Cottonseed meal may be related to the prevalence of the disease, because the signs can be produced by adding cottonseed meal to a chemically defined diet that is not known to produce visceral granuloma. In other studies, individual deletion of the various dry components of a meat and meal diet indicated that each of these components caused the disease to some degree. Information now available strongly suggests a basic relation between the nephrocalcinosis of rainbow trout and mineral balance. Although water chemistry may influence the occurrence of nephrocalcinosis and visceral granulomas in salmonids, an imbalance of dietary calcium, phosphorus, and magnesium is now believed to be the basic cause. In advanced cases, granulomas cause hypertrophy of the kidney and spleen and occur with increasing severity in most other organs.
Crystalline deposits disappear from the chronic lesions.
Cause of the Disease Prevalence of systemic granuloma in gilthead bream is correlated with long-term storage of formulated feeds, fish meal, or frozen fish. Alcohol extraction or autoclaving of a disease-producing feed did not affect the production of systemic granulomas. Related species that do not appear to concentrate tyrosine in this way do not develop systemic granuloma. It thus seems that excess tyrosine is a sign, but not a cause. All ages of animals appear susceptible and mortality can be high.