Origin and Evolution of Influenza Viruses

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To provide information how influenza A viruses are maintained among waterfowl reservoirs at their nesting areas in the northern territory and on the mechanism of perpetuation of the viruses in nature, virological surveillance of avian influenza was continued in Alaska during their breeding season in summer from 1993 to 1994. Phylogenetic analyzes of the genes of the isolates were then carried out.Influenza viruses were isolated mainly from fecal samples of dabbling ducks in their nesting places in central Alaska. From 541 fecal samples of ducks, 33 influenza viruses and 15 paramyxoviruses were isolated. The numbers of subtypes of influenza viruses consisted of 23 H3N8,8H4N61H2N3, and 1H7N3. Together with the results of our previous surveillance study, which was also supported by Grant-in-Aid for Scientific Research 03041013 by the Ministry of Education, Science and Culture in 1991 and 1992, the total number of influenza virus isolates from fecal samples is 108 comprising of 1 H2N3,37 H … More 3N8,55 H4N6,1H7N3,1 H8N2,1 H10N2,11 H10N7, and 1 H10N9 strains. These results indicate that influenza viruses have been maintained in duck population in the major breeding areas of central Alaska and that different subtypes of influenza A virus cocirculate.Influenza viruses were also isolated from water samples of the lakes where the ducks nest. Even in autumn of 1994 when they had left for migration to south, viruses were still isolated from the lake water. The present results support the notion that influenza viruses have been maintained in waterfowl population by water-borne transmission and revealed the mechanism of year by year perpetuation of the viruses in the lakes where they breed.Phylogenetic analysis of the NP and H3 HA genes of the representative isolates showed that they belong to the North American lineage of avian influenza viruses, suggesting that the majority of the waterfowls breeding in central Alaska migrate to North America and not to Asia. Thus phylogenetic analysis of various isolates from waterfowls made it possible to presume the migratory flyways of host birds carrying viruses. Less