Life-History of Liver Fluke

Life-History of Liver Fluke:

Liver fluke’s life-history is an extremely interesting process. Numerous eggs are laid in the bile ducts of the sheep: They are carried to the intestine and passed outside with the faeces. They develop only when they are placed in damp marshy places and if the temperature of the environment is above 50°F. If placed suit­ably a microscopic larva, called miracidium, hatches out of each egg in about ten to twelve days.

The miracidium is multicellular and ciliated. It is provided with a pointed knob at its anterior end, a nerve ganglion, two eyespots, two flame cells and a small gut. If it is hatched in water it swims actively in search of the inter­mediate host, that is the snail Limnaea truncatula. It dies within about eight hours unless it can bore its way into the pulmonary chamber of the snail.

In the soft tissues of the new host the mira­cidium loses its cilia, increases in size and is converted into a sac-like sporocyst. The sporocyst may divide transversely and produces other sporocysts. There are germ cells within the sporocyst each of which can develop by parthenogenesis, that is without fertilisa­tion, into another kind of larva called redia. A sporocyst usually produces four to eight rediae.

A redia is a hollow elongated struc­ture with a mouth and a short gut, a pair of blunt projections near the posterior end and a minute birth pore. Within a short time the rediae burst out of the sporocyst and migrate into the liver of the snail.

A redia has germ cells within its cavity which now develop parthenogenetically to produce other rediae. The daughter rediae escape from the parent through the birth pore. Several generations of rediae are produced in this way during the summer months.

Eventually, the redia produces the fourth type of larva, called cercaria. It has a flat heart-shaped body, with a slender tail which is twice as long, with two suckers and a forked gut.

The cercariae emerge from the redia by the birth pore, work their way out of the snail and swim through water by the lashing of the tail. Very soon it settles upon the blade of a grass, near the water surface, loses its tail, secretes a cyst around itself and becomes a meta- cercaria.

The encysted metacercariae remain alive for several months in damp places, if not subjected to very high temperature or drying. When infected vegetation is eaten by sheep or other suitable final host, the cyst wall dissolves and the larvae burrow through the intestinal wall into the body cavity. The bore their way into the liver in a day or two and are converted gradually into the adult fluke.