Persistence of a 1930 swine influenza A (H1N1) virus in Quebec.

Two antigenically distinct H1N1 influenza A viruses were isolated during an outbreak of respiratory disease in Quebec swine in 1990/91. Analysis of haemagglutinin and partial nucleoprotein sequences indicated that one was a variant of the swine H1N1 influenza virus circulating in the American Midwest whereas the other was very similar to virus isolated from swine in 1930. The existence of this latter isolate supports the concept that influenza viruses can be maintained for long periods in swine, perhaps in geographically limited pockets. Serological evidence indicates that these distinct strains continued to circulate widely in south-central Quebec until at least 1993.