In July, 1973, it was observed that many young carps in many farm ponds in Nagano, Yamagata and Akita prefectures were infected with a kind of cestode belonging to the genus Bothriocephalus. About 100 cestode specimens of different stages were collected from the intestines of 4 fishes (body weight : 20.7-31.6g). Ten of the largest size group among the mature worms obtained were examined morphologically and identified with B. opsariichthydis YAMAGUTI, 1934. The criteria for identification were as follows.: 1) As regards the shape and the structure of the body, the present species resembled to B. acheilognathi YAMAGUTI, 1934. B. opsariichthydis YAMAGUTI, 1934, B. fluviatilis YAMAGUTI, 1952 and B. gowkongensis YEH, 1955 which had all been found in the cyprinoid fish in Japan and China. 2) The present species differed from B. gowkongensis in the point that the egg was never embryonated when laid. 3) As regards the body length the present species (17.3 cm in average) was rather close to B. opsariichthydis (more than 10 cm long according to YAMAGUTI), but differed from B. acheilognathi (about 8 cm) and B. fluviatilis (2.2 cm). 4) It was thought that B. acheilognati and B. fluviatilis were young stages of B. opsariichthydis.
On September 27, 1973, 40 young carps of 4 months old were collected at random from a pond at Shioda in Nagano prefecture and examined for the presence of B. opsariichthydis YAMAGUTI, 1934 in their intestines. Nineteen fishes were found to harbour 1-57 worms (19.7 worms in average). The incidence rate was 47.5%. The intestine carrying many worms was extremly swollen and the yellowish-white worms inside were observable through the intestinal wall which became thin and trancelucent. Observations of cross sections of the infected intestine revealed that most of the tips of the villi intestinales were seriously destroyed. This destruction was thought to be induced by “a rasp effect” caused by lasting oppression and movement of the strobillae. At the same time, marked congestion was observed in the lamina propria mucosae and the tela submucosa, and coagulated materials including tissue debris were found on the surface of the villi intestinales. Moreover, hemorrhage, hemolysis or vesiculation was found to occur in various parts of the lamina propria mucosae and the tela submucosa. No deaths were reported from the above-mentioned farm pond, and it was said that affected fish continued to take foods actively, though the histopathological changes of the intestine of infected fish were fairly serious.