Fasciola Hepatica

F. hepatica

          Hosts : Most mammals, sheep and cattle are the most important.

          Intermediate host :

          Snails of the genus lymnaea. The more common – L. truncatula

Site :

          The adults are found in the bile ducts and the immature flukes in the liver parenchyma.

Occasionally aberant flukes become encapsulated in other organs such as the lungs.

Distribution : Worldwide

Indentification :

Gross : The young fluke at the time of entry into the  liver is 1.0-2.0 mm in length and lancet – like. When it has become fully mature in the bile ducts it is leaf – shaped , grey brown in colouur and is around 3.5 cm in colour and 1.0 cm in width. The anterior end is conical and marked off by distinct shoulders from the body. (Fig. 75-Gm-Urquhart)

Microscopic :

          The tegument is covered with backwardly projecting spines.

An oral and ventral sucker may be readily seen. The egg is oval, operculate , yellow and large (150mm × 90 µm)

Life – cycle :

          Eggs passed in the feces of the mammalian host develop and hatch releasing motile ciliated miracidia . This takes nine days at optimal temperatures of 22-26⁰C and little development occurs below 10⁰C .

          The liberated miracidium has a short life-span (1-3 days) and must locate a suitable snail within three hours.

In infected snails, development proceds through the sprocyst and the radial stages to the final stages in the intermediate host, the cecaria, these are shed from the snail as motile forms which attach themselves to firm surface such as grass blades, and encyst there to form the infective metacecariae. It takes a minimum of 6-7 weeks for completion of development from miracidium to metacecaria, although under favourable circumastances a period of several months is required. Infection of a snail with one miracidium can produce over 600 metacecariae. Meta cecariae ingested by the final host excyst in the small intestine,migrat thoug th gut wall,cross the peritoneum and penetrate the liver capsule. The young flukes tunel through the parenchyma or 6-8 weeks, then enter the small bile ducts where they migrate to the larger ducts and of occasionally the gall bladder. The prepatent period is 10-12 weeks.

The minimal period for completion of one entire life cycle of F.hepatica is therefore 17-18 weeks.

The longevity of F.hepaticain untreated sheep may be years; in cattle it is usually less than one year.