Investigation of a Cyprinid Herpesvirus 1 Disease Episode in a Group of Pond-Reared Koi

Fifteen adult koi (a variant of Common Carp Cyprinus carpio) simultaneously developed white cutaneous proliferations affecting up to 30% of their bodies. The onset of these lesions (in September 2014) was associated with their return to a remodeled backyard water garden after temporarily being maintained in a plastic swimming pool. A single water temperature taken during the outbreak read 21°C on November 17, 2014. The water garden had no extrinsic heat source, with average ambient temperatures ranging from 9.4 to 26.4°C during the outbreak (September 2014-January 2015). Representative skin biopsies were obtained from two fish; the histologic features included severe epidermal hyperplasia, dysplasia, keratinocyte apoptosis, decreased and haphazardly distributed goblet cells with the absence of club cells, keratinocyte hydropic degeneration, and moderate infiltration by lymphocytes and eosinophilic granular cells. Ultrastructural findings included intranuclear nonenveloped hexagonal nucleocapsids and abundant cytoplasmic-enveloped virions morphologically consistent with the Alloherpesviridae family. Polymerase chain reaction was performed on formalin-fixed, paraffin-embedded shavings from the two biopsied koi targeting the thymidine kinase gene of cyprinid herpesvirus 1 (CyHV-1). Together with the aforementioned findings, these results are consistent with an outbreak of CyHV-1 in a population of adult koi.