Genetic relatedness between influenza A (H1N1) viruses isolated from humans and pigs.

Complete nucleotide sequences were obtained from the nucleoprotein genes of three influenza A viruses and partial nucleotide sequences were obtained from the polymerase, neuraminidase, matrix, and non-structural protein genes of four influenza A viruses that had been isolated between 1931 and 1939 from clinically sick pigs in the United States or Europe. A phylogenetic analysis of the open reading frames of nine nucleoprotein genes showed that the U.S. swine influenza virus isolates from 1931 and 1937 originated from the classic swine viral nucleoprotein lineage, whereas the European swine strains fro 1938 and 1939 were placed among the early human influenza virus nucleoprotein lineage. All the partial gene sequences obtained from the two European swine strains fro 1938 and 1939 were also more closely related to early human H1N1 reference strains than to the U.S. swine isolates from 1931 and 1937, indicating that none of the four viruses isolated from swine had acquired genes from a heterologous lineage through reassortment.