Investigation of Cyprinid herpesvirus-3 genetic diversity by a multi-locus variable number of tandem repeats analysis

Cyprinid herpesvirus-3 (CyHV-3), or koi herpesvirus (KHV), is responsible for high mortalities in aquaculture of both common carp (Cyprinus carpio carpio) and koi carp (Cyprinus carpio koi) worldwide. The complete genomes of three CyHV-3 isolates showed more than 99% of DNA sequence identity, with the majority of differences located in short tandem repeats, also called VNTR (variable number of tandem repeats). By targeting these variations, eight loci were selected for genotyping CyHV-3 by multiple locus VNTR analysis (MLVA). CyHV-3 strains obtained after sequential in vivo infections exhibited identical MLVA profiles, whereas samples originating from a single isolate passaged 6 and 82 times in vitro exhibited mutations in two of the eight loci, suggesting a relatively slow genetic evolution rate of the VNTRs. The method was subsequently applied on 38 samples collected in Indonesia, France and the Netherlands. Globally, the isolates grouped in two main genetic clusters, each one divided in two subgroups including either CyHV-3-U/I or CyHV3-J. Interestingly, Indonesian strains were rather distant from CyHV-3-J isolate. The results of the present study indicate that these VNTR molecular markers are efficient in estimating the genetic diversity among CyHV-3 isolates and are therefore suitable for further molecular epidemiological studies.