Analysis of the influenza virus gene pool of avian species from southern China.

Although Southern China has been considered the epicenter of human influenza pandemics, little is known about the genetic composition of influenza viruses in lower mammals or birds in that region. To provide information on the molecular epidemiology of these viruses, we used dot blot hybridization and phylogenetic methods to study the internal genes (PB1, PB2, PA, NP, M, and NS) of 106 avian influenza A viruses isolated from a total of 11,798 domestic ducks, chickens, and geese raised in Southern China including Hong Kong. All 636 genes examined were characteristic of avian influenza viruses; no human or swine influenza genes were detected. Thus, influenza virus reassortants do not appear to be maintained in the domesticated birds of Southeast Asia, eliminating opportunities for further gene reassortment. Phylogenetic analysis showed that the internal genes of these viruses belong to the Eurasian avian lineage, supporting geographical separation of the major avian lineages. The PB1 genes were most similar to A/Singapore/57 (H2N2) and Hong Kong (H3N2) viral genes, supporting an avian origin for the recent human H2N2 and H3N2 pandemic strains. The majority of internal genes from avian influenza viruses in Southern China belong to the Eurasian lineage and are similar to viruses that have recently been transmitted to humans, swine, and horses. This study provides evidence that the transmission of avian influenza viruses and their genes to other species is unidirectional and that the transmission of mammalian influenza virus strains to domestic poultry is probably not a factor in the generation of new pandemic strains.