Kidney Diseases:
Neoplasm
Primary renal tumours in fish are rather uncommon, while secondary tumours of the kidney occur more frequently. The pathologist should have in mind that ectopic thyroid tissue may be found in several different locations, including the head kidney, and may produce functional thyroid tumours in aberrant locations. Secondary tumours resulting from spread from other tissues/organs to the kidney include fibromas and fibrosarcomas The most commonly encountered primary renal tumour in fish is probably the embryonic nephroblastoma (Wilms' tumour) developing from pluripotential blast-type cells. Such tumours are quite frequently seen in farmed rainbow trout, but may also be found in other species. Structures that may be recognized in such tumours include glomeruli and tubules in different stages of development and differentiation, connective tissue and cartilage. These tumours are frequently visible as they protrude from the abdominal surface. Teratomas have been identified with the same gross appearance as above, but composed of several tissues like e.g. gill filaments and cartilage, i.e. tissues not normally found in that organ. Tumours arising from haematopoietic tissue (kidney or spleen) include lymphosarcoma that may be found as ulcerating nodules in the skin and muscle, in later stages also in spleen, liver and kidney.

Nephroblastoma from a cultured mature rainbow trout.