The haemagglutinins (HAs) of five H3 influenza A viruses isolated from domestic ducks and one from a goose in southern China were analysed antigenically and genetically. The patterns of reactivity of two of the duck viruses and the goose virus with a panel of monoclonal antibodies to 10 different epitopes on the H3 HA were similar to those of influenza viruses isolated from wild ducks and pigs, as well as those of the earliest human H3 viruses. The other three isolates from domestic ducks were different from each other and from these viruses antigenically. Sequence analysis revealed that the HA genes of the two duck viruses and the goose virus were closely related to those of isolates from wild ducks and pigs; the identities between the deduced amino acid sequence of the HA of one of the isolates from domestic ducks and those of isolates from a wild duck and a pig were 98·7% and 99·5%, respectively. The antigenic and genetic similarity between these H3 HAs suggests that in southern China, the hypothetical influenza epicentre, domestic ducks may have played a role in the introduction of avian influenza viruses to pigs from feral ducks. The findings also support the hypothesis that the pig was a ‘mixing vessel’, producing a new human pandemic strain, A/Hong Kong/68 (H3N2), by genetic reassortment.


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