Recent H3N2 swine influenza virus with haemagglutinin and nucleoprotein genes similar to 1975 human strains

¹ Section de virologie, Faculté de médecine vétérinaire, Université de Montréal, Saint-Hyacinthe, Québec J2S 7C6
² Département de microbiologie, Faculté de médecine, Université de Sherbrooke, Sherbrooke, Québec J1H 5N4
³ Département de biologie, Faculté des sciences, Université de Sherbrooke, Québec J1K 2R1
and⁴ Influenza Reference Laboratory, Laboratory Centre for Disease Control, Ottawa K1A 0L2, Canada

Of the four pandemic strains of human influenza A virus observed this century, the 1977 virus strain was very similar in all genes to a 1950 isolate. Since mammalian influenza A viruses change annually by genetic drift, this reappearance could only be attributed at that time to conservation of the virus in a frozen state. We report here the isolation of swine influenza A viruses with haemagglutinin and nucleoprotein genes which are virtually identical to those of the human virus that circulated in 1975. We have also found serological evidence that this virus is circulating extensively in Quebec swine herds. We propose that human-like H3N2 influenza A strains may remain invariant for long periods in swine, which may serve as a reservoir for human pandemics.