Evolution and Prediction of Influenza Viruses

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On the basis of the results of epidemiological studies of influenza in birds, animals, and humans, we have proposed the route of introduction of the hemagglutinin(HA) gene of the A/Hong Kong/68(H3N2) strain, i.e., from a H3influenza virus circulating in migratory ducks to domestic ducks in Southern China, then to pigs where genetic reassortment occurred with the H2N2 Asian influenza virus to produce the new pandemic strain, A/Hong Kong/68(H3N2).To provide information on the distribution of influenza A viruses in waterfowls at their nesting points in the northern territory, virological survey was carried out in Alaska in summer of 1991 and 1992. From 2,579 fecal samples of birds comprizing 1,913 ducks, 646 geese, 6 swans, 7 dunlins, and 7 gulls, 75 influenza viruses and 82 paramyxoviruses were isolated. Most of the influenza virus isolates were obtained from those of ducks, consisting of 14 H3N8, 47 H4N6, 1 H8N2, 1 H10N2, 11 H10N7, and 1 H10N9 viruses. The antigenic subtypes and isolation rates of the viruses were different in the nesting points of ducks. In 1992, lake water samples were also collected for virus isolation. Out of 23 water samples two H4N6virus strains were isolated from different lakes. This support the notion of the mechanism of perpetuation of influenza viruses in nature, that viruses shed with feces from ducks and have been preserved in frozen lake water in winter in northern territory.Antigenic analysis of the hemagglutinin(HA) of H3 influenza virus isolates using a panel of monoclonal antibodies showed that they were closely related to that of the A/Hong Kong/68(H3N2) influenza virus. The present results also indicate that influenza viruses are maintained and conserved in duck population in their nesting areas. Continued surveillance of influenza viruses in ducks in their nesting areas must provide useful information for prediction of possible new human pandemic influenza virus strains.