Life cycle of fasciola hepatica:Trematods

Digenetic: Completes in two hosts

1. Life Cycle of Fasciola hepatica

2. Life cycle of Fasciola Hepatica The life-cycle of Fasciola hepatica is completed in two hosts. The primary or definite host is sheep or cattle in which the adult fluke lives, while the secondary or intermediate host is a snail in which numerous larval stages occurs. Copulation Copulation occurs in the bile duct of sheep. Copulation is mutual and cross-fertilization is the general rule. However , self-fertilization may also occur. During copulation the cirrus or penis of each worm is inserted into the vaginal opening of the other worm and sperms are ejaculated. It is also reported that the cirrus of a worm is inserted into the Laurer’s canal of other individual and sperms are ejaculated. The prostatic secretion enables the sperms to survive in the uterus. Sperms travel down the uterus .

3. Fertilization Fertilization is internal. Sperms received in Laurer’s canal enter the distal end of oviduct and fertilization occur. Capsule formation: The eggs become surrounded by the yolk cells produced by the vitelline glands in the ootype . The yolk cells contain yolk and many shell globules. The shell globules are released from the yolk cells. They form a capsule or the shell around the egg. The secretion of Mehlis gland lubricates the eggs and makes them slippery. A single fluke may produce about 2,00,000 eggs in about 11 years and 30,000 to 35,000 eggs per year.

4. The fully formed fertilized capsulated eggs are light brown, oval structures measuring about 130 x 150µ in size its capsule or shell is marked off into an operculum by a distinct line. Such eggs are called operculate . At a later stage of development, the eggs are deposited in the bile ducts of the host and carried to the intestine along with the bile and are finally passed out with faeces .

5. Cleavage : The division or the cleavage of the eggs starts while they are in the uterus. The first division is unequal as a result of which a small granular propagatory cell and a large somatic or ectodermal cells are formed. The somatic cell divides repeatedly to form the ectoderm of the larva. The divisions of the propagatory cell result in the formation of germ cells and somatic cells. The somatic cells form the body organs. Further development of egg does not take place in the uterus. The eggs are then released in the bile ducts of the host from where they travel to the intestine and come out of the body of the host with its faces. The eggs develop further if they are dropped in damp places. The development remains arrested while the embryos remain in the faeces . They may survive in wet faecal matter for several months. If washed free, the development of the embryo proceeds. The optimum temperature for development ranges from 10 –  30°C. At 30°C the encapsulated embryo differentiates into miracidium larva within 8 days.

6. Larval Stages: Miracidium Larva: It is the first larval stage in the life cycle of Fasciola hepatica . It swims actively in water in search of secondary host, the snail. External Structure: Miracidium of Fasciola is about 0.07mm long, oval or conical. It broad anterior end is produced into an apical lobe or apical papilla or terebratorium . Except the apical lobe, the rest of the body is ciliated. It is covered with 21 closely fitted hexagonal epidermal plates, arranged in five rows. The first row consists of six plates (2 dorsal, 2 ventral and 2 lateral ) T he second row of 6 cells (3 dorsal and 3 ventral) The third row or 3 cells (1 dorsal, 2 ventrolateral ) T he fourth row of 4 cells (2 right and 2 left) and T he fifth row of 2 cells (1 right and 1 left) only.

7. Internal Structure There is a pair of protonephridia and groups of germ cells. In addition, there is a pair of large pigmented eyespots, a large larval brain and a simple nervous system in the anterior part of body. There is a large apical gland and a pair of penetration glands or cephalic glands in the anterior part of body. These open on the apical lobe or cephalic cone. Miracidium does not feed. It dies within a maximum of 24 hours if it fails to reach its secondary host, which is usually snail. If it finds the snail, it penetrates through the soft skin and respiratory tissues of the snail. The penetration is helped by the apical lobe and tissue dissolving action of secretion of penetration glands. The snail may even swallow the miracidium or the capsule containing the miracidium . Inside the host tissue the larva discards its ciliated epidermis and penetrates deeper and finally the lymph vessels or pulmonary chamber of the host. Here it grows to form a sac-like body, the sporocyst .

8. 2. Sporocyst Larva Miracidium loses its apical gland, penetration grand, brain and eyespots and changes into a sac­like sporocyst larva. It looks like an elongated sac about 0.7 mm long. Its body wall retains all the layers of miracidium’s body wall except the ciliated epithelium. It consists of a thin cuticle, a layer of circular and longitudinal muscles. The wide interior is occupied by the protonephridia and germ cells. Each protonephridium now consists of two flame cells. These open on the surface by a common pore. A rudimentary gut is also found. The germ cells undergo repeated divisions to produce radia larvae, but rarely may also produce daughter sporocysts . A single sporocyst may contain 5-18 radia . The radia larvae migrate to the digestive gland of snail.

9. 3 . Redia Larva Redia is an elongated larva with a small mouth, a suctorial pharynx and a simple closed intestine in the anterior part of the body. Many unicellular pharyngeal glands open into the pharynx. These are a muscular, ring-like ridge, the collar, around the anterior region of the body. It helps in locomotion. Just behind the collar is a birth pore through which next generation of larvae come out. In the posterior region there is a pair of lobe-like processes, the lappets or procruscla . These are used an anchor. The larval excretory system consists of anterior and posterior flame cells which open to outside through a single nephridiopore on each side. The body spaces remain filled up with parenchyma. Redia also contains groups of germ cells.

10. Protonephridia divide further and form a much branched system. All the flame cells of one side open out through a common excretory duct. A birth pores in present lateral to and in proximity of the mouth. Within the brood chamber are found germ balls. Redia is very active little creature. It moves about by the contractions of the body and lappets. It nourishes itself by sucking fluid and cells of the host tissues with its muscular pharynx. It soon migrates into the digestive glands of the snail. During summer months when the food is sufficiently available, mother radiae mature in 12-18 days. The germ balls give rise to a second generation of daughter rediae . Germ balls of daughter rediae in the winter develop into next larval stage, the cercariae .

11. 4. Cercaria Larva The body of a fully developed cercaria larva is oval and has a long elastic tail extending from its posterior end. The body is covered over by a thin layer of cuticle in which many backwardly directed spines are present. A mouth, a muscular pharynx, a small oesophagus and a bifid intestine are present in the anterior region of the body. An oral sucker is present surrounding the mouth and a ventral sucker is found between the arms of the intestine. Many germ cells, flame cells and excretory ducts are found in the internal cavity. All the excretory ducts open into the excretory bladders which open to the outside through a small excretory duct.

12. From the digestive glands of the snail, the cercaria passes into the pulmonary sac and then escape into the surrounding water. Cercaris swims about five minutes to an hour in summer. Afterwards , it settles down on some green leaves of water plants. The excretory pore is situated at the base of tail. Many unicellular cystogenous glands are present below the body wall. The substance secreted by these glands form a cyst around the larva after which the cercaria changes into a metacercaria larva. The cercariae , coming out with the current of water, swim for some time and then attack themselves to the leaves of some aquatic plant. Their tails disintegrate and cysts are formed around them by the secretions of the cystogenous glands. Now the larva is known as metacercaria .

13. 5. Metacercaria The metacercaria is 0 .2 mm in diameter Somewhat rounded with thick outer covering of cuticle in the form of cyst. The cystogenous cells of the cercaria disappear and the flame cells increase in number Germ balls are present. Cyst provides protection against short period of desiccation.

14. Infection to the Primary Host The metacercaria enters the primary or definitive host, the sheep, when it grazes on the aquatic weeds and reaches the intestine. In the intestine the cyst is dissolved by the action of digestive enzymes and young fluke comes out. It reaches the liver through the hepatic portal system and starts its existence inside the bile passage.

15. Adult flukes in liver → copulation and fertilisation → laying of capsules in the bile ducts → capsules in the intestine (stages in sheep’s body) → capsules out in faeces → miracidia escape from capsules ( stages in open) → miracidia → sporocysts → rediae → cercariae → (stages in snail’s body) → cercariae → metacercariae (stages in open) → metacercariae young flukes → adult flukes (stages in a fresh sheep’s body )

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