Genetic characterization of H3N2 influenza viruses isolated from pigs in North America, 1977-1999: evidence for wholly human and reassortant virus genotypes
Ai. Karasin et al., Genetic characterization of H3N2 influenza viruses isolated from pigs in North America, 1977-1999: evidence for wholly human and reassortant virus genotypes, VIRUS RES, 68(1), 2000, pp. 71-85
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.