In this article we will discuss about:- 1. Habitat of Taenia Solium 2. External Feature of Taenia Solium 3. Alimentary System 4. Excretory System 5. Nervous System 6. Reproductive System 7. Life-History 8. Pathogenic Effects.
Contents:
- Habitat of Taenia Solium
- External Feature of Taenia Solium
- Alimentary System of Taenia Solium
- Excretory System of Taenia Solium
- Nervous System of Taenia Solium
- Reproductive System in Taenia Solium
- Life-History of Taenia Solium
- Pathogenic Effects Caused by Taenia Solium
1. Habitat of Taenia Solium:
The name tapeworm is due to its elongated tape-like appearance. It may attain a length of several metres. Sexual forms of Taenia Solium occur as endoparasites in the intestine of man.
Asexual forms are encountered in the muscles of pig or in exceptional cases in the muscles of man. Man is its primary host and pig is its secondary host. Distribution of Taenia Solium is world-wide and is most common in European countries. Nowadays Taenia solium occurs rarely in man.
2. External Feature of Taenia Solium:
A fully-developed Taenia Solium may attain a length of 3-5 metres. Its anteroposterior ends are clearly distinguishable but it is difficult to differentiate the dorsal from the ventral surface. The body is ribbon-like (Fig. 69A) and consists of a distinct head or scolex at the anterior region.
The tip of the head bears a conical elevation—the rostellum which can be retracted or extended. The rostellum bears 28 to 33 hooks arte of two types- larger and smaller and they alternate with each other. Each hook parts (fig 69B) has three a base or guard, a conical blade at the tip and a handle projected from the middle.
The hooks are arranged in two rows. When the contractile rostellum is withdrawn the hooks become anteriorly directed and get fixed into the host tissue. The head bears in the middle four cup-like suckers or Acetabulum.
Rostellum and suckers act as organs of attachment to the intestine of the host. Behind the head there is a narrow and small tubular region—the neck or the zone of proliferation. The rest of the body or tape is called strobila. The strobila is segmented in appearance.
The chain like strobila is made up of numerous segments or sexual units called proglottids. The proglottids progressively increase in size and mature towards the posterior extremity. The youngest or newly formed proglottid occupies a position just beneath the neck while the oldest one is at the posterior end. The number of proglottids varies from 800-850 in a full-grown worm.
A proglottid from the middle region of the strobila offers a rectangular outline. The surface is lined by cuticle (recently renamed as epidermis). It is thick and perforated at intervals by fine canals, at the bottom of which either gland cells or nerve endings are situated.
It is followed by longitudinal and circular layers of muscles. The circular muscles divide the parenchyma cells into an outer cortex and inner medullary regions (Fig. 69C).
Towards each lateral margin is found the longitudinal nerve and just median to them lies the longitudinal pair of excretory vessels. A transverse excretory canal is situated at a posterior position of the proglottid.
The anterolateral borders of the medulla are housed with testes and posterior lateral borders are with ovary. The male and female genital ducts open to a chamber called genital atrium which is situated in the middle of one of the lateral margins. The atrium opens to the exterior through an aperture called genital pore.
The body is covered by a thick epidermis. The epidermis is many-layered and perforated. It is impregnated with calcium carbonate. The epidermis remains sunk iii the parenchyma. Longitudinal muscles run under the epidermis.
The parenchyma is divided into an outer cortical zone and inner medullary zone by the circular muscles. Nervous, reproductive and excretory organs are situated within the medullary zone.
For a long time it was regarded that the body of tapeworm is covered by a thick cuticle. But recently electron microscopic studies have revealed that the outer layer of the body of all cestodes contains mitochondria and remains continuous with processes emerging from the cytoplasm of some underlying cells.
There is a recent trend to replace the term ‘cuticle’ by epidermis or tegument. The ontogenetic development of the epidermis is not exactly known. The epidermis bears microvilli which increase the absorptive area of the tapeworms where gut is absent. Beneath the epidermis lies the prominent basement membrane.
3. Alimentary System of Taenia Solium:
It is completely absent. This absence is due to the parasitic life of the tapeworm. It absorbs nutrients from the intestinal contents of the host.
4. Excretory System of Taenia Solium:
There are two pairs of main longitudinal trunks. Each pair runs along the lateral margins of the strobila. The paired state of the longitudinal trunks is well recognised at the anterior part of the strobila and in the posterior part one from each of the pairs becomes lost.
The two pairs of the longitudinal trunks are connected with each other in the head region by a ring-like vessel. Similar connections by straight transverse vessels are seen in the posterior regions of each proglottid.
In the last and penultimate proglottids the longitudinal trunks open into a pulsatile caudal vesicle which opens to the outside by a single median aperture. When the last proglottid is cast off at maturity, the longitudinal trunks open to the exterior separately and independently. The main trunks give off branches from which numerous fine canaliculies each terminating in a flame cell arise.
5. Nervous System of Taenia Solium:
The nervous system consists of a pair of ill-defined ganglia situated in the head or scolex. The ganglia are connected to each other by a broad transverse commissure of slender nerves. Each sucker is provided with a pair of nerves arising from the ganglion.
Two longitudinal nerves of considerable thickness arise, one each from the ganglion and run along the lateral margin up to the last proglottid. Each longitudinal nerve gives a pair of accessory nerves.
6. Reproductive System in Taenia Solium:
Taenia Solium is hermaphrodite. Male and female reproductive organs occur in the same individual. Each proglottid behind the first 200 is equipped with a set of reproductive organs. Male reproductive organs develop first in each proglottid and then female organs make their appearance.
Male Reproductive Organs:
There are numerous rounded testes distributed along the length and breadth of the proglottid. Each testis is provided with a fine efferent duct. The efferent ducts of neighbouring regions join together and form larger ducts.
These larger ducts open into the vas deferens or main duct of the testes. The vas deferens is convoluted, transverse in position and extends towards the lateral margin (left or right) of the proglottid. The tip of the vas deferens is narrow and it pierces a narrow protrusible process the cirrus and opens at its extremity in the genital atrium (Fig. 69D). The base of the cirrus is enclosed in a muscular sac—the cirrus sac.
Female Reproductive Organs:
There is a pair of bilobed (paired according to some) ovaries or germaria situated in the posterior region of the proglottid. The two lobes are unequal in size and lie one on each side of the median line. The ovaries are made up of numerous branching tubules which converge to form the oviducts.
The two oviducts meet and form a common median oviduct. A single vitelline gland or yolk gland consisting of few lobules is situated at the posterior border of the proglottid and opens into the median oviduct through yolk duct. Numerous round shell glands (also designated as Mehlis’ gland) are situated round the tapeworm yolk duct and the shell gland ducts open into the oviduct by the side of the yolk duct.
The specialised part of the oviduct where shell gland ducts and yolk duct open is called ootype. The ootype passes anteriorly into a median, elongated and blind uterus. A fertilizing r spermatic duct arises from the ootype.
The anterior part of the spermatic duct becomes dilated to form the receptaculum seminalis. From the receptaculum seminalis arises the vagina which runs forward and lateral and opens into the atrium. In a mature or gravid proglottid the uterus becomes distended and branched and occupies the major space inside the proglottid. Consequently other structures become reduced and modified.
7. Life-History of Taenia Solium:
Taenia Solium practices self-fertilization, i.e., eggs are fertilized by sperms from the same proglottid or one proglottid may be inseminated by a proglottid situated anterior to it. This is achieved by the bending of the strobila into folds. The possibility of cross-fertilization is remote since no host will be in a position to house two large tapeworms at a time.
Fertilization occurs inside the ootype and fertilized eggs become surrounded by mass of yolk cells secreted by the yolk gland. The fertilized ovum and yolk cells are subsequently enclosed in a thick egg shell secreted by the shell glands. Finally the eggs pass to the uterus.
In the uterus the development of the egg starts (Fig. 69E). The first cleavage results a large megamere and a small embryonic cell. Both the megamere and the small embryonic cell divide and the descendants of the megamere ultimately form an envelope called embryophore round the embryonic cells.
From the embryonic cells develop the embryo proper. Later on, six hooks develop in the posterior pole of the embryo and it is now called a hexacanth embryo [Fig 69E (A-B)].
The whole structure containing the embryo, embryophore and egg-shell is called onchosphere. Terminal proglottid containing onchospheres break off from the strobila (four or five at a time) and pass out of the body of the host along with the faeces. A newly-shed proglottid wriggles for some time but eventually dies and disintegrates.
The onchospheres are not affected but further development within them do not occur until they enter the body of the secondary host (pig) through its food. In the stomach of the pig, as the egg-shell and embryophore get digested, the hexacanth embryo is released.
The embryo with the help of its hooks bores into the wall of the gut and reaches the blood stream. Then it reaches the voluntary muscles or tongue or any other muscular tissue via heart and becomes encysted.
Inside the cyst the embryo increases in size. A large cavity filled with watery fluid is formed inside the cyst and the whole structure assumes a bladder-like appearance. At one point in the bladder an invagination occurs and at the bottom of the invagination a small scolex—the proscolex is formed [Fig. 69E (C-D)].
The embryo at this stage is called cysticercus or bladder-worm. It does not develop further. Pork meat infected with bladder-worms is spotted in appearance and if such imperfectly cooked meat is eaten, the bladder-worms enter the body of man. The invagination is everted and the scolex attaches itself to the wall of the intestine, develops the neck which buds off proglottids.
8. Pathogenic Effects Caused by Taenia Solium:
Taenia Solium may cause:
(i) Cerebral cysticercosis,
(ii) May bring about reduction and complete occlusion of the lumen of intestine,
(iii) Anaemia and
(iv) Gastric disturbances leading to regurgitation of gravid proglottids.