Abstract
At an outbreak of collective injury among rainbow trout in a hatchery in Iwate Prefecture, 96 cases were examined pathologically. After comparing the pathological findings made in the cases of the affected fish and the surviving ones, it was confirmed that the injury was caused by octomitiasis produced by infection with the Octomitus protozoa in the intracellular stage. The principal results obtained are as follows: 1. A large number of epithelial cells of the pyloric ceca and the anterior end of the intestine were infected with protozoan cysts, which caused damage to the epithelial lining, such as perforation, necrobiotic or necrotic change, and desquamation. Infiltration of eosinophilic leucocytes was also noted in the submucosa of the affected areas. 2. Severe damage of the epithelial lining was found also in the middle and posterior portions of the intestine where the invasion of the cyst was extremely slight. Besides, some inflammatory processes, such as congestion and leucocyte infiltration, were noted in the submucous layer. These changes were considered to be due to the effect of a debris of epithelial cells shed from the anterior portion of the intestine. 3. Necfobiotic changes were confirmed in the hepatic parenchyma in a fairly large number of affected fish. 4. The stomach wall was invaded by a sort of fungi in most of the affected fish, and it was increased in thickness due to the proliferation of connective tissue and macrophages as well as eosinophilic leucocytes. 5. From the present study the authors are convinced that octomitiasis may occur also in Japan and, under certain conditions, cause serious damage among young trout.
