The Variation Of Very Virulent Infectious Bursal Disease Virus Gx Strain

One strain vvIBDV Gx was isolated and identified from china according with European and OIE standard. This part research aims at seeking the variation of very virulent infectious bursal disease virus(vvIBDV) in molecule level through the attenuation study.The vvIBDV Gx strain was attenuated through replicating in SPF embryo and CEF. The change pattern of main structure protein VP₃ and no structure protein VP₅ nucleotide and the deduced amino acid sequences were obtained from this strain of vvIBDV to attenuated one during the process of attenuation.Sequence analyses of selected passage viruses here showed that there was no change apparent in the VP₃ gene before CEF-7. A few nucleotide changes in the VP₃ gene but no amino acid substitution by the CEF-8 passage level meant that the homology of amino acid sequence with reference vvIBDV HK46 strains being maintained at 99. 6%. There were many changes were detected in the VP₃ gene of CEF-9 , and there was many substitutions of amino acid. The VP₃ gene sequence remained the same from CEF-10 to CEF-20, the homology of amino acid sequence with reference IBDV attenuated strain P2 was up to 100%. 28H, 163E, 226P, 235V and 250A of VP₃ gene converted to Q, D, L, A and T respectively, at CEF-10. and 28H is peculiar one of vvIBDV Gx. Sequence analyses of selected passage viruses here showed that there was no change apparent in the VP₅ gene before CEF-7. A few nucleotide changes in the VP₅ gene but no amino acid substitution by the CEF-8 passage level meant that the homology of amino acid sequence with HK46 strains remained at 98. 7%. There were many changes of VP₅ gene of CEF-9, and there was some substitutions of amino acid. The VP₅ gene did not change further between CEF-10 and CEF-20, the homology of amino acid sequence with P2 being maintained at 99.3%. 18E, 49R, 78F, 91E, 104G, 122Y, 129P and 137W of VP₅ gene converted to K, G, I, G, C, H, S and R respectively,at CEF-10. 104G and 122Y are peculiar of vvIBDV Gx.So we draw conclusion, during the process of attenuation, vvIBDV converted to attenuated IBDV strains from CEF-8 to CEF-10, through the transition of intermediate, VP2, VP3 and VP5 are possessed vvIBDV molecule characteristic converted to possessed the molecule characteristic similar to other attenuate strains. The trend of virulence is becomes weak gradually, turn into attenuate strain finally. The molecule characteristic was remain stable since CEF-10. One amino acid of VP:i and two amino acid of VP5 are peculiar of vvIBDV Gx. It offers very good basis for identified among vvIBDV and attenuated stains.The studies of the pathogenecity of the Gx strain showed that the levels of mortality induced in 4-week-old SPF chickens by the parent strain was 64% and even at CEF-5, CEF-8, the virus remained very virulent, producing mortality levels of 60%. At the CEF-9 passage level however, pathogenicity was reduced with mortality at 28%, while the virus at CEF-10, CEF-15 and CEF-20 were essentially non-pathogenic. It is felt that the decline in pathogenicity observed here is most likely to be reflecting the genetic molecular features of IBDV. The CEF-20 could not atavistic after propagated 6 generation in SPF chickens. It can be seen from VN tests that the antigenicity does not change significantly during passage in CEF cells.The results of research on CEF-9 molecular biology and biological characteriza- tions shows that: the VP3 and VP5 gene of CEF-9 has some characters of vvIBDV strain as well as some characters of attenuated strain, pathogenicity of CEF-9 is between vvIBDV strain and attenuated strain; and CEF-9 is not stable due to the fact that it could be attenuated in vitro and could rapidly atavistic in vivo. All the results demonstrate that CEF-9 is an intermediate form from high virulence to low virulence during the attenuation.All the results shows that vvIBDV gene is able to change through this attenuated way and vvIBDV could change into an intermediate and an attenuated strain. This has maken materials and theoretical foundation for the further research on the virulence of IBDV.