AMINO ACID CHANGES IN VP5 OF TWO INFECTIOUS BURSAL DISEASES VIRUSES ORIGINATING FROM A COMMON SOURCE WITH DIFFERENT VIRULENCE

Infectious bursal disease virus (IBDV) causes a highly contagious immunosuppressive disease in chickens by targeting developing B- lymphocytes. In this study an IBDV was isolated and purified in BGM-70 cell line from an outbreak. The virus was passaged seven times in BGM-70 cell line and ten times in SPF chickens to obtain two viruses with low and high virulence, respectively. The VP5 gene of the in vivo and in vitro passaged viruses were amplified, cloned and sequenced. Comparison of the predicted amino acid sequences revealed that changes were only in residues at positions 80,124 and 125. Amino acids at positions 80,124 and 125 were Gly, Ala and Ser in in vitro passaged virus whereas these were Arg, Thr and Pro in in vitro passaged virus, respectively. Comparison of the VP5 amino acid sequences among these viruses, attenuated and virulent IBD viruses showed that only amino acid change at position 125 (Ser to Pro) might have a role in pathogenicity of the IBD virus. To the best of our knowledge it is the first report for existence of virulence amino acid marker in VP5 of IBDV.