Fascioliasis is caused by two species of parasitic flatworms or trematodes that mainly affect the liver. It belongs to the group of foodborne trematode infections and is a zoonosis, meaning an animal infection that may be transmitted to humans.

The two species of trematodes that cause fascioliasis (F. hepatica and F. gigantica) are leaf-shaped worms, large enough to be visible to the naked eye (adult F. hepatica measure 20–30 mm x 13 mm; adult F. gigantica measure 25–75 mm x 12 mm). The disease they both cause is similar.

Until recently, human cases occurred occasionally but are now increasingly reported from Europe, the Americas and Oceania (where only F. hepatica is transmitted) and from Africa and Asia (where the two species overlap). WHO estimates that at least 2.4 million people are infected in more than 70 countries worldwide, with several million at risk. No continent is free from fascioliasis, and it is likely that where animal cases are reported, human cases also exist.

The life-cycle of fascioliasis is complex. It involves a final host (where the adult worm lives), an intermediate host (where the larval stages of the worm develop) and a carrier (entailing suitable aquatic plants). The process starts when infected animals (cattle, sheep, buffaloes, donkeys and pigs but also horses, goats, dromedaries, camels, llamas and other herbivores) defecate in fresh-water sources. Since the worm lives in the bile ducts of such animals, its eggs are evacuated in faeces and hatch into larvae that lodge in a particular type of water snail (the intermediate host).

Once in the snail, the larvae reproduce and eventually release more larvae into the water. These larvae swim to nearby aquatic or semi-aquatic plants, where they attach to the leaves and stems and form small cysts (metacercariae). When the plants with the small cysts attached are ingested, they act as carriers of the infection.

Watercress and water-mint are good plants for transmitting fascioliasis, but encysted larvae may also be found on many other salad vegetables. Ingestion of free metacercariae floating on water (possibly detached from carrier plants) may also be a possible mode of transmission.