The relationship of hepatoma in rainbow trout to aflatoxin contamination and cottonseed meal.

Summary A test diet was formulated to approximate the commercial feed associated with the 1960 trout hepatoma epizootic in California. The same diet without the cottonseed meal portion served as the control. A third ration with food grade cottonseed flour substituted for the cottonseed meal was also tested. The trout on the 2 diets with the cottonseed products showed a high incidence of hepatoma. These results confirm previous observations associating the development of hepatoma in trout with the feeding of cottonseed meal. Chemical analyses relate the occurrence of hepatoma to aflatoxin B1 in the cottonseed components of the test diets. These observations provide cumulative evidence that the hepatoma epizootic in California trout resulted from aflatoxin contamination of cottonseed meal ingredient. Confirmation of the carcinogenicity of the cottonseed flour sample was obtained in a second experiment using a semipurified ration. Initial analysis of the cottonseed flour detected no aflatoxin. Subsequently an improved method detected 2 parts per billion of aflatoxin B1; this is equivalent to 0.4 ppb in the experimental diet. Results from other studies indicate that the naturally occurring cyclopropene fatty acids and gossypol in the cottonseed components probably enhanced the carcinogenic action of the minute quantity of aflatoxin.