Life cycle of the parasitic Chinese Liver Fluke (Clonorchis sinensis), the cause of clonorchiasis in humans
The life cycle of the Chinese Liver Fluke (Clonorchis sinensis) involves an intermediate snail host. Embryonated eggs are discharged in the biliary ducts and stool (1) of a human host. If these eggs are ingested by a suitable snail intermediate host (2), the eggs release miracidia (2a), which go through several developmental stages (sporocysts (2b), rediae (2c), and cercariae (2d)). The cercariae are released from the snail and after a short period of free-swimming time in water, they may come in contact with and penetrate the flesh of freshwater fish. Here they encyst as metacercariae (3). Infection of humans occurs by ingestion of undercooked, salted, pickled, or smoked freshwater fish (4). After ingestion, the metacercariae excyst in the duodenum (5) and ascend the biliary tract through the ampulla of Vater (6). Maturation takes approximately one month. The adult flukes (which measure 10 to 25 mm by 3 to 5 mm) reside in small and medium-sized biliary ducts. In addition to humans, carnivorous animals can serve as reservoir hosts.