Comparisons of Field Strains of Infections Bursal Disease Viruses for Their Pathogenicity and vp2 Genes*

Four field strains of Infections bursal disease viruses (IBDV) isolated in China during 1997～1999 were compared for their pathogenity and sequences in high variable (hv) region of vp2 genes. It was indicated that GX8/99 was a very virulent IBDV strain (vvIBDV), pathogenicity of other 3 strains was not as the typical vvIBDV but much higher than classical or standard virulent IBDV strains. The hv regions of vp2 of the 4 strains have very high homology with vvIBDV reference strain HK46, 96.8%～99.5% at DNA level and 96.6%～100% at amino acid (aa) level. But their homology with vaccine strain D78 was very low, as only 91.7%～93.6% or 91.8%～93.2% at DNA level or aa level, respectively. It seems like that there was some relationship between the sequences of hv region and pathogenicity. Relatively, the new vvIBDV strain GX8/99 had less homology of 96.8%～97.2% at DNA or aa level to vvIBDV reference strain HK46 and other 3 field strains, than the homology among HK46 and other 3 strains, which were more than 98.4% or 98.6% at DNA or aa level. There was a 100% homology of aa sequence among SD3/98、JS30/99 and HK46. It was demonstrated that the new vvIBDV strains GX8/99 had changed in both pathogenicity and hv region of vp2 gene relative to reference strain HK46 and other field strains.