Observations on the Occurrence off Hepatomas in Rainbow Trout

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Abstract
Rainbow trout reared in hatcheries throughout the United States suffer from an epidemic of primary liver cancer, which in some instances seems to affect practically 100 percent of the trout population over 3 years of age. The hepatocarcinomas apparently grow rather slowly and are not rapidly killing the trout, since large tumor masses may exist in the liver without seriously interfering with the general well-being of these animals. The carcinomas may directly invade adjacent skeletal muscle and form metastases in the spleen and kidney. Among the various causal agents suspected, virus infection and nutritional factors related to the recent introduction of processed commercial feed have attracted the main attention. The bulk of available evidence at present incriminates some constituent, or constituents, in the feed as a nutritive factor, a chemical additive, or a medicinal agent. The occurrence of this epidemic among edible fish subjected to an artificial nutritive regimen provides a serious warning of the possible future production of a similar cancer epidemic in the human population through an increasing contamination of the human environment with some of the many industry-related carcinogenic chemicals.