Clonorchis sinensis is a digenetic trematode living in the bile passages of man and other mammalian hosts, particularly dogs and cats. The infection is found endemically only in the Far East. Although infection in reservoir hosts is common throughout the Sino-Japanese areas, that in man is confined almost entirely to one restricted area in Japan, to the southern half of Korea, to Kwangtung Province, China, and to Tonkin Province, French Indo-China. Cases found outside these endemic areas have without exception been traced back to them.The Process of Encystment. The life cycle of Clonorchis involves a snail, Bythinia striatula, as first intermediate host and fresh-water fishes as second intermediate hosts. The second larval stage, the cercaria, attacks the fish, attempting to burrow under the scales and into the flesh. During this process it drops its tail and, after penetrating as far as it can into the new host, secretes a viscous fluid which gradually hardens to form a spherical cyst wall, the inner (tr...