Spontaneous nephroblastoma in a hatchery rainbow trout Oncorhynchus mykiss

Finding of a nephroblastoma in a juvenile rainbow trout Oncorhynchus mykiss is described; the total body length of the fish was 140 mm and weight 45 g.The fish had been fed dry pellets.The tumour was of a considerable size and weight (28.8 % of the fish's body weight) which caused lordosis of the host's spine.The tumour's expansive growth pushed the internal organs deep into the abdominal part of the body.The tumour structure comprises 2 components: epithelial and mesenchymal.Its epithelia1 parts prevailed, no other structures (myeloblastic.neuroblastic and cartilaginous) being present.The tumour was a demarcated formation with no haemorrhages, no necroses and metastases that would penetrate into the organs.No haematopoietic tissue and no melanin were detected in the neoplastic tissue.