The Attenuation of vvIBDV Gx Strain

Very virulent infectious bursal disease virus (vvIBDV) Gx strain with high pathogenicity was attenuated through replicating in SPF embryo and CEF. The change pattern of VP2 nucleotide and the deduced amino acid sequences were obtained from this strain of vvIBDV,during the process of attenuation. The results of sequencing , analyzing of every generation viruses showed that there was no changes on VP2 gene before 7th generation, there was a few changes in VP2 gene but not on amino acid at 8th generation , the homology of amino acid sequence with European reference vvIBDV strain was up to 100%;The virus of 9th generation was an intermediate with complex changes. 10th generation had become cell_adapted strain, the homology of amino acid sequence with European reference IBDV attenuated strain Cu_1 was up to 97%. VP2 gene did not changed any more from 10th to 20th generation. The results of pathogenicity tests showed that the original (Gx) was typical vvIBDV with the mortality up to 64%, the virus of 5th generation was still vvIBDV with the mortality up to 60% in 4_week_old SPF chickens, while the virus of 20th generation was no pathogenicity and could not atavistic after propagated 6 generation in SPF chickens.