Fasciola hepatica is a parasite found in sheep and cows and is more commonly known as the sheep liver fluke. These worms can measure up to 30 x 13 millimeters, and may infect the host for up to 10 years. Ironically, the sheep liver fluke resides in its host's bile ducts, not the liver.

An adult liver fluke has an oval, flat body with a cephalic cone that contains the powerful oral suckers it uses to cling to and feeding off of the lining of bile ducts. In addition to producing inflammation, these parasites may release a substance that stimulates collagen production causing fibrosis in the host's tissue; such a condition may proceed to atrophy the liver and lead eventually to cirrhosis. Symptoms of worm infestation can include anaemia, severe weight loss, and low milk yields in a host sheep or cow, possibly leading to its death.

During its life cycle, the liver fluke travels through many environments including an intermediate host before arriving at its final destination, the bile ducts of a definitive host. When worms residing in bile ducts reach sexual maturity, they produce eggs that pass out with the feces. The eggs hatch miracidia, and each miracidium is mobile and small enough to penetrate a snail that may cross its path. Once inside the snail, through a series of steps, the parasite advances to the larval stage of cercaria, thereupon leaving the snail to attach to vegetation and encapsulate itself in a cyst transforming into a metacercaria. The definitive host, a sheep or cow, eats the infected vegetation thereby allowing the fluke to travel through its digestive tract. The metacercaria excysts or exits its cyst inside the small intestine where it burrows through its host's intestinal wall. The juvenile worm then migrates into the liver feeding and growing for about two months before making its way to the bile ducts where it feeds on the lining of the biliary ducts, reaches sexual maturity, produces eggs and restarts the cycle. Diagnosis is arrived at by examination of the feces for the presence of eggs.