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Abstract
Since 1997, novel viruses of three different subtypes and five different genotypes
have emerged as agents of influenza among pigs in North America. The appearance of
these viruses is remarkable because there were no substantial changes in the overall
epidemiology of swine influenza in the United States and Canada for over 60 years
prior to this time. Viruses of the classical H1N1 lineage were virtually the exclusive
cause of swine influenza from the time of their initial isolation in 1930 through
1998. Antigenic drift variants of these H1N1 viruses were isolated in 1991-1998, but
a much more dramatic antigenic shift occurred with the emergence of H3N2 viruses in
1997-1998. In particular, H3N2 viruses with genes derived from human, swine and avian
viruses have become a major cause of swine influenza in North America. In addition,
H1N2 viruses that resulted from reassortment between the triple reassortant H3N2 viruses
and classical H1N1 swine viruses have been isolated subsequently from pigs in at least
six states. Finally, avian H4N6 viruses crossed the species barrier to infect pigs
in Canada in 1999. Fortunately, these H4N6 viruses have not been isolated beyond their
initial farm of origin. If these viruses spread more widely, they will represent another
antigenic shift for our swine population, and could pose a threat to the world's human
population. Research on these novel viruses may offer important clues to the genetic
basis for interspecies transmission of influenza viruses.