantage that could in part account for the establishment of novel swine influenza genotypes carrying these avian gene segments.