The genus Cyprinivirus consists of a growing list of phylogenetically related viruses, some of which cause severe economic losses to the aquaculture industry. The archetypal member, cyprinid herpesvirus 3 (CyHV-3) causes mass mortalities worldwide in koi and common carp. A CyHV-3 mutant was described previously that is attenuated in vivo by a deletion affecting two genes (ORF56 and ORF57). The relative contributions of ORF56 and ORF57 to the safety and efficacy profile of this vaccine candidate have now been assessed by analysing viruses individually deleted for ORF56 or ORF57. Inoculation of these viruses into carp demonstrated that the absence of ORF56 did not affect virulence, whereas the absence of ORF57 led to an attenuation comparable to, though slightly less than, that of the doubly deleted virus. To demonstrate further the role of ORF57 as a key virulence factor, a mutant retaining the ORF57 region but unable to express the ORF57 protein was produced by inserting multiple in-frame stop codons into the coding region. Analysis of this virus in vivo revealed a safety and efficacy profile comparable to that of the doubly deleted virus. These findings show that ORF57 encodes an essential CyHV-3 virulence factor. They also indicate that ORF57 orthologues in other cypriniviruses may offer promising targets for the rational design of attenuated recombinant vaccines.

Cyprinivirus属包括越来越多的系统发育相关病毒，其中一些对水产养殖业造成严重的经济损失。原型成员，塞浦路斯疱疹病毒3（CyHV-3）在全世界的锦鲤和鲤鱼中造成大规模死亡。先前描述了在体内减毒的CyHV-3突变体通过影响两个基因（ORF56和ORF57）的缺失。现在已经通过分析针对ORF56或ORF57单独删除的病毒，评估了ORF56和ORF57对该候选疫苗的安全性和功效特征的相对贡献。将这些病毒接种到鲤鱼中表明，ORF56的缺失不影响毒力，而ORF57的缺失导致的衰减与双倍缺失病毒的衰减相当，虽然稍有下降。为了进一步证明ORF57作为关键毒力因子的作用，通过将多个读码框内的终止密码子插入编码区，产生了一个保留ORF57区但无法表达ORF57蛋白的突变体。体内分析这种病毒揭示了与双重删除病毒相当的安全性和有效性。这些发现表明，ORF57编码必需的CyHV-3毒力因子。他们还表明，其他犬肺病毒中的ORF57直向同源物可能为合理设计减毒重组疫苗提供有希望的靶标。