FASCIOLA NOTES

External morphology and life history of Fasciola hepatica
PHYLUM-PLATYHELMINTHES
CLASS-TREMATODA
ORDER-DIGENEA
1. Habit and Habitat of Fasciola Hepatica:
 Fasciola hepatica (L., fasciola = small bandage; Gr., hepar = liver), the sheep liver fluke, lives as an endoparasite in the bile passages of sheep.
 Fasciola hepatica, in addition to sheep, also infects other vertebrates like goat, deer, horse, dog, ass, ox and occasionally man. Fasciola hepatica is worldwide in distribution, particularly sheep and cattle raising areas are the primary zones where human beings are also infected.
 Its other Indian species, F. gigantica (= indica) is found in the bile passages of buffaloes, cow, goats and pigs.
2. Shape, Size and Colour:
 F. hepatica has a thin, dorsoventrally flattened, leaf-shaped, elongated and oval body. It measures about 25 to 30 mm in length and 4 to 12 mm in breadth.
 The maximum width is at about anterior third of the body from where the body tapers anteriorly as well as posteriorly, however, the anterior end is somewhat rounded, while it is bluntly pointed posteriorly.
 3. External Morphology:
 Shape and size: body of F. hepatica is soft, oval in outline, dorso-ventrally flattened and leaf like. It is about 1.8 to 3cm, which is a little infront of the middle region of body. From this region body tapers away anteriorly as well as posteriorly. Anterior end is somewhat broad and rounded, while posterior end is bluntly pointed.
 Colouration: Colour is usually pinkish, but the transparency of body wall enables the observer to see the blackish or brownish vitelline glands along the lateral margins, and the alimentary canal, which appears brownish due to ingested bile of the host.
 Oral cone: anterior end of body is drawn out into a prominent conical projection, termed the oral cone or head lobe, bearing at its tip a somewhat triangular aperture, the mouth.
 Suckers: there are two small suckers, anterior and ventral. Both are devoid of hooks and spines.
 Anterior sucker: Mouth is situated at the centre and bottom of a cup-shaped muscular organ, known as the anterior or oral sucker. It has a diameter of about 1 mm. Muscles of oral sucker radiate from margin of mouth to the periphery of sucker. Oral sucker acts as an ideal suctorial organ serving for adhesion as well as ingestion.
 Ventral sucker: About 3 to 4 mm. behind the oral sucker, situated mid-ventrally, is another bowl-like adhesive sucker, the ventral or posterior sucker, also known as acetabulum. It is without an aperture and has a diameter of about 1.6mm.











4. Life cycle and development:
 Digenetic life cycle. Life cycle of F. hepatica is complex and completed in two hosts. Primary host, in which the adult fluke lives, is sheep. While the intermediate host, in which numerous larval stages are passed, is a snail (Lymnaea, Planorbis, etc.). This type of life cycle, involving two different kinds of hosts, is termed digenetic.
 Copulation. Self-fertilization is of rare occurrence in liver flukes though they are hermaphrodite. In F. hepatica, copulation takes place in bile ducts of the host.
 Two flukes in copulation bring their genital pores in opposition. Cirrus of one fluke, everted through its gonopore, penetrates the Laurer’s canal of the other through the latter’s temporary opening, and injects spermatozoa. Secretion of prostate glands, and perhaps also of the Mehlis’s glands, keep the sperm active for fertilization.
Fertilization. Fertilization is internal. In cross-fertilization, sperms received in Laurer’s canal during copulation, enter the distal end of oviduct where fertilization occurs. During self-fertilization, sperms enter the uterus of same fluke through female genital aperture and pass down to fertilize the egg.
Capsule formation. Each fertilized egg or zygote is surrounded by yolk cells, which provide yolk and shell material. Shell- globules of yolk cells contain proteins and a phenol.

According to Stephenson (1947), phenol is oxidized to a quinine in the proximal part of uterus. Quinine then tans the protein, producing a hard, resistant and lethary sclerotin like that of insects. This sclerotin forms the shell around fertilized eggs.
 Capsules. Shelled eggs are termed capsules or simply eggs. A shell or capsule is yellow or brown, in colour and oval in shape. It is about 130 to 150µ long and 60 to 90µ wide.
 It is operculate, i.e., provided with a lid or operculum. Situated immediately beneath the operculum, at the terminal end of egg is a viscous and granular cushion. About 3000or more such capsules may occur at a time in the uterus of a single fluke.
 There may be as many as 200 flukes in the liver of one sheep. If each fluke produces 500,000 eggs, a single infected sheep may disperse 100 million fertile eggs. This vast capacity for egg production is necessary in view of the complicated life cycle and slim chances of survival.








Cleavage and embryonic development. Cleavage starts while eggs are still in uterus. Cleavage is holoblastic and unequal. First division of zygote results in two unequal cells, a larger somatic cell and a smaller propagatory cell. Subsequent divisions of somatic cell form larval ectoderm and tissues. Propagatory cell divides further into two daughter cells.
 One daughter cell by its divisions finally produces the larval body. Other daughter cell divides several times to form a mass of smaller germ cells which cluster in the posterior part of larval body.
 Encapsulated embryos or capsules or simply eggs do not develop further in fluke’s uterus.
 A very large number of capsules leave fluke’s body through its gonopore into host’s intestine, and finally ejected out with its faeces.
 Further development takes place when capsules come in contact with water (or Damp areas with at least 60% moisture content) which is slightly acidic (pH 6.5). Optimum temperature for development ranges from 220C to 250C.

Larval stages of Fasciola hepatica
Miracidium Larva. It is the first larval stage involved in life cycle. When suitable conditions become available, the encapsulated embryo, in 4-15 days, differentiates into a miracidium larva. It hatches out and swims in water. Hatching is initiated by a proteolytic hatching enzyme. It dissolves the cementing material by which operculum is attached, thus releasing the operculum.






Characters:
1) It is 150 microns in length. It is small. It is conical in shape.
2) It is covered by ciliated epidermal cells. ,
3) The body is covered by 21 ciliated cells which are arranged in five rows.
a) First row - six cells
b) Second row - six cells
c) Third row - three cells
d) Fourth row - four cells
e) Fifth row - two cells
4) With the help of cilia it swims in the water
5) At the apex of the larva an apical papilla or boring papilla is present.
6) 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.
7) A brain is present. Above the brain two eye spots are present.
8) A pair of flame cells are present which open out laterally towards the posterior end. The larva shows a number of germ cells.
9) The miracidium larva lives only for 8 hours. During this time it will swim in search of the secondary host.

Transmission to secondary host:
 The secondary host of liver fluke is Limnea truncatula or Planorbis (fresh water snailj.When the miracidium larva comes 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. In the body of snail miracidium develops into sporocyst stage.

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. It is covered by thin cuticle. It shows a collar which is muscular. It helps in locomotion.
 Near the collar a birth pore is present. The next larval stage will go out through the birth pore. The larva shows a gut which opens out through mouth. Mouth opens into pharynx which leads into intestine.
 Many flame cells are present. 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 birth pore.











Cercaria:
The redia larva will give 15 to 20 cercaria larvae. They are liberated from the redia larva through birth pore.
 1) It is oval in shape with tail.
 2) It is 0.25mm to 0.35mm in length.
 3) The cuticle covering will show backwardly directed spines.
 4) Two suckers are present, a) Oral sucker around mouth, b) ventral sucker.
 5) The digestive system starts with mouth, opens into pharynx, oesophagus and intestine. Intestine divided into two branches.
 6) 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.
 7) Germ cells are present.
 The completely developed cercaria 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:
As many as a thousand metacercariae may be found attached to a single grass blade. They have a rounded form with a diameter of about 0.2mm.
 Meta cercaria is in fact the juvenile fluke, also called marita. It differs from cercaria in that it has a rounded form, a thick hard cyst and large number of flame cells. It lacks a tail and cysogenous gland cells and its excretory bladder opens out directly through a single pore.
 Germ cells or the genital rudiments are present as such. Cyst provides protection against short periods of desiccation.







Infection of primary host.
 When the sheep eats the plants with metacercaria stages they enter into its digestive system. The cyst wall is digested in intestine, finally dissolves in proximal part of intestine and liberate the larva. It penetrates through the intestine wall and gets onto coelomic cavity.
 Now it infect the liver, feeds on its tissue, and grows in size in five to six weeks. It then takes up its position in the bile duct, where it finally attains sexual maturity. In 11 to 13 weeks. After entering the body of host, it starts laying eggs.