Abstract
Naturally diseased amago (Oncorhynchus rhodurus f. macrostomus) with no external symptoms other than marked petechiae on the gill and acute death were histopathologically examined. The following histopathological changes were observed. 1) Micro infection in the epithelium of a gill lamella, 2) bacterial embolism in gill lamellae which, causing disturbance in blood circulation as well as the proliferation of branchial epithelial cells, further developed to involve the gill-filamental artery, 3) metastatic lesions in the heart with well developed bacterial colonies, marked tissue destruction and mononuclear infiltration, and 4) metastatic lesions in the liver, the spleen and the kidney with either small of large bacterial colonies and with marked tissue destruction.
From these histopathological observations we concluded that with this type of diseased fish Aeromonas salmonicida entered the fish through branchial epithelium. We further suspected that, as an infected lesion in the gill developed into the initial septicemic lesion and a metastatic lesion in the heart into the secondary one, there appeared general dissemination, which later induced the secondary septicemia.
