Avian Influenza Virus as the Origin of Human Pandemic Strains

In Japan, no influenza epidemics have been recognized in domestic fowls since the last fowl plague outbreak caused by H7N7 influenza virus in chickens in 1924-1926. On the other hand, influenza viruses of different subtypes have been isolated from wild ducks, geese, and swans. Antigenic and genetic analyses of H3 influenza viruses isolated from migratory ducks captured on the Pacific flyway in Japan have shown that the hemagglutinins (HAs)of some of these viruses are closely related to those of the earliest human H3N2 viruses and are highly conserved in nature. I t has also been shown that the HAs of some H3N2 influenza viruses isolated frompigs in southern China are closely related not only to those of the earliest human H3N2 influenza viruses but also to those of the strains from migratory ducks. The HAs of some H3 influenza viruses isolated from domestic ducks in southern China were closely related to those of isolates from migratory ducks and pigs antigenically and genetically. The antigenic and genetic similari ty between these H3HAs indicate that in southern China, the hypothetical influenza epicenter, domestic ducks may have played a role in the introduction of avian influenza viruses to pigs from migratory ducks. Experimental infection of pigs with HI and H3 influenza viruses of different origin showed that the upper respiratory tract of pigs was sensitive not only to the HA of viruses of porcine and human origin but also to that of duck origin. Pigs infected concurrently with avian and porcine virus strains produced genetic reassortants between those. The results indicate that domestic pigs have played a role as a 'mixing vessel', producing a new human pandemic strain by exchange of gene segments between viruses which infect birds and those which infect man. Experimental infection studies of pigs with influenza viruses of avian