Visceral granuloma in brook trout (Salvelinus fontinalis)

Brook trout were fed a synthetic diet with suspect diet components added or a meat-meal diet with individual components removed to determine their effect on the incidence of visceral granuloma. Removal or addition of cottonseed meal had the greatest effect on incidence. Gossypol was not the causative agent. The disease is characterized, histologically, by the presence of Langhans-type giant cells and concretions like Schaumann bodies giving an apparent similarity to sarcoidosis in humans. Gross symptoms of the disease are papillary growths of connective tissue on the serosal surface of the stomach, and calcification of the posterior kidney. In severe cases, lesions may be present throughout the visceral mass. Visceral granuloma is similar to nephrocalcinosis in rainbow trout and may be the same disease modified by difference in species response.