This presentation describes a tumor in the posterior kidney of an adult red oscar, Astronotus ocellatus.

The fish was a pet which shared a 55 gallon aquarium with a plecostomus catfish. The water quality parameters were normal and the owner noted that the fish had been healthy for years until its condition began to deteriorate several months before its death. The onset of illness appeared to coincide with feeding the oscar some small fishes which were captured in a local canal in Naples, Florida.

Approximately three months prior to death the fish began to show abdominal distension. The owner brought the fish to a local pet store where the proprietor advised withholding food for two weeks. No improvement was noted and the fish became depressed, debilitated and began listing to one side. Upon presentation 60 milliliters of dark brown sero-sanguinous transudate was aspirated from the caudoventral abdomen. Chloramphenicol (50.0 mg/kg) and dexamethasone (2.0 mg/kg) were administered intramuscularly but the fish died 12 hours after presentation.

A post mortem revealed an enlarged, fluid-filled posterior kidney. All other internal organs appeared normal on gross inspection. Histologically, no normal kidney tissue was present. Mildly pleiomorphic epithelial cells were forming acini varying in size and shape. Islands of large, foamy, mildly pleiomorphic macrophages containing large amounts of granular pigmented material were scattered throughout the tissue. Some of this material stained positively for hemosiderin. This histologic appearance is consistent with an adenoma or well differentiated adenocarcinoma, either of renal or thyroid origin. Ectopic thyroid tissue is present in the kidneys of many fish.

In addition to a detailed discussion of the above case study the incidence and types of renal tumors encountered in various species of tropical fish will be addressed.