Development of a PCR assay to detect cyprinid herpesvirus 1 in koi and common carp

Cyprinid herpesvirus 1 (CyHV1) infects all scaled and color varieties of common carp Cyprinus carpio, including koi. While it is most often associated with unsightly growths known as 'carp pox,' the underlying lesion (epidermal hyperplasia) can arise from a variety of disease processes. CyHV1-induced epidermal hyperplasia may occur transiently in response to water temperature, and thus histopathology cannot be used in isolation to assess CyHV1 infection status. To address this problem, here we describe a PCR assay targeted to the putative thymidine kinase gene of CyHV1. The PCR assay generates a 141 bp amplicon and reliably detects down to 10 copies of control plasmid DNA sequence (analytic sensitivity). The PCR does not cross-detect genomic DNA from cyprinid herpesvirus 2 and 3 (analytic specificity). The CyHV1 PCR effectively detected viral DNA in koi and common carp sampled from various locations in the UK, USA, Brazil, and Japan. Viral DNA was detected in both normal appearing and grossly affected epidermal tissues from koi experiencing natural epizootics. The new CyHV1 PCR provides an additional approach to histopathology for the rapid detection of CyHV1. Analysis of the thymidine kinase gene sequences determined for 7 PCR-positive carp originating from disparate geographical regions identified 3 sequence types, with 1 type occurring in both koi and common carp.