Genetic relatedness between A/Swine/Iowa/15/30(H1N1) and human influenza viruses

The nucleotide sequences of the M and NS1 genes of influenza virus A/Swine/Iowa/15/30 (A/SW/IW/30)(H1N1) were determined with cloned DNAs and compared with reported sequences of human and avian influenza viruses. A/SW/IW/30 virus was found to be closely similar to A/PR/8/34(H1N1) virus in the nucleotide sequences of the M and NS1 genes, the base differences between the two strains being 64 out of 1027 nucleotides in the M gene and 52 out of 740 in the NS1 gene. Based on the assumptions that these two viruses were derived from a common ancestor and that the rate of base changes per year was the same in man and in swine, it was estimated that the progenitor virus was in circulation during the period from 1915 to 1920. This estimation was compatible with the epidemiological findings suggesting that the progenitor of the swine influenza virus was the agent of the 1918 influenza pandemic. Furthermore, the M and NS1 gene sequences of A/FPV/Rostock/34(H7N6) virus were much closer to those of A/SW/IW/30 and A/PR/8/34 viruses than to A/duck/Alberta/60/76(H12N5) virus, but not as close as the A/SW/IW/30 virus was to A/PR/8/34 virus.