Classification: ICD-9 121.3; ICD-10 B66.3
Syndromes and synonyms: Sheep liver fluke disease, pharyngeal fascioliasis.
Agent: Large trematode liver flukes living in blile ducts: Fasciola hepatica and Fasciola gigantica. F. hepatica is 20 to 30 mm long and F. gigantica can be up to 75 mm long.
Reservoir: Sheep, cattle, water buffalo, and other large herbivores. Occasionally humans.
Vector: Freshwater (pond) snails (Lymnaeidae).
Transmission: Accidental ingestion of metacercariae via contaminated water, watercress, or other contaminated plants (lettuce, alfalfa juice, etc.). Rarely via consumption of raw sheep or goat liver (pharyngeal fascioliasis). There is no person-to-person transmission.
Cycle: Eggs hatch in water and release miracidia larvae that penetrate the snail, where they develop to cercariae that encyst on aquatic plants (e.g. watercress) and become desiccation-resistant metacercariae. After the plants are eaten by herbivores (e.g. sheep, cattle), or water containing metacercariae is drunk, the larvae pass through the intestinal wall into the peritoneal cavity, enter the liver, and lay eggs in the bile duct, which are then excreted in the feces. The whole cycle takes 3 to 4 months.
Incubation period: 3 to 4 months, but is highly variable.
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