对4个IBDV野毒株的致病性和它们的vp2基因高变区序列同时做了比较分析。结果表明， 4个IBDV毒株在致病性程度上存在较大差异。其中有一个是真正的超强毒，即GX8/99，其他几个虽然达不到真正超强毒的毒力，但也比经典的标准毒的致死性高得多。在vp2基因高变区，这4个IBDV野毒株与超强毒参考株HK46同源性很高，在DNA水平为96.8%～99.5%,在氨基酸（aa）水平为96.6%～100%。而与疫苗毒D78有很大差异，分别只有91.7%～93.6%和91.8%～93.2%。说明IBDV毒株的vp2基因高变区确实与其致病性有一定关系。特别是SD-1/97、SD-3/98、JS-30/99株之间及其与HK46在DNA和aa水平的同源性高达98.4%和98.6%以上，SD-3/99、JS-30/99株之间及其与HK46的氨基酸同源性为100%。然而,致病性特别高的GX8/99株病毒与其他3个野毒株及HK46在DNA和氨基酸水平的同源性相对较低,在DNA和aa水平的同源性只有96.8%～97.2%和96.6%～97.9%。相对于国内的流行毒株和香港超强毒参考株HK46, GX8/99株在致病性和VP2高变区都已发生了一定的变异。

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.