Continued evolution of H1N1 and H3N2 influenza viruses in pigs in Italy.

Swine influenza viruses possessing avian genes were first detected in Europe in 1979 (Scholtissek et al., 1983, Virology, 129, 521-523) and continue to circulate in pigs in that region of the world. To characterize the molecular epidemiology of swine influenza viruses currently circulating in Europe, we used dot-blot hybridization and sequence analysis to determine the origin of the genes encoding the nonsurface proteins ("internal" genes) of 10 H1N1 and 11 H3N2 swine influenza viruses isolated in Italy between 1992 and 1995. All of the 126 genes examined were of avian origin; thus the currently circulating H3N2 strains which possess A/Port Chalmers/1/73-like surface glycoproteins appear to be descendants of the reassortant human-avian viruses that emerged between 1983 and 1985 in Italy. Sequence analysis of matrix (M), nonstructural, and nucleoprotein genes, as well as phylogenetic analysis of M gene showed that the H1N1 and H3N2 viruses from the pigs were closely related to recent isolates of the avian-like swine H1N1 influenza strain currently circulating in northern Europe and were distinguishable from the genes of viruses isolated from European swine in 1979. To evaluate the frequency of transmission of swine H1N1 and H3N2 viruses to man, we tested 123 human sera for hemagglutination-inhibiting antibodies against avian and mammalian H1N1 and H3N2 virus strains. Our findings indicate that swine influenza viruses possessing A/Port Chalmers/1/73-like hemagglutinin may have transmitted to approximately 20% of young persons under 20 years of age who had contact with pigs. Thus, H3N2 swine viruses, possibly possessing avian-derived internal genes, may be entering humans more often than was previously thought. We strongly recommend that pigs be regularly monitored as a potential early warning system for detection of future pandemic strains.