Fasciola hepatica, also known as the common liver fluke or sheep liver fluke, is a parasitic trematode (fluke or flatworm, a type of helminth) of the classTrematoda, phylumPlatyhelminthes.
It infects the livers of various mammals, including humans. The disease caused by the fluke is called fasciolosis or fascioliasis, which is a type of helminthiasis and has been classified as a neglected tropical disease.
Fasciolosis is currently classified as a plant/food-borne trematode infection, often acquired through eating the parasite’s metacercariae encysted on plants.
Copulation and Fertilization of Fasciola Hepatica
Though F. hepatica is hermaphrodite even then cross- fertilisation is of common occurrence. Hence, before fertilisation copulation occurs; during copulation, which occurs in the bile duct of the sheep, the Cirrus of one Fasciola is inserted into the Laurer’s canal of other Fasciola and the sperms are deposited into the oviduct, so that crossfertilisation takes place.
During self- fertilisation, which occurs only when cross-fertilisation does not take place, the sperms from the same Fasciola enter the female genital aperture and pass down the uterus to fertilize the eggs in the oviduct.
Formation of Egg Capsules in Fasciola Hepatica
The eggs are brownish in colour, oval in shape and measure about 130 to 150 µ in length and 63 to 90 µ in width.
As referred to, the eggs are fertilised in the oviduct, the fertilised eggs receive yolk cells from vitelline glands and they get enclosed in a chitinous shell formed by granules in the yolk cells giving out droplets, the shell hardens and becomes brownish yellow; the shell has an operculum or lid. Mehlis’s glands play no role in the formation of the shell.
The completed ‘eggs’ are called capsules which are large in size and they pass into the uterus where development starts. Capsules come out of the gonopore into the bile duct of the sheep, they reach the intestine and are passed out with the faeces. The capsules which fall in water or damp places will develop at about 75°F. Capsules are produced throughout the year, and one fluke may produce 500,000 capsules.
Development of Fasciola Hepatica
Development starts in the uterus and is continued on the ground. The fertilised egg divides into a small propagatory cell and a larger somatic cell. The somatic cell divides and forms the ectoderm of the larva. Later the propagatory cell divides into two cells, one of which forms the endoderm and mesoderm of the larva, and the other forms a mass of germ cells at the posterior end of the larva.
This method of development takes place in the formation of all larval stages during the life history. In two weeks time, a small ciliated miracidium larva is formed and it comes out of the shell by forcing the operculum. The miracidium produces a proteolytic enzyme which erodes the lower surface of the operculum.
Miracidium Larva
Miracidium larva is a minute, oval and elongated, free-swimming stage, it is covered with 18 to 21 flat ciliated epidermal cells lying in five rings.
The first ring is made of six plates (two dorsal, two lateral and two ventral), second ring has again six plates (three dorsal and three ventral), third ring has three plates (one dorsal and two ventrolateral), fourth ring has four plates (two right and two left) and fifth ring has two plates (one left and one right).
A sub-epidermal musculature, consisting of outer circular and inner longitudinal fibres, is situated beneath the epidermal cells. The subepidermal musculature is followed by a layer of cells constituting the sub-epithelium. All these, i.e., epidermal cells, sub-musculature and sub- epithelium, together form the body wall of miracidium.
Anteriorly it has a conical apical papilla, and attached to it is a glandular sac with an opening called apical gland.
On each side of the apical gland is a bag-like penetration gland. There are two pigmented X-shaped eye spots and a nervous system. There is a pair of protonephridia, each with two flame cells. The flame cells open to the exterior by two separate excretory pores or nephridiopores situated laterally in the posterior half of the body.
Towards the posterior side are some propagatory cells (germ cells), some of which may have divided to form germ balls which are developing embryos. The miracidium does not feed, it swims about in water or moisture film, but it dies in eight hours unless it can reach a suitable intermediate host, which is some species of amphibious snail of genus Limnaea or even Bulinus or Planorbis.
After getting a suitable host the miracidium adheres to it by its apical papilla and enters the pulmonary sac of the snail, from where it penetrates into the body tissues with the aid of penetration glands and finally reaches to snail’s digestive gland. In the tissues the miracidium casts off its ciliated epidermis, loses its sense organs and it swells up and changes in shape to form a sporocyst.
Sporocyst
The sporocyst is an elongated germinal sac about 0.7 mm long and covered with a thin cuticle, below which are mesenchyme cells and some muscles.
The glands, nerve tissue, apical papilla and eye spots of miracidium disappear. The hollow interior of sporocyst has a pair of protonephridia each with two flame cells it has germ cells and germ balls. The germ cells have descended in a direct line from the original ovum from the miracidium developed.
The sporocyst moves about in the host tissues and its germ cells develop into a third type of larva called redia larva. A sporocyst forms 5 to 8 rediae. The rediae larvae pass out of the sporocyst by rupture of its body wall into the snail tissues with the aid of the muscular collar and ventral processes, then the rediae migrate to the liver of the snail.
Redia
The redia is elongated about 1.3 mm to 1.6 mm in length with two ventral processes called lappets or procruscula near the posterior end and a birth pore near the anterior end.
Body wall has cuticle, mesenchyme and muscles, and near the anterior end, just in front of the birth pore, the muscles form a circular ridge, the collar used for locomotion. Redia has an anterior mouth, pharynx in which numerous pharyngeal glands open, sac-like intestine and there is a pair of protonephridia with two pairs of flame cells. Its cavity has germ cells and germ balls.
The germ cells of redia give rise during summer months to a second generation of daughter rediae, but in winter they produce the fourth larval stage, the cercaria larva. Thus, either the primary redia or daughter redia produce cercaria larvae which escape from the birth pore of the redia into the snail tissues. Each redia forms about 14 to 20 cercariae.
Cercaria
The cercaria (Fig. 41.19) has an oval body about 0.25 mm to 0.35 mm long and a simple long tail. Its epidermis is soon shed and replaced by cuticle; below the cuticle are muscles and cystogenous glands. It has rudiments of organs of an adult; there are two suckers (oral sucker and ventral sucker) and an alimentary canal consisting of mouth, buccal cavity, pharynx, oesophagus and a bifurcated intestine.
There is an excretory bladder with a pair of protonephridial canals (excretory tubules) with a number of flame cells. An excretory duct originates from the bladder, travels through the tail and bifurcates to open out through a pair of nephridiopores.
There are two large penetration glands, but they are non-functional in the cercaria of Fasciola.
It also has the rudiments of reproductive organs formed from germ cells. The cercariae escape from the birth pore of the redia, then migrate from the digestive gland of the snail into the pulmonary sac from where they pass out into surrounding water. The time taken in snail from the entry of miracidia to the exit of cercariae is five to six weeks.
Metacercaria
The cercariae swim about in water for 2 to 3 days; they then lose their tails and get enclosed in a cyst secreted by cystogenous glands.
The encysted cercaria is called a metacercaria (Fig. 41.20) which is about 0.2 mm in diameter and it is in fact a juvenile fluke. If the metacercariae are formed in water they can live for a year, but if they are formed on grass or vegetation then they survive only for a few weeks, they can withstand short periods of drying.
The various larval stages (the miracidium, sporocyst, redia, and cercaria) are all formed in the same way from germ cells which are set aside at the first division. There is, thus, a distinction between germ cells and somatic cells, and germ cells alone form the various larval stages.
Infection of the primary host (Sheep)
Further development of the metacercaria takes place only if it is swallowed by the final host, the sheep.
Metacercariae can also infect man if they are swallowed by eating water cress on which cercariae encyst, but such cases are rare. But the metacercarie are not infective until 12 hours after encystment. In the alimentary canal of a sheep, the cyst wall is digested and a young fluke emerges and bores through the wall of the intestine to enter the body of the host.
After about two to six days they enter the liver and their movements in the liver may cause serious injuries.
The young flukes stay in the liver for seven or eight weeks feeding mainly on blood and then they enter the bile duct and bile passages. The young flukes have been growing in the liver and after several weeks in the bile duct they become sexually mature adults. The period of incubation in the sheep takes 3 to 4 months.
However, the life history of Fasciola hepatica (Fig. 41.21) can be summarised as under
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).
Characteristic Features of Life History of Fasciola Hepatica
Life history of Fasciola Hepatica is complicated because of parasitism. A sheep harbours about 200 flukes which will produce about 100 million eggs. The miracidium larva is free living and is structurally adapted to seek out an intermediate host, a snail Limnaea, which is found conveniently in water and damp places in grass in wide areas where sheep graze.
The sporocyst forms 5 to 8 rediae, each of which produces 8 to 12 daughter rediae, each daughter redia forms 14 to 20 cercariae; so that about a thousand cercaria larvae are produced from each egg. From this large number some cercariae are bound to infect a new sheep, thus, ensuring a continuance of the race.
Life history of Fasciola Hepatica affords an example of alternation of generations. The fluke is the sexual generation and it alternates, not with an asexual generation, but with parthenogenetic generations of sporocysts and rediae. Such an alternation of a sexual generation with a series of parthenogenetic generations is called heterogamy.
This theory of parthenogenetic development in the various larval stages is now discounted, and formation of various larvae from germ cells is regarded as simple mitotic asexual multiplication; this asexual multiplication of various larvae is called polyembryony.
Thus, there is a period of asexual multiplication during larval stages, followed by sexual reproduction in the adult fluke. This may be regarded as an alternation of generations, but more probably it is continuous life history in which asexual multiplication occurs in the larval stages due to parasitism.
The free swimming larval stages, miracidia and cercariae of F. hepatica, are morphologically more advanced than the adult fluke because they bear organs of locomotion, sense organs, cellular epidermis and a well developed body cavity.