Isolation and Identification of Type A Swine Influenza Virus from Shandong Province and Sequence Analysis of HA Gene

10 influenza viruses were isolated from sicked pigs in different places of Shandong province from autumn of 2002 to spring of 2003. They were all identified as Influenza A virus of H9N2 subtype by National Influenza Research Center. The complete HA gene of one isolate Sw/SD/1/2003 (H9N2) was cloned and sequenced and compared with other H9N2 isolates of avian and swine in GenBank. The results indicated that the isolate Sw/SD/1/2003 (H9N2) was similar to Ck/GX/1999 (H9N2) and Ck/YN/2000 (H9N2 ) in HA gene; Sw/SD/1/2003 might come from an avian influenza H9N2 subtype according to the phylogenetic analysis of HA genes; the amino acid sequence at the HA cleavage site in Sw/SD/1/2003 is R-S-L-R-G, but the sequences of all other H9N2 subtypes of porcine isolates and avian isolates are R-S-S-R-G.