Pandemic influenza: a zoonosis?

In the last two decades, influenza A viruses have been found to occur throughout the animal kingdom, mainly in birds, notably aquatic ones, in which infection is largely intestinal, waterborne, and asymptomatic. The domestic duck of southern China, raised in countless numbers all year round mainly as an adjunct to rice farming, is the principal host of influenza A viruses. Studies based on Hong Kong H3N2 viruses from southern China suggest that pandemic strains originate from the domestic duck there and are transmitted to humans via the domestic pig, which acts as a "mixing vessel" for two-way transmission of viruses. This provides further support for the hypothesis that the region is a hypothetical influenza epicenter. Rural dwellers in the epicenter show serological evidence of contact with non-human influenza A viruses. Two hypotheses are advanced for the range of hemagglutinin (HA) subtypes of viruses that can cause pandemics (1) circle or cycle limited to H1, H2, and H3 subtypes, thereby implying that a virus of the H2 subtype will cause the next pandemic; and (2) spiral, by which any one of the 14 HA subtypes recorded to date may be involved. Consideration is given to the temporal and geographical factors and range of hosts, namely the duck, pig, and human, that need to be submitted to virus surveillance in China and beyond to attempt to anticipate a future pandemic. Evidence is presented that points strongly to pandemic influenza being a zoonosis.