| Avian influenza (AI) is infection and/or syndrome of poultry caused by influenza A viruses while highly pathogenic avian influenza (HPAI) is an extremely contagious systemic disease of poultry leading to high morbidity and mortality caused by some H5 and H7 subtypes of influenza A viruses. The natural host for influenza A virus is believed to be wild waterfowls, gulls and shorebirds. Domestic ducks are a great pool of AIVs and considered as an intermediate host between wild waterfowls and land-based poultry. It s possible that AIVs of H5 subtype causing outbreaks of high morbidity and mortality in chickens in China be directly derived from domestic waterfowls. In this study, we investigated the distribution of AIVs in domestic ducks in Eastern China during 2002 and 2003. Moreover, we characterized the pathogenicity and transmission of the representative AIVs of H5 subtype isolated from chickens, ducks and geese . and analysed the sequences of HA and NA genes of these viruses. In addition, phylogenetic relationships among the HA genes of 26 AIVs of H5 subtype isolated from different poultry species during 2000 and 2003 were analysed.1 . Distribution of avian influenza viruses of different HA subtypes among domestic ducks in Eastern ChinaTo determine the status of influenza A viruses circulation and distribution in domestic ducks in China, virological monitoring for influenza A viruses was performed. In July and August of 2002, 673 duck cloacal swabs were collected from five provinces of Eastern China. Additional 930 duck cloacal swabs were collected from Yangzhou live bird market (LBM) once a month from December 2002 to November 2003. All 1603 cloacal swabs were tested for hemagglutinating viruses by inoculating embryonated hens eggs. Each sample underwent at least two passages, and influenza A viruses were identified by AGP and HI test. Hemagglutinin subtypes of these isolates were characterized by HI test and RT-PCR with subtype specific pairs of primers. A total of207 avian influenza viruses were isolated from 1603 duck cloacal swabs. Of those isolates, there were eight HA subtypes, including HI, H3, H4, H5, H6, H9, H10 and H11. The number of viruses in each subtype showed that the H5 subtype avian influenza viruses were the predominant subtype(81/207), followed by H3 subtype isolates (63/207). Only two viruses of H9 subtype were isolated in this investigation. The isolation rate in Winter and early Spring months (from January to March 2003, 56.4%, 40.0% and 40.5% , respectively) was by far higher than that in other seasons , while the isolation rate in Summer was the lowest (less than 6.0%). The isolation rate in other months was between 8.0% and 14.7%. The isolation rates of H5 subtype viruses were 42.6%, 16.2% and 30.4% respectively in January, February and March of 2003. Much less H5 subtype viruses were isolated in other months. Therefore, circulating season of H5 subtype viruses was similar to that of all AIVs in general. H3 subtype viruses almost could be isolated every month, but the isolation rate in Spring and Autumn months was higher than in other seasons s. The prevalent season of AIVs in domestic ducks in China was the winter and early spring in contrast the summer was prevalent season for AIVs in wild aquatic fowl in other parts of the world. Our findings speculated that H5 subtype viruses circulating in the domestic ducks might be related to outbreaks of H5N1 HPAI in chickens in winter and spring in China.2. Characterization of H5 subtype avian influenza viruses isolated from ducks, geese and chickens from Eastern ChinaThe isolation rate of H5 subtype avian influenza viruses is very high in domestic ducks in Winter and early Spring, while H5N1 HPAI outbreaks frequently occurs in chickens in the same season in China. To demonstrate the possibility that H5 viruses prevalent in chickens were directly derived from domestic waterfowls, we characterized six representative H5 viruses isolated from healthy dometic ducks, geese and diseased chickens in the same period and location. These viruses w...
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