Evidence for interspecies transmission and reassortment of influenza A viruses in pigs in southern China.

The Asian/57, Hong Kong/68, and Russian/77 pandemics of this century appeared or reappeared in China. Interspecies transmission and genetic reassortment of influenza viruses have been implicated in the origin of these human pandemic influenza viruses. Pigs have been suspected to be the "mixing vessel" where reassortment occurs. To investigate this possibility, 104 porcine influenza viruses collected at random from Southern China from 1976 to 1982, including 32 H3N2 isolates and 72 H1N1 isolates, were studied using dot blot hybridization, partial sequencing, and phylogenetic analysis. There were 29 of 32 H3N2 isolates characteristic of viruses originally derived from humans; the other 3 isolates were reassortants containing genes from porcine and human influenza viruses. Phylogenetic analyses of the polymerase B1 (PB1) genes showed that interspecies transmission from humans to pigs has happened multiple times in pigs in Southern China. All 72 H1N1 isolates were of porcine origin characteristic of classical porcine H1N1 influenza virus. Analysis of 624 genes of porcine influenza viruses from Southern China failed to detect any evidence for avian influenza virus genes. This contrasts to what is currently found in Europe, where the majority of porcine influenza virus isolates are of avian origin.