Influenza infection in humans and pigs in southeastern China

The three last pandemic strains of influenza A virus -Asian/57, Hong Kong/68 and Russian/77 - are believed to have originated in China. The strains responsible for the 1957 and 1968 human pandemics were reassortants incorporating both human and avian influenza viruses, which may have arisen in pigs. We therefore undertook a population-based study in the Nanchang region of Central China to establish the prevalence, types and seasonal pattern of human influenza infection and to screen serum samples from animals and humans for evidence of interspecies transmission of influenza viruses. Two definite influenza seasons were demonstrated, one extending from November to March and the other July to September. The profile of antibodies to commonly circulating human influenza viruses was no different in Nanchang and neighboring rural communities than in Memphis, Tennessee, USA. In particular, Chinese women who raised pigs in their homes were no more likely to have been exposed to influenza virus than were subjects who seldom or never had contact with pigs. However, we did obtain evidence using isolated H7 protein in an enzyme-linked immunoabsorbent assay for infection of pig farmers by an avian H7 influenza virus suggesting that influenza A viruses may have been transmitted directly from ducks to humans. The results of the serological survey also indicated that pigs in or near Nanchang were infected by human H1N1 and H3N2 influenza viruses, but not with typical swine viruses. We found no serological evidence for H2 influenza viruses in humans after 1968.