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The Lanceolate Liver Fluke (Dicrocoelium dendriticum) is a trematode flatworm that sometimes infects humans, causing dicrocoeliasis. It has a complex life cycle, involving two intermediate hosts in addition to the definitive host. Ruminant mammals are the usual definitive hosts for Dicrocoelium dendriticum, although other herbivorous mammals, carnivores, and humans can alse serve as definitive hosts.
Embryonated eggs are shed in feces. The eggs are ingested by a snail. Many snail species may serve as the first intermediate host, including Zebrina spp. and Cionella spp. When the miracidia hatch, they migrate through the gut wall and settle into the adjacent vascular connective tissue, where they become mother sporocysts. The sporocysts migrate to the digestive gland, where they give rise to several daughter sporocysts. Inside each daughter sporocyst, cercariae are produced. The cercariae migrate to the respiration chamber, where they are shed from the snail in a slime ball. After a slime ball is ingested by an ant, the cercariae become free in the intestine and migrate to the hemocoel, where they become metacercariae. A variety of ant species may serve as the second intermediate host, especially members of the genus Formica. After an ant is eaten by the definitive host, the metacercariae excyst (emerge from the cyst) in the small intestine. The worms migrate to the bile duct, where they mature into adults. Humans can serve as definitive hosts after accidentally ingesting infected ants.
Dicrocoelium dendriticum is known from Europe, northern Asia, America, and northern Africa.
For a whimsical but informative look at the life cycle of the Lanceolate Liver Fluke, see this animation from CreatureCast.