An Example of Phylum Platyhelminthes: Liver Fluke

 In this article we will discuss about Liver Fluke:- 1. Habit and Habitat of Liver Fluke 2. Structures of Liver Fluke 3. Body Wall 4. Digestive System 5. Excretory System 6. Nervous System 7. Reproductive System 8. Life History 9. Pathogenicity 10. Clinical Features and Treatment of Disease.

Contents:

1. Habit and Habitat of Liver Fluke
2. Structures of Liver Fluke
3. Body Wall of Liver Fluke
4. Digestive System of Liver Fluke
5. Excretory System of Liver Fluke
6. Nervous System of Liver Fluke
7. Reproductive System of Liver Fluke
8. Life History of Liver Fluke
9. Pathogenicity of Liver Fluke
10. Clinical Features and Treatment of Disease Caused by Liver Fluke

1. Habit and Habitat of Liver Fluke:

Liver flukes are typical digenean trematodes. It was reported in the year 1379 and as such is the first described trematode. It lives as an endoparasite in the bile duct of sheep and is scientifically known as Fasciola hepatica. The adult flukes are typical para­sites of vertebrate animals (primary host) but one stage of their life history is invari­ably spent in an invertebrate host—a mol­lusc (secondary or intermediate host).

This alternation of hosts suggests that they have digenetic life histories. The group to which they belong has been named as Digenea. Sometimes the adult flukes invade other cattle and men and cause serious loss of human life and domestic animals. The disease caused by the parasites is known as liver rot.

2. Structures of Liver Fluke:

Shape and Size:

Liver Fluke is a soft bodied, flattened leaf-like animal (Fig. 14.10) and exhibits bi­lateral symmetry. The size varies from about 1.0 to 2.5 cm in length but the width does not generally exceed 1.0 cm.

External Morphology:

The anterior end has a conical projec­tion—the head lobe, and at its apex is situ­ated the oral or anterior sucker perforated by the mouth.

On the ventral surface, a little behind the head lobe, the ventral or poste­rior sucker or acetabulum is situated. It is bigger in size than the anterior sucker. Be­tween the two suckers and close to the pos­terior sucker there is the genital opening or gonopore through which the penis some­times protrudes.

The excretory aperture is single and lies at the extreme posterior tip of the body. The opening of canal of Laurer, a temporary opening which appears during the breeding season, opens on the middle of dorsal surface. The body surface is marked by the presence of a number of conical pro­jections—the spinules or papillae which are extensions of cuticle surrounding the body.

3. Body Wall of Liver Fluke:

The architecture of the body wall is pecu­liar to some extent. In histological sections (Fig. 14.11B), it appears that the body wall is made up of a homogenous cuticle (now called epidermis) from which spinules arise.

The cuticle or epidermis is also called tegument which is thick, non-ciliated syncy­tial layer containing scleroprotein and resist­ant to digestive enzymes. Beneath the tegument there are circular muscle fibres followed by longitudinal muscle fibres and oblique or diagonal muscle fibres. The muscle layers bear under­neath a number of unicellular gland cells. These cells open to the outside by long ducts.

Interspaces between the organs are packed with mesenchyme or parenchyma tissue. The mesenchyme is in the form of loose connec­tive tissue that occupies the fluid-filled space between the outer body wall and the endoderm of the gut and supports the organ systems.

The epidermal layer is absent but many ectodermal cells are seen to sink into the parenchyma and are connected to the cuticle by protoplasmic projections (Fig. 14.11).

4. Digestive System of Liver Fluke:

The mouth is situated in the middle of the anterior sucker and opens into rounded bulb­like pharynx which is muscular and sucto­rial (Fig. 14.12A). Next to the pharynx is the oesophagus which is very short and is fol­lowed by the intestine. The intestine is bifur­cated into two limbs—right and left, and runs to the posterior side of the body. Each limb ends blindly and gives off numerous blind branches or caeca.

The caeca of the inner side are short and simple while those of the outer side are large and branched. Practically every region of the body is traversed by the caeca. The intestine and its branches are very promi­nent as they remain filled with biliary matter, desquamated epithelium and blood on which the animal feeds. There is no aperture be­tween the intestine and the exterior, i.e., the anus is absent.

5. Excretory System of Liver Fluke:

The excretory system or system of water vessels consists of a median longitudinal excretory canal or protonephridial tubule which opens to the posterior by means of the excretory pore situated at the posterior tip of the body. From the anterior region of the main canal four large canals are given off.

Each canal gives repeated branches. These branches finally end in fine microscopic ves­sels or capillaries which enter the incon­spicuous flame cell or flame bulb (Fig. 14.13). The excretory canals normally contain fat droplets. Each flame bulb is a large cell bearing many cytoplasmic projections. The nucleus is displaced to one side of the cell and the cytoplasm bears secretory droplets as usual like other groups of flat worms.

Each cilium or flagellum is composed of tubulin contained microtubules which make 9 + 2 arrangements.

6. Nervous System of Liver Fluke:

The nervous system consists of a pair of prominent nerve ganglia situated one on either side of the oesophagus (Fig. 14.12B). The ganglia are joined together by a nerve ring around the oesophagus. From the nerve ganglia nerves are given off to the head lobe and to the posterior part of the body.

Amongst these posterior nerves a pair become larger and stouter than the rest and run posteriorly as lateral nerve cords and bear branches. The presence of a rather well-developed nervous system in Fasciola is puzzling because sense organs in them are lacking and the sluggish movement of the animal does not demand a large correlation centre. Absence of sense organs is due to parasitic mode of living.

7. Reproductive System of Liver Fluke:

Liver Fluke reproduces sexually and the same individual bears both the sexes (hermaphro­dite) (Fig. 14.12B).

Male:

Male reproductive system consists of a pair of testes which are in the forms of much branched tubules and occupy a major part of the middle region of the body. From each testis is given off a vas deferens which runs towards the anterior region.

The two vasa deferentia unite and form a median coiled and dilated vesicula seminalis or called copulatory bursa or bursa copulatrix. It serves for the storage of sperms. From it a narrow tube—the ejaculatory duct leads to the male aperture situated at the tip of cirrus or penis. The penis opens into genital atrium.

Female:

The female reproductive organ is a single ovary or germarium which pro­duces eggs. The ovary is in the form of a much ramified tube situated in the right anterior region and above the middle line of the body. The tubules of the ovary open into a single median oviduct. Vitelline glands or yolk glands or called vitellaria are numer­ous, minute and round and open by small ducts into a large duct.

There are two such ducts. The two ducts meet and form a lateral vitelline duct runs transversely and opens into a small chamber called yolk reservoir. Form it comes out a median vitelline duct which opens into the oviduct. The vitelline glands release yolk and the shell material for the eggs.

Around this junction there are groups of unicellular shell glands (accessory female glands) which open by small ducts into the lumen of the oviduct. The lumen of the oviduct at this region is dilated and called ootype. The uterus is a wide convo­luted tube formed by the oviduct and the median vitelline duct and it opens in the genital atrium near the base of the cirrus.

There are a mass of numerous unicellular glands, situated around the junction of me­dian vitelline duct, oviduct and the uterus, called Mehlis’s glands. The secretion of these glands is to lubricate the passage of eggs in the uterus and also probably activates the spermatozoa.

A canal, termed Laurer’s canal leads from the junction of oviduct and median vitelline duct and opens externally on the dorsal surface. During breeding season it receives the copulatory organ during mating and transfers sperm to the eviduct.

The common genital aperture is situated on the mid-ventral line between the suckers and close to the ventral sucker.

8. Life History of Liver Fluke:

Liver Fluke is a digenetic trematode parasite because it completes its life cycle through two hosts. The definitive (or called primary) host is sheep or cattle than the man although several hundred cases of human fascioliasis have been reported from Latin America. The intermediate (= secondary) host is a gastropod snail known mainly Lymnaea truncatula from India and S.E. Asia.

The secondary or intermediate host is essential for the completion of the life-cycle of the parasite but it does not become sexu­ally mature within secondary host.

The development is indirect because there are several larval stages between egg and adult. The completion of life cycle (Fig. 14.14) depends upon the transference of the para­site from one host to the other.

The life history of the liver fluke was studied first by A. P. Thomas in 1883. A list of intermediate snail hosts with geographical distribution is given in Table 14.1.

Copulation:

Fasciola is hermaphrodite (= monoecious) but cross-fertilization between individuals is a rule. It has also been reported that during copulation the cirrus (penis) of one is inserted into the temporary opening of the Laurer’s canal of the other individual.

Fertilization:

The eggs become fertilized by the biflagellate sperm in the lower part of the oviduct and then pass on to the uterus.

Capsule formation:

The fertilized egg is surrounded by a fair amount of yolk (vitel­line secretion) secreted from yolk glands containing yolk cells. The yolk cells (or called vitelline cells) secrete granules of material which consider the precursor material of chitinous shell. The shell finally encloses the fertilized egg, accompanying the yolk cells while passing through the ootype.

The shell becomes hard when it enters the uterus. The hardening of the shell is caused by the action of quinone. The shelled eggs are termed capsules or commonly called eggs.

Eggs:

A capsule or an egg is oval in shape and yellowish or light brown in colour. It measures about 130-150 µm by 63 to 90 µm. It bears a small lid or operculum for the exit of the future larva.

Segmentation:

The cleavage of the ferti­lized egg (zygote) starts when the eggs re­main inside the uterus.

The first cleavage is complete but unequal which produces two unequal cells:

(i) a small granular propagatory cell and

(ii) a large somatic or ecto­dermal cell.

The somatic cell by repeated divisions leads to the formation of the ectoderm of larva.

The propagatory cell by further divisions forms two groups of cells— propagative cells and somatic cells. The somatic cells form the body of the larva and the propagative cells form the germ cells.

Development of the Shelled Embryos of Liver Fluke:

Development within eggs or shelled embryos continues under favourable condi­tions. The shelled embryos come out of the uterus and are carried to the digestive tract via the common bile duct and finally get out of the body along with the faeces of the vertebrate host.

Conditions for the Survival of the Shelled Embryos:

The eggs can survive only if the host defecates in water or on the banks of a stream where subsequent rains wash the faeces into the river. The eggs can also sur­vive for several months in wet faecal matter.

Factors for the development of Miracidium Larva:

1. The development of the eggs proceeds in water where a pH range varies 4.2 to 9.

2. The optimum temperature for devel­opment varies from 10 °C-30 °C.

3. At 30 °C of temperature the develop­ment completes in 8 days but at 10 °C of temperature the completion of the development of embryos takes 23 weeks.

4. Above 37 °C of temperature the devel­opment stops.

5. The availability of O₂ is related to the development of the shelled embryos of Fasciola, and in the aerobic condition the eggs of Fasciola, take 1/5 of the time rather than the time required in oxy­gen deprived condition.

Under these optimum conditions the em­bryos develop into a ciliated miracidium. When the capsulated embryo comes in con­tact with water, the operculum dissolves and the miracidium larva comes out and swims actively with the help of cilia.

Larval Stages of Liver Fluke:

The life history of F. hepatica was studied by Leuckart in 1881 and mechanism of hatch­ing of different larval stages was studied by Wilson (1968).

The life cycle of liver fluke is completed through five larval stages, such as:

(i) Miracidium larva,

(ii) Sporocyst larva,

(iii) Redia,

(iv) Cercaria, and

(v) Metacercaria.

(i) Miracidium larva:

Characters:

1. The miracidium is small and conical in appearance.

2. The anterior end of the body is broader than the posterior end.

3. It bears cilia round the body.

4. The anterior end has a non-ciliated conical projection, called the head lobe or apical papilla or terebratorium.

5. Behind the head lobe there is a pair of eye spots situated above the brain which are considered as primary receptors of the larva.

6. A large brain or nerve ganglion is situated antero-dorsally near the ante­rior end.

7. The body wall is covered by a layer of 21 epidermal plates, arranged in 5 rows.

8. In the first row there are 6 plates, second row consists of 6 plates, third row includes 3 plates, fourth row has 4 plates and fifth row also has 2 plates.

9. Within the body just below the epider­mis there are delicate layers of circular and longitudinal muscle fibres.

10. At the anterior end there is a sac-like multinucleated gland, called apical gland, which opens into the apical papilla by a duct. The gland is consid­ered to secret the proteolytic enzyme helps in the penetration of the snail’s body.

11. The head lobe bears a pair of sac-like penetration glands lie on the sides of the apical gland and also open into apical papilla. These glands secrete histolytic enzymes, causing in the lysis of the host’s tissue.

12. A pair of protonephridia, each with two flame cells are present in the posterior half of the body which open to the outside by nephridiopores. The protonephridia serve as excretory as well as osmoregulatory organs.

13. The germ cells en bloc are present in the posterior part of the body (Fig. 14.14).

Infection to the secondary host (inter­mediate host):

1. The miracidium larva swims freely in water or crawls over damp surface for some-time and dies in case it does not find its secondary host—the fresh­water snail, Lymnaea truncatula and its related Bulinus, Planorbis, within a few hours.

2. The ciliated miracidium larva can swim 2 mm per second and at 22 °C of temperature they can survive about 4-12 hours.

3. Miracidium is gutless and does not feed, and can metabolize glucose which is added to the swimming medium.

4. The incomplete tricarboxylic cycle is seen in the miracidium of Fasciola.

5. During swimming the miracidium is not attracted by the mucus that is secreted by the intermediate host— Lymnaea truncatula.

6. On meeting the snail (usually Lymnaea truncatula) the miracidium bores into by the operation of the penetration glands of miracidium and enters into the host’s body through the skin and reaches the internal organs, specially the pulmonary sac.

After the entry into the snail’s body the cilia are cast off, and eye spots, brain and cephalic glands are degenerated, and meta­morphoses into a sac-like sporocyst larva.

(ii) Sporocyst larva:

Characters:

1. The sporocyst is a sac-like body with no gut.

2. The body is covered with a single layer of thin cuticle.

3. The subepithelial cells and muscles are present below the cuticle.

4. Each protonepridium on each side is divided into 2 flame cells and they open to the outside by a single nephridiopore.

5. Birth pore may be present or absent.

6. The internal cavity contains about 25-40 germ cells or germ balls.

7. The sporocysts live in the digestive glands or in the pulmonary sac.

They can survive about 154 days in the snail’s body. The germ cells and the lining cells are budded off which undergo segmen­tation and produce first generation or mother rediae. Each sporocyst produces 5-8 mother redia larvae (Fig. 14.14).

(iii) Redia larva:

The redia larvae come out by rupture of the sporocyst and migrate to the digestive gland (hepatopancreas) of the snail.

Characters:

1. A redia is elongated in appearance measuring about 1 to 3 mm in length.

2. The body is covered with a thin cuticle and bears a muscular circular ridge, a little behind the anterior end, called collar, helps in locomotion. It is formed by the bulging of the body wall.

3. Below the collar an aperture is found, called birth pore through which next larval stage (daughter larvae) comes out.

4. The redia is provided with mouth, a muscular pharynx and a sac-like in­testine. It can feed on host tissue.

5. It bears a pair of short muscular ven­tral projections at the posterior side, called lappets or procruscula, are used as anchor in the host’s tissues during locomotion.

6. Two protonephridia with many flame cells open through nephridiopores.

7. Many germ cells are found in the body cavity of redia larva.

8. These germ cells in winter months may give rise to a second generation or daughter rediae.

9. The germ cells within the body of daughter redia undergo segmentation and give rise to a tailed form known as cercaria larva.

10. Each redia produces about 14 – 20 cer­caria larva.

11. These cercaria larvae come out through the birth pore.

(iv) Cercaria larva:

Characters:

1. The larva is oval in shape with a long contractile tail.

2. It measures about 0.25 mm to 0.35 mm in length.

3. The body is non-ciliated and covered with a thin cuticle and bears two suck­ers—(i) oral and (ii) ventral sucker or called acetabulum.

4. Alimentary canal is well-developed and consists of mouth, pharynx and a bifid intestine.

5. Paired excretory tubules and groups of germ cells are present inside the body cavity.

6. Many structures such as penetration glands, mucous glands and eye spots are found.

7. The body also contains cystogenous glands whose secretion forms the cyst around the cercaria larva and trans­forms into metacercaria.

8. The cercaria comes out of the body of snail and swims in the surrounding water by means of muscular tail.

Factors affecting the extrusion of cer­caria from the snail’s body:

1. The light and environmental tempera­ture (9 °C-26 °C) are related for the extrusion of the cercaria from the snail’s body.

The cercarial larvae first gather in the perivisceral space around the distal part of the intestine. When the pneumostome of the host’s snail closes, the cercariae are extruded through the thin temporary papillae or through the weak muscle around the anus.

The cercariae after extrusion swim in water for 2-3 days and then takes refuse on aquatic vegetation. Ultimately the tail is shed at the first stage of cyst formation and it encysts only on aquatic vegetation. The rest cercaria in encysted condition is called metacercaria.

(v) Metacercaria larva:

The process of encystment takes place in such ways:

1. The cercaria attaches itself to the sub­stratum by the ventral sucker.

2. Then the body becomes flattened and produces the outer cyst layer,

3. Gradually the cyst wall consists of 4 layers. This cyst wall is produced from the secretion of cystogenous glands.

4. From outside the cyst layers are— (a) an outer tanned protein layer, (b) a fibrous layer, (c) a mucopolysaccha­ride layer and (d) an innermost kera­tin layer (Crompton and layer, 1980).

5. Now the encysted metacercaria be­comes round in shape and 0.2 mm in diameter.

6. Many cercarial structures such as pen­etration glands, mucous glands and eye spots disappear.

7. Germ cells are located within the body cavity.

8. The metacercaria on vegetation may survive one year at low temperatures but two to three weeks at 25 °C.

Transference to Final (definitive) Host (sheep):

When the sheep feeds on metacercaria infested green plants or water plants, the metacercaria larva enters the gut. Here the cyst wall of the larva dissolves and the young fluke liberates. The cyst wall is dis­solved by a lytic enzyme of the worm which destroys the mucopolysaccharide layers of the cyst, and other factors are higher tem­perature (38 °C), a high CO₂ tension, low redox potential, and also bile help for excitement. The young fluke penetrates the wall of the intestine and finally migrates to the bile duct where it grows into an adult fluke.

They attain sexual maturity and begin to lay eggs 12-14 weeks after ingestion. They can live in the final host’s body for several years and a new cycle begins with the libera­tion of eggs.

Alternation of Generations of Liver Fluke:

The life cycle of Liver Fluke repre­sents an alternation of generations. In Fas­ciola alternation takes place between sexual and a series of parthenogenetic generations (larval). Here sexual generation is repre­sented by adult Fasciola and larval genera­tions reproduce asexually by parthenogen­esis called heterogamy. Sporocysts and redia develop parthenogenetically.

9. Pathogenicity of Liver Fluke (Effect on the Host):

When sheep or cattle swallow the metacercariae infested water vegetation dur­ing grazing, the metacercariae become excysted in duodenum and migrate towards bile duct, penetrating the intestinal wall.

During sojourn in the liver, the young liver fluke actively consumes the tissue of liver causing a disease/called liver rot or fascioliasis. The adult F. hepatica live within the bile duct causing the inflammation in the bile ducts and the epithelium of the bile ducts are destroyed.

Humans become infected when they eat salads prepared by vegetables, particularly by watercress. When the excysted young liver flukes pass through the liver, consume the liver tissue causing the liver rot. The people of Lebanon and Syria prefers to a consump­tion of rotton liver of sheep which suffer a disease known as pharyngeal fascioliasis.

10. Clinical Features and Treatment of Disease Caused by Liver Fluke:

The liver rot is a common disease of sheep and cattle. The symptoms appear about one month after infection. The symptoms are dullness, swelling of the abdomen, loss of weight, and sockets become pale and ulti­mately the animal dies.

The fascioliasis in humans is character­ised by acute hepatic pain, continued high fever about 3 months and coughing during the initial stages. During severe infestation, there causes enlargement of liver and spleen; eosinophilia and jaundice appear.

Treatment:

In sheep and cattle, oral administration of hexachloroethane and carbon tetrachloride are useful to kill the parasites. Human fascio­liasis are treated by intramuscular injections of emetine hydrochloride for 10 days.

To eradicate the intermediate hosts, cop­per sulphate and pentachlorophenate can be used in the ponds, streams or in other snail’s habitats, which may kill the snail hosts.