Summary
A malignant tumor, measuring 27 x 18 x 12 mm, was reported on the first branchial arch of an Amazonian fish (Cichlidae, Chastobranchus semifasciatus Steindachner). The tumor had an irregular epithelioid covering and a dense interior packed with nuclei and fibers. Patches of pigment cells and areas of cartilage were also seen. The tumor was classified as an epithelio-chondroma. A small (3 mm) satellite tumor, having the same composition, was found on the same gill, and 19 metastatic growths on the gill filaments of both sides were also found. These consisted of 4 with multiple branching of the filament, 4 with terminal knobs and 11 with subterminal swellings. Cancerous cells and metacercariae of a trematode (Heterophyidae, Ascocotyle sp.) were present together in 18 of these sites. It was concluded that a relationship between the cancer and the trematode existed in this case. Since destruction of the metacercarial cyst wall by the cancerous cells was observed, it was presumed that this process could proceed to the point of destroying, or displacing, the trematode as well. The absence of helminths in the principal tumors could be explained in this way. It was suggested that the invasion of the parasites, in the presence of some genetic anomaly in the host, could have caused the carcinoma.