Life cycle of the trematode parasites Fasciola hepatica and F. gigantica
Immature eggs are discharged in the biliary ducts and in the stool (1). Eggs become embryonated in water (2) and release miracidia (3), which invade a suitable snail intermediate host (4), including snails in the genera Galba, Fossaria, and Pseudosuccinea. In the snail, the parasites pass through several developmental stages (sporocyst (4a), redia (4b), and cercaria (4c)). The cercariae are released from the snail (5) and encyst as metacercariae on aquatic vegetation or other surfaces. Mammals acquire the infection by eating vegetation containing metacercariae. Humans can become infected by ingesting metacercariae-containing freshwater plants, especially watercress (6). After ingestion, the metacercariae excyst in the duodenum (7) and migrate through the intestinal wall, the peritoneal cavity, and the liver parenchyma into the biliary ducts, where they develop into adults (8). In humans, maturation from metacercariae into adult flukes takes approximately 3 to 4 months. The adult flukes (Fasciola hepatica: up to 30 mm by 13 mm; F. gigantica: up to 75 mm) reside in the large biliary ducts of the mammalian host. Fasciola hepatica infect a variety of mammals, but mostly herbivores.