一株鸡新城疫强毒引起非典型发病的机理探讨

Mechanism of Atypical Newcastle Disease in Layers Caused by a Virulent Strain

对 1株从不表现鸡新城疫临床症状 ,只引起产蛋下降的鸡群中分离的新城疫野毒株 (PMV_CO2 )开展了研究。经测定 ,该毒株的颅内接种致病指数 (ICPI)为 1.85 ,静脉接种指数 (IVPI)为 2 .42 ,确定为强毒型。对该病毒的融合蛋白裂解基因的核酸序列进行了测定 ,其推测的F1/F2裂解位点附近的氨基酸序列为112 R_R_Q_K_R116,这一序列附合典型鸡新城疫强毒112 R/K_R_Q_K/R_R116,的普遍规律 ,确定该毒株不是超强毒株。为进一步了解该病毒对鸡的非典型发病的原因 ,我们通过人工发病方法对不同ND抗体水平的产蛋鸡进行了攻毒试验。结果该毒株致使ND抗体阴性或抗体水平极低的蛋鸡 (HI效价 1∶5 )发病致死并呈现典型的ND病症和病理变化 ,低抗体水平鸡 (HI效价 1∶10～ 1∶40 )呈现一过性产蛋停止 ,并在 2周后恢复 ,未表现出典型的ND症状 ,高水平抗体鸡 (HI效价 1∶80～ 1∶32 0 )则未见异常。从而推断抗体抑制为引起本病毒非典型ND发病的原因而非毒株变异引起。

A field Newcastle disease(ND)virus was isolated from a layer flock that only showed egg dropping and without any typical ND symptom was studied.ICPI and IVPI are 1.85 and 2.42 respectively.It means this isolate is virulent strain.The nucleotide sequence was analyzed and the deduced amino_acid sequence near cleavage site is 112 R_R_Q_K_R 116 .It follows the rule from the former work on the most virulent NDV stains,which is 112 R/K_R_Q_K/R_R 116 .We think it is not a variant strain. For understanding the reason why this strain could cause atypical ND,we implemented an artificial infection experiment that three different ND antibody levels' layers were used and challenged with this isolate.The results showed that the layers with low HI titer of ND(HI titer 1∶20～40)only reveal 2 weeks egg drop stop and no typical ND symptom was found.The layers with high HI titer(HI titer 1∶80～320)did not show any symptom of disease during experiment.The layers with very low titer or negative antibody layers(HI titer1∶5)were dead by attacked this virus.We concluded that antibody impression is the reason to cause atypical ND in this case, not caused by variant virus.