Horse Liver as the Causative Factor in White Blindness of Hatchery Brook Trout

White blindness, a dietary disease peculiar to brook trout, was previously reported by the author as having appeared in the early spring of each year from 1946 through 1949 at certain fish hatcheries throughout Michigan. Brown and rainbow trout were not affected by the disease. White blindness is characterized by a very pale body color, and a milky opaqueness of the crystalline lens of the eyes which renders the fish blind. Analysis of the diets at hatcheries where the malady appeared, and where it was absent, strongly suggested that horse products were the cause of it, but the diversity of products included in the diets made it impossible to determine whether one or all horse products were involved. The present experiments were conducted in an effort to determine which horse product was responsible for producing white blindness. Cooked horse skeletal muscle, horse hearts, and horse livers each mixed with pork melts on a 50–50 basis were tested. Brewerˈs yeast at a 5 percent level was added to the diet of one group of fish receiving each diet. White blindness was first observed on December 19, 1949, among the group of brook trout fed the horse liver‐pork melt diet continuously since June, 1949, and increased in incidence until the experiment was terminated on May 3, 1950. No symptoms of the disease were present among trout fed any other diet in the experiment. Results of this experiment indicate that horse livers may safely comprise up to 50 percent of the diet of brook trout during summer months but will cause white blindness when used at that level during prolonged periods of low water temperatures.