Phylogenetic analysis of H1N2 isolates of influenza A virus from pigs in the United States.

Twenty-four H1N2 influenza A viruses were newly isolated from pigs in the United States. These isolates originated from 19 farms in 9 different swine producing states between 1999 and 2001. All farms had clinical histories of respiratory problem and/or abortion. The viral isolates were characterized genetically to determine the origin of all eight gene segments. The results showed that all H1N2 isolates were reassortants of classical swine H1N1 and triple reassortant H3N2 viruses. The neuraminidase (NA) and PB1 genes of the H1N2 isolates were of human origin, while the hemagglutinin (HA), nucleoprotein (NP), matrix (M), non-structural (NS), PA and PB2 polymerase genes were of avian or swine origin. Fifteen of the 24 H1N2 isolates were shown to have a close phylogenic relationship and high amino acid homology with the first US isolate of H1N2 (A/SW/IN/9K035/99). The remaining nine isolates had a close phylogenic relationship with classical swine influenza H1N1 in the HA gene. All other genes including NA, M, NP, NS, PA, PB1 and PB2 showed a close phylogenic relationship with the H1N2 (A/SW/IN/9K035/99) strain and triple reassortant H3N2 viruses. However, PB1 genes of two isolates (A/SW/KS/13481-S/00, A/SW/KS/13481-T/00) were originated from avian influenza A virus lineage. These results suggest that although there are some variations in the HA genes, the H1N2 viruses prevalent in the US swine population are of a similar genetic lineage.