"A few years later, a new avian-origin influenza A (H1N1) virus was introduced in the European pig population from wild ducks.34 By 1979, this strain had largely replaced the classical North American A/H1N1 strain."
Zimmer & Burke 2009
Historical Perspective — Emergence of Influenza A (H1N1) Viruses
http://www.nejm.org/doi/full/10.1056/NEJMra0904322

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"Swine influenza viruses possessing avian genes were first detected in Europe in 1979 (Scholtissek et al., 1983, Virology, 129, 521 – 523) 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"
Continued Evolution of H1N1 and H3N2 Influenza Viruses in Pigs in Italy
https://ac.els-cdn.com/S0042682297985147/1-s2.0-S0042682297985147-main.pdf