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An influenza virus strain isolated in Northern Italy in 1977 was identified as belonging to the H3N2 subtype (A/swine/Italy/1850/77). A close antigenic relationship to the human strain A/England/42/72 was demonstrated. Serologic surveys of 548 pig sera from 53 farms in two provinces in Northern Italy demonstrated the existence of antibodies to the swine/Italy/1850/77 strain and to A/Hong Kong/1/68, A/Victoria/3/75 and H 1 N 1 (Hsw 1 N 1) strains in the pig population.

To investigate the possible mechanism of maintenance of old human influenza A (H3N2) viruses in pigs, the haemagglutinins (HAs) of seven isolates from swine were studied by analysis of nucleotide and deduced primary amino acid sequences, as well as reactivity of the HA molecule to chicken antisera and monoclonal antibodies. The swine HAs were closely similar to the HA of the A/Victoria/3/75 human variant as regards antigenic and molecular characteristics. These findings are consistent with the hypothesis that the swine HA genes were transmitted from an early human H3 virus to pigs, where they survived with limited mutations over a period of 5 years, The sequence data were also compared with swine H3 sequences to investigate genetic relationships between the H3 genes from swine viruses isolated in different geographical areas. An evolutionary tree, constructed from the nucleotide sequences of viruses isolated from pigs in China and in Italy, illustrated that, depending on the country of their isolation, the HA genes of swine influenza A (H3N2) viruses have different origins, e.g. human and avian, and evolved independently in different lineages. The study provides direct support for the hypothesis that pigs might serve as a 'mixing vessel' for the generation of pandemic strains of human influenza viruses.