Morphological and histological characteristics of spontaneous testicular neoplasms in koi carp (Сyprinus carpio var. Koi) and associated changes in the liver and spleen

Describes cases of spontaneous occurrence of testicular neoplasms in four male koi carp (Cyprinus carpio var. koi). Macroscopic and microscopic examination of the tumours and adjacent tissues (liver and spleen) was performed. The size of the neoplasms varied from 8 to 12 cm with a weight of 30– 45 g. Two histological types of neoplastic structure were observed. The first type was characterised by a multilobular structure with dense and moderate cellular areas, pronounced pleomorphism of the cells and the presence of areas of necrosis. The second type was characterised by the presence of parallel strands of pleomorphic cells with large multifocal foci of necrosis and a thick connective tissue capsule. The average size of the nuclei in the first type of tumour was 6.9×5.9 μm (length × width), in the second type 6.8×5.9. In addition, in the tissue of the second type of tumour, clusters of basophilic cells with pronounced karyomegaly were detected, forming multifocal foci in the form of curved strands. In all cases, micro- and macroscopic examination showed no evidence of metastatic development. Based on the results obtained, the neoplasms studied can be classified as germ cell tumours with a predominance of seminoma (gonadoblastoma). No differences in the histogenesis or developmental stages of the studied tumours were observed. Histopathological changes were found in the liver (vacuolization of hepatocytes, fibrosis) and spleen (disruption of the pulp structure, adenomatous formations), probably related to tumour progression. Among the possible aetiological factors of carcinogenesis, the following have been identified: genetic predisposition, housing conditions and the use of gonadotropic drugs.