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.
Immature eggs are discharged in the biliary ducts and in the stool.
After development in water, each egg releases a miracidium which invades
a suitable snail intermediate host.
In the snail the parasites undergo several developmental stages
(sporocysts, rediae, and cercariae).
The cercariae are released from the snail and encyst as metacercariae on
aquatic vegetation or other surfaces.
Mammals acquire the infection by eating vegetation containing
metacercariae.
After ingestion, the metacercariae excyst in the duodenum and migrate
through the intestinal wall, the peritoneal cavity and the liver parenchyma into the
biliary ducts, where they develop into adults.
Fasciola hepatica infect various animal species, mostly
herbivores.
Humans can become infected by ingesting metacercariae-containing
freshwater plants, especially watercress.
In humans, maturation from metacercariae into adult flukes takes
approximately 3 to 4 months.
