Fasciola hepatica

Phylum : Plalyhelminthes.

Class : Trematoda

    Fasciola hepatica is commonly known as liverfluke.It is a parasitic trematode seen in sheep,cattle and other mammals.It resides in the bile duct of sheep.It is a digenetic parasite .The defenite host is sheep and intermediate host is a gastropod namely  Lymnae.The body is soft, leaf-like, bilaterally  symmetrical and dorso-ventrally flattend.The size varies from 1.0 mm to 2.5 cm in length.The anterior end of the body has a conical  projection called the head lobe.

    At the tip of the head lobe is the mouth surrounded by the oral or anterior sucker . On the ventral surface behind the head lobe is the posterior or ventral sucker (acetabulum). It is bigger in size than the  anterior sucker. Between the two suckers and close to the posterior sucker is the genital porehe excretory opening is situated at the posterior tip of the body.

    Their body is covered by a tegument, a  peculiar kind of epidermal arrangement. The tegument consists of an outer syncytial layer and  an inner layer where the main cell bodies are found. The tegument serves the functions of protection, excretion and gaseous exchange. Beneath the the tegument there are circular, longitudinal and oblique muscles.The digestive system consists of the mouth, a pharynx and a bifurcated intestine, which is highly branched. The excretory system consists of longitudinal excretory canals  and the associated flame cells.Fasciola reproduces sexually .

    The adults are hermaphrodites, but cross fertilization is the rule. The fertilized eggs leave the vertebrate host through faeces and urine. Each egg hatches out as free swimming, ciliated larva called miracidium . This larva happens to penetrate the body wall of an aquatic snail that comes in contact with it. Once inside the snail, the miracidium metamorphoses into a sac like sporocyst without any gut. The sporocyst contains several embryos. Each embryo develops into another sporocyst or into a redia (after the Italian Scientist, Redi), which has a mouth and a gut. Within the redia are several embryos, which develop into cercaria. The cercaria has a gut, suckers and a tail. The cercaria leaves the snail host and attach itself to green leaves. it becomes encysted to form a metacercaria. When a sheep feeds on such infested green leaves, metacercaria enters the gut. The metacercaria now escapes from the cyst (excysts), migrates to the bile duct and grows into the adult.

Since the life cycle includes two hosts and at least two infective stages, Fasciola is usually termed as a digenetic trematode (belonging to the subclass Digenea).

Polyembryony

During the transition from miracidium to sporocyst, polyembryony occurs. The embryo repeatedly divides and gives rise to numerous sporocysts of similar kind.Polyembryony is an adaptation of parasitic mode of life. This ensures prolific breeding and maintenance of sufficient number of the parasite through the different stages of parasiticlife.

Pathogenicity

The disease caused by the infection of Fasciola hepatica is known as fascioliasis.lt mainly affects the liver but it also causes inflammation of the bile duct causing loss of its epithelium. The bile duct soon becomes calcified and gall stones are formed in it. Heavy infections cause haemorrhage. The young flukes undertake a migration through the liver tissue, during which the liver is extensively damaged. The liver function is seriously affected and the sheep is said to have the disease "liver rot". The affected sheep becomes dull and sluggish. This is followed by swelling and pain in abdomen, weight loss, inflammation of liver and finally death ofthe sheep.

Prevention

· Avoid grazing of sheep on the banks of ponds and streams, especially during drought season when the cercariae emerge from the snails.

· Sacrificing heavily infected sheep and destroying the parasites will help a lot in preventing the transmission of the parasite.

· Destruction of the intermediate host (snail) by appropriate means, is also an effective control measure. Snails can be killed by the application of copper sulphate in ponds and ditches.

· Allowing ducks to forage on the paddy fields and ponds can help in checking the population of snails.

· Proper washing of green vegetables and appropriate cooking will eliminate the chances of transmission of the parasite to man

· Fasciola hepatica can be eliminated from the final hosts by the use of anthelmintics such as carbon tetrachloride, emetine hydrochloride tetrachlorethane and hexachloroethane.