An Example of Phylum Platyhelminthes: Echinococcus

In this article we will discuss about Echinococcus Granulosa:- 1. Habitat of Echinococcus Granulosa 2. Structure of Echinococcus Granulosa 3. Life Cycle 4. Transmission of Disease 5. Other Cestodes.

Habitat of Echinococcus Granulosa:

Echinococcus Granulosa lives in the small intes­tine of dogs and allied animals. The dog is the optimum definitive host. The larval stage is passed in sheep, cattle, pig or man which represent the intermediate hosts of the para­site. These animals, therefore, serve as the common reservoirs of the hydatid diseases.

Structure of Echinococcus Granulosa:

This parasite is the smallest tapeworm (in adult stage) of medical importance.

Echinococcus Granulosa measure 5-8 mm in length and are usually with three segments plus a scolex and a neck (Fig. 14.22). It has a typical taenioid scolex with four deep, well-developed suckers and an anteriorly located retractile rostellum beset with 30-36 hooklets. The scolex contains internal con­centration of nervous and excretory systems.

Behind the scolex lies the un-segmented proliferous neck region. The first proglottid has no definite organisation but the begin­ning of genital organs is marked by the presence of germinal mass. The second proglottid is mature and contains fully de­veloped genital organs. The last proglottid is gravid and relatively larger than other two segments. It contains only the uterus filled with eggs.

Eggs:

The eggs of Echinococcus Granulosa are ovoid in shape. The egg contains hexacanth embryo with 3 pairs of hooks.

Life Cycle of Echinococcus Granulosa:

The eggs produced by Echinococcus Granulosa in the intestine of dog or any other suitable primary host pass out along with the faeces and are ingested by the intermediate host with contaminated food or drink. The eggs, when swallowed, pass down the oesophagus into the stomach. Inside the stomach the shell wall is digested and the active hexacanth embryos (onchospheres) hatch out in the duodenum.

The liberated onchospheres bore their ways through the intestinal wall and enter the radicles of the portal vein. Whenever the onchospheretles, it forms a hydratid cyst. The young larva transforms into a hollow bladder. The cyst is double-walled. The outer layer is laminated while the inner one is germinative. Fig. 14.23 shows the scheme of a hy­datid cyst formation.

The young larva of Echinococcus Granulosa changes into a hollow bladder around which the host adds an enveloping fibrous cyst wall. With the advent of maturity, the inner surface begins to produce hollow brood cap­sules.

The brood capsule is formed by the proliferation of the cells from the germinative layer about eight months after the begin­ning of the cyst formation. It starts as small nuclear mass which grows and becomes vacuolated to form a small vesicle.

The brood capsules remain attached by slender stalks and often set free into the fluid-filled cavity of the mother cyst. As the cyst grows larger, more brood capsules develop.

The older brood capsules begin to differentiate a number of scolices on their inner walls. The mother cysts may, as a result of intracystic pressure, develop hernia-like buds which detach themselves and continue their devel­opment independently as daughter cyst.

Formation of Daughter Cysts:

The mother hydatid cyst develops daughter cysts. The daughter cysts are generally endogenous in origin while the exogenous daughter cysts occur rarely. The endogenous cysts arise from the germinative layer. The daughter cyst is almost identical to the mother cyst. Each scolex developed from the daughter cyst has the property of becoming a new hydatid in an intermediate host.

Transmission of Disease by Echinococcus Granulosa:

The end product of the hydatid cyst is the production of the scolices and each scolex has the power to develop into the strobilate worm, when ingested by any primary host (especially dog). As a sin­gle hydatid may contain thousands of brood capsules and each brood capsule may give origin to a large number of scolices, the number of strobilate worms resulting from such ingestion of a single hydated cyst be­come numerous.

Attainment of strobilate stage in the intestine of dog requires 3-10 weeks after ingestion. Man becomes infected in a variety of ways—viz., by swallowing food or drinks contaminated with infested canine faeces or by handling the infested dogs. The life cycle of Echinococcus granulosa is schematically represented in (Fig. 14.24).

Other Cestodes of Echinococcus Granulosa:

Over 1,500 species of cestodes are known to live as parasites on different animals rang­ing from fish to mammals (Fig. 14.25). Pri­mary host harbours the adult stages while embryonic stages occur in secondary hosts. Infection is caused when the secondary hosts are eaten by the primary hosts.

Hymenolepis nana:

Intermediate host is usually absent. Adults occur in the lumen of the intestine of man, and the larvae live in the intestinal villi. Proglottids numbering about 200 are popu­larly known as dwarf tapeworm. It meas­ures about 10-45 mm.

Taenia saginata:

Adult infects man, larvae live in cattle, proglottids are 2,000 in number (Fig. 14.25). T. saginata is a more common cestode parasite in man, because it uses cattle as its intermediate host. It resembles T. sobium but the rostellum is without hooks and the mode of branching of gravid uterus is different.

Taenia pisiformis:

Adults occur in the dog or cat, larvae stay in the liver and mesenteries of rabbit.

Moniezia expansa:

Adults reside in sheep, larvae remain in mites. It extends up to six metres in length (Fig. 14.25).

Diphyllobothrium latum:

Diphyllobothrium latum (also known as Dibothriocephalus latum), commonly called the fish-tapeworm, is the largest known cestode measuring about 18 metres in length. The number of proglottids varies from 3000- 4000. It has an extremely elongated slender neck.

The scolex has two slit-like bothria (Fig. 14.25) but hooks are absent. The sec­ondary hosts of this form are crustaceans (first host) and various freshwater fishes are the second host. Man or other carnivorous animals are its primary hosts.

Adaptive Features in Cestodes:

(A) The covering of the cestodes is not digestable by the host’s digestive juices and it is permeable to water,

(B) The osmotic pressure inside the body of the parasite is lower than the sur­rounding medium,

(C) pH tolerance is high,

(D) Glycogen and lipid contents in the body tissues of cestodes are high and protein content is less,

(E) In the absence of oxygen it can re­spire anaerobically,

(F) The bladder of cysticercus is digested by the digestive juice but the scolex escapes digestion,

(G) The eversion of scolex is accelerated by the bile of the host.