Development in Secondary Host or Snail (With Diagram)

Sporocyst Larva:

It looks like an elongated sac, about 0.7 mm long. Its body-wall retains all the layers of body wall of Miracidium except the ciliated ectoderm. During metamorphosis into Sporocyst, the outer coat of ciliated cells is thrown off, the pigment cells of the eye-spot separate from one another and loose their crescentic form and the organism becomes a mere sac.

It is known that glands, brain, eye-spots and apical papilla of Miracidium degenerate and disappear in the Sporocyst. This sac like structure is covered by cuticle. Protonephridium on each side divides into two flame cells which open outside by a common excretory pore. The Sporocyst contains germ cells which by division form Redia larvae or daughter sporocysts. The Redia larvae when fully formed come out by rupturing the Sporocyst wall (Fig. 8.5).

Redia Larva:

Rediae emerge from the Sporocyst by rupture of its body wall. Each Redia is an elongated oval hollow body covered with thin cuticle. At the anterior end there is a muscular collar. The mouth is at the anterior end which opens into muscular pharynx, which in its turn opens into a small intestine. Little behind the muscular collar is the birth pore.

Posterior part of the body has a pair of blunt conical processes called lappets. Above each lappet lies the excre­tory pore. The Redia larva comprises of germ cells which by division produces Cercaria larvae in autumn. These come out through birth-pore (Fig. 8.6).

The rate and extent of development depend mainly on two factors:

(i) The available food reserves in the gland, and

(ii) The size of the infec­tion.

In hibernating and starved snails, develop­ment is greatly retarded but recovers rapidly when the snail resumes feeding. Since large snails are provided with large digestive glands it is not surprising to observe that the number of Rediae formed by a snail bears a direct relationship to its size, as measured by shell length.

Cercaria Larva (In Water):

Each Redia produces 14 to 20 Cercaria larvae. These larvae come out through the birth pore of Redia larva and enter into the snail’s digestive gland. A fully grown Cercaria is a flattened, heart shaped body with an excee­dingly contractile tail, more than double as long as the body proper.

There is an oral sucker at the anterior end and a little behind there is a ventral sucker, placed in the middle of the lower surface of the body. In the centre of the oral sucker there is a mouth which leads into the pharynx. The pharynx finally opens into bifid intestine. The body contains flame cells (Protonephridia), germ cells and gland cells (Cystogenous gland cells) (Fig. 8.8).

The Cercariae are gymnocephalous in type (i.e., they possess two almost equal suckers, no stylet, well developed pharynx, oesophagus, and intestine and simple tail) and have well developed cystogenous glands. When fully formed the Cercaria emerges from the snail body and anchors itself by means of its oral sucker to a suitable sub­strate, such as grass, loses its tail and transforms into a Metacercaria, which becomes enclosed in a meta-cercarial cyst secreted by its cystogenic gland.

Metacercaria:

When fully formed the Cercaria attains a length of about 0.28 mm, then it emerges through the birth pore of Redia. With the help of its suckers and contractile tail, it wriggles out of the tissue of the host. It leads a free existence for about 2 or 3 days and it finally gets attached to some aquatic plant and wet grass.

Shortly, it ceases to move about and rests on a leaf of the aquatic plant or a blade of the grass. It loses the tail and starts encystment. During encystment the cystogenous cells secrete an abundant slimy material which hardens immediately and in a few minutes the ani­mal is covered by a thick hard cyst and gets firmly attached to the aquatic plants. This encysted larva is called Metacercaria. It is in fact the juvenile fluke, also known as Marita (Fig. 8.9).

Metacercaria differs from Cercaria in that it has a rounded form, a thick hard cyst wall and large number of flame cells. It lacks a tail and its excretory bladder opens out directly through a single pore. Like Cercaria, pharynx and bifid intestine are present.

Infection of Definitive Host (Sheep):

When a sheep feeds upon the aquatic plants contaminated with the Metacercaria, the Metacercaria reaches duodenum of the sheep. In the proximal part of the intestine the cyst-wall is dissolved and the larva is liberated in the intestine. It penetrates the intestinal wall to reach the peritonial cavity and the young fluke reaches the liver through the hepatic portal system.

Here the fluke grows rapidly and takes up its position in the bile duct where it finally attains sexual maturity. It begins to lay eggs after 3-4 months of infection and the eggs are liberated in the faeces through bile. The cycle is then repeated (Fig. 8.11).

On ingestion by the primary host (Sheep), Metacercariae excyst in the proximal part of the intestine. Excystment in Liver fluke is an active pro­cess, taking place in two stages: activation and emergence. Activation appears to be initiated by a high concentrations of CO₂ and reducing conditions (i.e. a low Eh) and a temperature of about 39°C. The emergence phase of Liver fluke is triggered by bile. This bile probably plays a role of non­specific stimulation of muscular activity. There is evidence that glycine conjugates of cholic acid are more effective than the taurine conjugates and glycocholic acid alone gives 71.4% emergence of Metacercariae.

Fasciola Gigantica, Cobbold (1885):

This is the common liver fluke of domestic stock in Africa; it is found frequently in the Indian sub-continent, Formosa and has also been repor­ted from Hawaii and the Philippines. F. gigantica resembles F. hepatica but is readily recognised by its larger size being 2.5-7.5 cm x 1.2 cm. The anterior cephalic lobe is smaller than that of F. hepatica, the shoulders are not as prominent and the body is more transparent. The eggs mea­sure 156-197 μm by 90-104 μm.

Life cycle:

The life cycle of F. gigantica resem­bles that of F. hepatica. Kendall (1965) considers F. gigantica to be transmitted throughout the world by members of the super species Lymnaea auricularia. They are true aquatics and hence are generally different from the snail hosts of F. hepa­tica. Lymnaea natalensis is a convenient laboratory host.

There is little evidence that the snail vectors of F. gigantica can aestivate unlike the snail hosts of F. hepatica.

The eggs of F. gigantica hatch within 17 days at 26°C.

It has been seen that in the warm sector in East Africa, 75 days are required for development of F. gigantica in the snail, thus being extended up to 175 days in the cold season. Dinnik and Dinnik (1964) found that one to six first generation Rediae may develop from a Sporocyst of F. gigantica at 26°C, each Redia forming daughter Rediae and then Cercariae.