CASE STUDY: THE LIVER FLUKE (Fasciola hepatica)
THE liver fluke is a parasitic flatworm. A parasite is an organism which lives in or on another organism known as the host causing it harm. Endoparasites such as worms live inside the organism and usually become problematic after turnout while ectoparasites which live on the outside typically manifest themselves at housing.
Unusually, the liver fluke requires two hosts to complete its life cycle — a primary host and a secondary host. The adult flukes most commonly live in the liver and bile ducts of the primary host mainly cattle and sheep, but other hosts include pigs, goats, horses and rabbits.
The parasite greatly inhibits the functioning of the liver and affected animals suffer weight loss resulting in an emaciated appearance. Liver fluke may also increase the susceptibility of the host animal to Salmonella infection, also known as Salmonellosis.
Although liver fluke occurs nationwide, the highest prevalence is in the West. The liver fluke is hermaphrodite, but does not possess a circulatory system or coelom. It has suckers, which aid attachment to its host and a cuticle, which protects it from the host’s defences.
Life Cycle of the Liver Fluke
Fertilised eggs are usually passed to the exterior via the faeces of the host.
Under suitable conditions, mainly moisture and warmth (temperature > 10.3oC a small ciliated larva called the miracidium emerges from the egg.
The miracidium will die within hours unless it finds the secondary host ie the mud-snail. Once inside the snail, it develops into a sporocyst which through a simple cell dividing process becomes the redia.
Each redia further divides giving rise to cercariae which resemble miniature tadpoles. The cercariae leave the pond snail (which then dies) and eventually come to rest on grass where they lose their tails and become encysted, waiting to be eaten by the final host — grazing livestock mainly cattle and sheep. Control measures against liver fluke include the following 1. Efficient pasture management including effective drainage and adequate fencing. 2. Biological control through the introduction of a predator to control the mud-snail — in this case ducks, geese or pheasant. 3. Spraying molluscicides to kill the snail. 4. Dosing animals to kill the adult flukes.