Swine infection with H9N2 influenza viruses in China in 2004.

In 2004, H9N2 influenza A viruses were isolated from pigs with respiratory syndrome in commercial swine farms in Henan province, China. Antigenic and genetic characterization were performed for seven swine H9N2 influenza viruses. The hemagglutinin antigenicity of swine H9N2 viruses was similar to those of avian H9N2 viruses of A/duck/Hong Kong/Y280/1997 (Dk/HK/Y280/97)-like sublineage prevalent in China. It is noteworthy that the neuraminidase of these isolates had no deletions in the stalk, which was seldom observed in those viruses of Dk/HK/Y280/97-like sublineage. Genetic analysis revealed that all seven isolates had an -R-S-S-R- motif at the HA cleavage site, which was the same as those of Dk/HK/Y280/97-like viruses established in avian population in China. Phylogenetic analyses showed that the seven swine H9N2 viruses were completely derived from avian influenza viruses of Dk/HK/Y280/97-like sublineage. The present results indicated that avian-to-pig interspecies transmission of H9N2 viruses continued to exist in China through 2004; therefore, surveillance of swine influenza should be given a high priority.