Liver fluke is a parasitic disease affecting the liver and bile ducts of a large variety of warm-blooded animals, domestic and wild, and of man, causing enlargement and thickening of the walls of the bile ducts and fibrosis of the liver tissue, and resulting in loss of condition, digestive disorders, anaemia, and other symptoms of parasitism. In sheep, extensive liver damage caused by flukes contributes directly to 'black disease', (infectious necrotic hepatitis), which does not occur in healthy animals. Economic losses arise from emaciation of domestic animals, and from production of liver unfit for human consumption.
Liver fluke is a widespread disease present on all continents. Liver flukes are hermaphroditic and release their eggs into the bile ducts of the host, which are then passed out through the faeces. In the three most common species, the eggs embryonate when they reach water, where they hatch in 2 to 6 weeks and find suitable snails in which to mature. Transmission comes via the snails, from which the fluke larvae are deposited on grass or other vegetation or under the surface of the water, and are then ingested by the host animals. Carnivores or man may become infected from eating the livers of diseased animals, or from water intake. There are many species of liver flukes and since all need intermediate hosts, it is hard to develop control measures which apply to all. Four common species are: the common liver fluke Fasciola hepatica, large American liver fluke Fascioloides magna, giant liver fluke Fasciola gigantica, and lancet fluke Dicrocoelium dendriticum. The lancet fluke differs from the other three in so far as the eggs are eaten by land snails in which they mature into cercariae which escape from the snails and are eaten by ants. The ants in turn are eaten by cattle and other host animals while grazing. Adult flukes have a long life cycle inside the infected animal.