Isolation and genetic analysis of human origin H1N1 and H3N2 influenza viruses from pigs in China

Influenza A viruses of subtypes H1N1 and H3N2 have been reported widely in pigs, associated with clinical disease. These mainly include classical swine H1N1, avian-like H1N1, and human-like or avian-like H3N2 viruses. From 2005 to 2006, we carried out swine influenza virus surveillance in eight provinces of China. Here we report, for the first time, the isolation and genetic analysis of a human-like influenza H1N1 virus from a pig in a farm of Guangdong province of southern China, a host suspected to generate new pandemic strains through genetic reassortment. Each of the eight gene segments is of human origin. Phylogenetic analysis indicates that these genes form a human lineage, suggesting that this virus is the descendant of recent human H1N1 influenza viruses. In addition, four swine H3N2 viruses were also isolated. The three H3N2 viruses from Guangdong province are descendants of recent human viruses, while an H3N2 virus from Heilongjiang province derives from early human viruses. Isolation and genetic analysis of human H1N1 and H3N2 influenza viruses from pigs in China provides further evidence about the interspecies transmission of human influenza viruses to pigs and emphasizes the importance of reinforcing swine influenza virus surveillance.