Genetic characterization of H3N2 influenza viruses isolated from pigs in North America, 1977-1999: evidence for wholly human and reassortant virus genotypes

Since 1998, H3N2 viruses have caused epizootics of respiratory disease in p igs throughout the major swine production regions of the U.S. These outbrea ks are remarkable because swine influenza in North America had previously b een caused almost exclusively by H1N1 viruses. We sequenced the full-length protein coding regions of all eight RNA segments from four H3N2 viruses th at we isolated from pigs in the Midwestern U.S. between March 1998 and Marc h 1999, as well as from H3N2 viruses recovered from a piglet in Canada in J anuary 1997 and from a pig in Colorado in 1977. Phylogenetic analyses demon strated that the 1977 Colorado and 1997 Ontario isolates are wholly human i nfluenza viruses. However, the viruses isolated since 1998 from pigs in the Midwestern U.S. are reassortant viruses containing hemagglutinin, neuramin idase and PBI polymerase genes from human influenza viruses, matrix, non-st ructural and nucleoprotein genes from classical swine viruses, and PA and P B2 polymerase genes from avian viruses. The HA proteins of the Midwestern r eassortant swine viruses can be differentiated from those of the 1995 linea ge of human H3 viruses by 12 amino acid mutations in HAI. In contrast, the Sw/ONT/97 virus, which did not spread from pig-to-pig, lacks 11 of these ch anges. (C) 2000 Elsevier Science B.V. All rights reserved.