Genetic variation in swine influenza virus A isolate associated with proliferative and necrotizing pneumonia in pigs.

A new antigenic variant of H1N1 swine influenza virus A (Sw/QC/5393/91 [QC/91]) has been found to be associated with porcine proliferative and necrotizing pneumonia. Analysis of its genomic RNA by T1 oligonucleotide mapping revealed that considerable genomic divergence exists between QC/91 and the swine influenza viruses currently circulating in North American swine herds. Analysis of the nucleotide sequence of the HA1 region of the hemagglutinin RNA of QC/91, in comparison with those of most common H1N1 human and swine influenza A viruses, showed the presence of multiple point mutations. Two amino acid substitutions appeared to be located in antigenic sites Sb and Ca. This correlates with antigenic variations demonstrated between A/NJ/8/76, A/Sw/WI/49/76, and Québec isolate A/Sw/QC/5393/91 of swine influenza virus A. Another mutation was responsible for the loss of a glycosylation site, which may have also affected the antigenicity. The other mutations seem to have been accumulated progressively over time. This significant constancy in the fixation of mutations with time suggests that genetic diversity of these viruses may best be interpreted as the result of drifts in the population of circulating swine influenza viruses in Québec.