1. In the human host, the adult worm releases embryonated eggs through the biliary ducts into the intestine. They are excreted in feces. 2. Eggs are ingested by a snail (the intermediate host); > 100 species of snails can act as intermediate hosts. Each egg releases a miracidium (the first larval stage), which develops into a sporocyst, then a redia, then a cercaria. 3. Cercariae are released from the snail, and after a short period of free swimming in water, they come in contact with and penetrate the flesh of freshwater fish, where they encyst as metacercariae. 4. Humans are infected by eating undercooked, salted, pickled, or smoked freshwater fish. 5. After ingestion, the metacercariae excyst in the duodenum. 6. They then enter the common bile duct through the ampulla of Vater and migrate to smaller intrahepatic ducts (or occasionally the gallbladder and pancreatic ducts).
Image from the Centers for Disease Control and Prevention Image Library.