Lecture notes on Life cycle of Fasciola hepatica

The life cycle of Fasciola hepatica:Fasciola  is a digenetic parasite and the life cycle completed within two hosts. The primary hosts, is the sheep, while the intermediate host, is a snail. This type of life cycle, involving two hosts, is termed as digenetic parasite.The liver fluke undergoes copulation in the bile duct of sheep. During copulation the penis of one liver fluke is inserted into laurer's canal of another  and sperms are transferred. Fertilization takes place in the oviduct. In liver fluke cross fertilization takes place by copulation and self-fertilization also takes place.

The fertilized egg is surrounded by yolk cells and is developed into shelled egg or capsule. The shelled egg is light brown in colour. It shows a lid. It is 130 to 150 microns in length. These eggs will reach intestine through bile duct. They will be sent out of the body of sheep through fecal matter. Thousands of eggs are liberated by sheep every day. These eggs will undergo early development when the environmental conditions are favorable. At 25 degrees centigrade the egg gives rise to Miracidium larva in 9 to 15 days.

MIRACIDIUM LARVA:

In the life history of liver fluke the first larval stage is miracidium when the egg is placed in water the lid opens and miracidium comes out. This larva swims in the water.It is small,conical and 150 microns in lengthBody is covered by ciliated epidermal cells. ,The body is covered by 21 ciliated cells which are arranged in five rows. Consists of first row  of six cells, second row of six cells,third row of  three cells,fourth row of four cells and fifth row of two cellsIt swims in the water with the help of cilia.At the apex of the larva an apical papilla or boring papilla is present.An apical gland is present in the miracidium larva which opens into the apical papilla. On either side of it two penetration glands are present.A brain is present. Above the brain two eye spots are present.A pair of flame cells are present which open out laterally towards the posterior end. The larva shows a number of germ cells.The miracidium larva lives only for 8 hours. During this time it will swim in search of the secondary host. The secondary host of liver fluke is Limnea truncatula or Planorbis (fresh water snail.When the miracidium larva come in contact with the snail, it pierce into the soft body of snail. Apical papilla and secretions of penetration gland will help the larva to bore into the body of snail. Sporocyst :In the body of snail miracidium enters into pulmonary sac. There miracidium will loose its ciliated epidermis. It becomes a bag like structure. It looses all the structures except flame cells and germ cells. The germ cells will undergo parthenogenesis and give rise to the next larvae called Redia. The sporocyst absorbs nourishment from the host tissues and often causes destruction to the host.

Redia :In the sporocyst five to eight redia larvae are produced. They come out of the sporocyst by rupturing the wall of the sporocyst. This larva is elongated in structure covered by thin cuticle. It shows a collar which is muscular which  helps in locomotion. Near the collar a birth pore is present through which the next larval stage will go out. The larva shows a gut with mouth, pharynx and intestine. The flame cells of one side will open into a common excretory duct which opens out through a single nephridiopore. The mesenchyme of the larva shows germ cells.The germ cells will undergo parthenogenesis and give rise to the next larval stage called cercaria in the winter season. These cercaria larvae will come out of the redia through birthpore.

Cercaria :The redia larva will give 15 to 20 cercaria larvae. They are liberated from the redia larva through birth pore. It is oval in shape with tail and  is 0.25mm to 0.35mm in length.The cuticle covering will show backwardly directed spines.Two suckers are present, a) Oral sucker around mouth, b) ventral sucker.The digestive system starts with mouth, opens into pharynx, oesophagus and intestine. Intestine divided into two branches.More flame cells are present. All of them open into excretory tubules. The two excretory tubules will unite at the posterior end and become excretory bladder. It gives an excretory tube. It divides into two, which opens out through nephridiopore.Germ cells are present.The completely developed cercariae will enter into water from the body of snail. They swim for 2 or 3days in the water and settle on a water plant.

Metacercaria :

Cercaria larva after attaching to a water plant loses its tail and develops a cyst around itself. It is called Metacercaria. it is 0.2 mm in diameter. These stages can develop only when they enter into sheep. This stage can survive for few weeks if they are present near water.

Transmission:

When the sheep eats the plants with metacercaria stages they enter into its digestive system. The cyst wall is digested in intestine, it penetrates through the intestine wall and reaches the liver. It takes six weeks to grow into adult. It takes 12 weeks to attain sexual maturity.