First report of turbot circovirus in cultured turbot (Scophthalmus maximus) in Korea

Turbot circovirus (TCV) is a recently identified virus associated with tumor in farmed turbot (Scophthalmus maximus). However, its distribution, pathogenicity, and genetic characteristics remain poorly understood. In this study, we report the first detection and characterization of TCV from diseased turbots farmed in Jeju, Korea, where juvenile stocks are commonly imported from China. Affected fish exhibited external and internal signs, including jaw tumors and co-infections with bacteria and parasites. Real-time PCR targeting the capsid (cap) gene confirmed high viral loads in tumor tissues compared to other organs. Among ten tissues analyzed from TCV-only infected individuals, both kidney and spleen showed relatively high viral loads, suggesting systemic infection. Histopathological examination revealed glomerular atrophy with Bowman's space expansion in the kidney, melanomacrophage centers and lymphoid hyperplasia in the spleen, and hepatic necrosis in the liver. Tumors were histologically diagnosed as fibromas. Phylogenetic analysis based on the cap gene sequence indicated a shared origin between the Korean isolate, TCV-KJ1 and the Chinese strain TurCV10LN-18/2021 due to their clustering together with 99.7 % sequence identity. These findings suggest that TCV is an emerging virus in Korean turbot aquaculture. Continuous surveillance and further investigation into host immune responses under single and co-infection conditions are needed to develop effective disease management strategies.