Excystment phenomena in clonorchis sinensis.

Abstract

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 (true) cyst capsule. The presence of the encapsulated larva in the tissues of the fish, and the excretion of its waste products into the host's tissues causes the latter to lay down an outer false wall which is fibrous in nature and adheres firmly to the inner capsule. The process is comparable to the encystment of trichina larvæ that have migrated into the striped muscles of mammals.

Japanese investigators have found that various cyprinoid fishes are involved in this phase of the Clonorchis life cycle, but our studies have shown that practically any fresh-water fish which is exposed to the infection may serve as the host of the encapsulated larvae. While the encysted larvz are usually found some distance beneath the epidermis, a certain proportion fails to penetrate the superficial layers of the fish and becomes encapsulated either on the epidermis or attached to the under side of the scales.