Abstract

Aflatoxicosis in rainbow trout was a significant economic problem during the 1960's after dry pelleted foods containing a high level of seed oil came into use. It was described as a powerful carcinogen responsible for the widespread occurrence of hepatocarcinoma in cultured trout. Aflatoxin is a mycotoxin produced by strains of the blue-green fungi Asperigillus flavus. Aflatoxin B1 is a common contaminant of oilseed crops such as cottonseed and groundnut meal, corn and to a lesser extent, wheat, sunflower and soybean. Alfatoxin B1 is known as one of the most potent naturally occurring animal carcinogens.¹ Aflatoxin levels as low as 0.01ppb can induce neoplastic changes in trout over a short period.

Aflatoxicosis is now rare in the rainbow trout industry due to strict regulations enforced by the FDA for aflatoxin screening in oilseeds, corn and other feed ingredients that are able to grow A. flavus. Because of their frequent occurrence, especially in the Southeastern United States, agricultural commodities are routinely monitored for the presence of aflatoxins. Although there are no official tolerance levels for mycotoxins, the FDA allows a maximum of 20ppb in feeds or feed ingredients in interstate commerce.¹ However, in countries where there are no strict regulations, occurrences of aflatoxicosis causing hepatic carcinomas have been on the rise especially in the Tilapia industry. In tropical conditions this is a major problem because of difficulties in storage of oil seeds under humid conditions. Temperature (> 80°F) and humidity (moisture levels >14-15%) are two major environmental factors that promote the growth of the fungi.

Reported pathological changes generally include poor growth, anemia, impaired blood clotting, damage to liver and other organs, decreased immune response and increased mortality. Prolonged feeding of low concentrations of B1 cause liver tumors which may appear as pale yellow, well-vascularized nodular lesions contrasting sharply with the reddish brown color of normal tissue. A second type of tumor can be found that is grayish white, translucent and smooth in appearance. These eventually become multinodular, large necrotic tumors.² In tilapia culture, the toxin is associated with renal tubular carcinoma, lymphoma and hepatoma.³

References

1.  Jacobsen, B. J., K. L. Bowen, R. A. Shelby, U. L. Diener, B. W. Kemppainen, and J. Floyd. 1993. Mycotoxins and Mycotoxicosis. Alabama Cooperative Extension System. Alabama University press Circular ANR-767.

2.  Ashley, L. M. 1970. Pathology of fish fed aflatoxins and other antimetabolites. In: A symposium on diseases of fishes and shellfishes. AFS special pub. No 5. Washington D.C.

3.  Haller R. D., and R. J. Roberts. 1980. Dual neoplasia in a specimen of Saratherodon spilurus spilurus (Gunther) (Tilapia spilurus ) J. Fish Dis. 3: 63-66.