GENETIC RELATEDNESS BETWEEN INFLUENZA-A (H1N1) VIRUSES ISOLATED FROM HUMANS AND PIGS

Complete nucleotide sequences were obtained from the nucleoprotein gen es of three influenza A viruses and partial nucleotide sequences were obtained from the polymerase, neuraminidase, matrix, and non-structura l protein genes of four influenza A viruses that had been isolated bet ween 1931 and 1939 from clinically sick pigs in the United States or E urope. A phylogenetic analysis of the open reading frames of nine nucl eoprotein genes showed that the U.S. swine influenza virus isolates fr om 1931 and 1937 originated from the classic swine viral nucleoprotein lineage, whereas the European swine strains from 1938 and 1939 were p laced among the early human influenza virus nucleoprotein lineage. All the partial gene sequences obtained from the two European swine strai ns from 1938 and 1939 were also more closely related to early human H1 N1 reference strains than to the U.S. swine isolates from 1931 and 193 7, indicating that none of the four viruses isolated from swine had ac quired genes from a heterologous lineage through reassortment.