Taenia solium Linnaeus: History and Functions

In this article we will discuss about:- 1. History of Taenia solium 2. Systematic Position of Taenia solium 3. Geographical Distribution 4. Functions 5. Reproductive System 6. Protandrous Condition  7. Distinction between T. Solium and T. Saginata 8. Symptoms of Taeniasis Disease 9. Treatment.

History of Taenia solium:

Taenia solium Linnaeus, 1758, is a common tape­worm found parasitizing in the intestine of man. The parasite is of veterinary importance because the larval stage is seen in the pig and dog. The pork tapeworm is the armed tapeworm of man.

Tapeworms are ribbon like or tape-like animals which remain as parasites in the intestine of verte­brates. Tapeworm is a common name. Flatworms that have long, flat and ribbon like body are collectively known as tapeworms.

Many species are included in the tapeworms. They belong to the phylum Platyhelminthes and class Cestoda, which has undergone profound modifications in the struc­tural set-up due to its parasitic mode of life. There are various types of tapeworms, of which the pork tapeworm (Taenia solium) is a common parasite of human being. Taenia solium, Taenia saginata, Hymenolepis nana, Echinococcus granulosus like flat- worms are commonly known as tapeworms.

The disease that is caused by Taenia in man is known as Taeniasis. This disease is mainly caused by two species, Taenia solium and Taenia saginata. According to Leuckart, the specific name solium has been coined from a Syrian word signifying a chain. Both are commonly termed as tapeworms as they are segmented ribbon shaped worms. They generally live as endoparasites in the intestine of vertebrates.

They have no alimentary system and absorb nutrition through their surface from the host. They also do not possess any cloaca but are provided with cuticular body and the body is divided into many segments or proglottids. For anchorage to the intestinal walls they possess hooks, suckers and other organs.

Though Taenia solium and Taenia saginata are living in the intestine of man yet their develop­mental stages are taking place in the body of pig and beef respectively. As the developmental stages of Taenia solium pass through the body of pig; hence this parasite is commonly known as Pork tapeworm.

In case of T. saginata, the deve­lopmental stages occur in the body of cattle, so it is regarded as Beef tapeworm. These parasites are regarded as digenetic parasites, as they require two hosts, such as man and pig, in case of T. solium and in case of T. saginata are man and cattle respec­tively. It is known that Taenia solium is a typical intestinal tapeworm of human beings and so the structure, reproductive system and life history of this parasite.

Systematic Position of Taenia solium:

Phylum – Platyhelminthes

Class – Cestoda

Subclass – Eucestoda

Order – Taenoidea or Cyclophyllidea

Genus – Taenia

Species – solium

Geographical Distribution of Taenia solium and an Account of its Habit and Habitats:

Geographical distribution:

Taenia solium is cosmopolitan in distribution. However, its infection is fairly abundant in India and even in some European countries, America, Latin-America and Africa, because they are consuming pig as their daily food material.

The infection of pork tape­worm is common amongst those eating raw or insufficiently cooked “measly” pork. It is uncom­mon in Jews and Mohammedans who are not generally pork-eaters. In India also the distri­bution of pork tapeworm is restricted.

Habit and habitat:

Adult worm lives as an endoparasite in the intestine of man and in the muscle of pig. So they are regarded as digenetic parasite i.e. requires two hosts to complete its life cycle. Taenia solium adheres to the mucous mem­brane of the intestine of man and injures it. The disease is characterised by loss of apetite, indiges­tion, abdominal pains etc.

Here sexual reproduc­tion also occurs, so man is the definitive host or final host. The larval stage occurs in the muscles of pig which is regarded as intermediate host or secondary host. Taenia solium is found specially in European countries where eating of pork without thorough cooking is a regular practice. But now it is com­paratively rare. The adult remains attached to the intestinal mucosa of man by its scolex and the develop­mental stages occur in pig.

Gross Structure of a Pork Tapeworm:

The body of tapeworm is ribbon-like or tape­ like dorsoventrally flattened and much elongated. It may attain a length of about 3-4 meters. The anterior and posterior ends of tapeworm can be easily recognised by the dorsal and ventral sur­faces and are difficult to distinguish (Fig. 7.1).

The body is differentiated into three distinct regions viz.—head or scolex, neck and strobila.

i. Head or scolex:

The anterior end of this tapeworm is modified into a knob like structure called the head or scolex. It bears two types of hooks (smaller and larger) and four distinct cup like suckers for the attachment to the body of the host but the mouth is absent.

The anterior round prominences at the tip of the scolex is known as rostellum. Approximately 30 to 40 hooks are arranged in two circles. The hooks of the inner circlet are alternately arranged with those of the outer circlet. In T. saginata the scolex is devoid of hooks.

The rostellum with hooks and the suckers act as organs of attachment to the host’s intestinal mucous membrane. So we can say that the scolex is the organ of adhesion (holdfast organ) and it plays no role in ingestion or feeding (Fig. 7.2).

ii. Neck:

Behind the scolex, there is a well- defined, narrow, short and un-segmented tubular region, known as neck. It is dorso­ventrally flattened and is commonly known as the zone of over growth or zone of proliferation. As a result, new segments or proglottids are developed and segments are pushed backwards. Usually 7-8 proglo­ttids are formed every day to form the neck.

iii. Strobila:

The rest of the body is called strobila which is composed of a chain of segments or proglottids (800-900 proglo­ttids in a full grown tapeworm). The proglottids are budded off from the neck region which gradually increase in size and mature towards the posterior end of the body.

There are three kinds of proglottids found in a tapeworm:

(a) Immature proglottids:

These are present near the neck region. These are broader than long and are devoid of sex organs.

(b) Mature proglottids:

These are present in the middle regions of the body and squarish in shape. These are provided with male and female reproductive organs making them hermaphrodite. A mature proglottid is a complete reproduc­tive unit and produces eggs which are fertilised by its own sperms (self-fertilization) or by those of other mature proglottids (cross-fertilization) (Fig. 7.3).

(c) Gravid proglottid:

The oldest proglot­tids present towards the end of the body, last 150 to 350, are termed gravid proglottids. Such proglottid is larger, longer, broad and devoid of reproductive organs. Each gravid proglottid con­tains a branched uterus filled up with fertilised eggs. They get detached from the body of tape­worm and pass to the exterior along with the faeces of the host (Fig. 7.4).

Apolysis:

Small groups of gravid proglottids are regularly detached from the termi­nal part of a strobila and are eliminated to the out­side along with the faecal matter of the host (man). This process of shedding of gravid proglottids is known as apolysis and the tapeworm exhibiting this phenomenon is called apolytic in contrast to the anapolytic tapeworms, which retain all their proglottids throughout life.

This phenomenon exhibits a two-fold pur­pose:

(i) It helps to transfer the growing embryos to the exterior, where they can be ingested by the intermediate host, and

(ii) It limits the size of the body which may otherwise attain enormous length owing to continued proliferation of the neck region.

Utilities of Apolysis in T. solium:

(i) Embryos in the gravid proglottids are getting chance of transference to intermediate host (pig).

(ii) The length of pork tape-worm is maintained owing to the process of apolysis.

Structure of a Mature Proglottid of Taenia solium:

Structure of mature proglottid:

A proglottid from the middle region of the strobila of tape­worm is known as mature proglottid and it exhibits the following features:

(a) The body wall of tapeworm is made up of three main layers, such as cuticle, mus­cular layer and parenchyma.

(b) There are two longitudinal nerve cords, placed laterally, one on each side of the proglottid.

(c) Inside each longitudinal nerve cord there is a longitudinal or lateral excretory duct. These excretory ducts are joined by a transverse connection situated at the base of each proglottid.

(d) The reproductive organs are well deve­loped in mature proglottid. It bears both male and female reproductive organs.

i. The male reproductive system con­sists of numerous testes, vasa efferentia, vas deferens, cirrus sac and genital atrium.

ii. The female reproductive system com­prises of a bilobed ovary, oviduct ootype, uterus, vagina, genital pore, seminal receptacle, vitelline gland, Mehli’s gland etc.

iii. The vas deferens and the vagina open into a genital atrium which opens to the exterior by a genital pore.

The digestive, respiratory and circulatory systems are completely absent in each proglottid.

Functions of Each Part in Taenia solium:

Testes:

They produce spermatozoa.

Vasa efferentia and vas deferens:

Spermato­zoa are carried through the vasa efferentia and vas deferens to the cirrus.

Cirrus:

It acts as a copulatory organ.

Ovary:

It produces ova.

Oviduct:

It carries the ova from ovary to ootype.

Vitelline gland:

The gland secretes yolk materials that surround the fertilized egg.

Ootype:

It is the site where fertilization takes place.

Uterus:

It accommodates thousands of ferti­lized eggs.

Mehli’s gland:

This gland secretes a subs­tance that forms a thin membrane surrounding the fertilized egg.

Female genital pore:

It receives the sperma­tozoa through the cirrus.

Seminal receptacle:

It receives the spermato­zoa.

Vagina:

Tubular pathway which transport sperms towards seminal receptacle.

Reproductive Systems of Taenia solium:

T. solium is a hermaphrodite animal as the male and female sex organs occur within the same individual. The anterior proglottids are devoid of reproductive organs. It is known that male sex organs develop from about 200th proglottid. Subsequently about 350 proglottids possess complete set of male and female repro­ductive organs.

In Taenia the male reproductive organs are differentiated before the female repro­ductive organs, so the animal is termed as protandrous. The rest of the proglottids are gravid proglottids as they contain only fertilised egg.

Male reproductive system:

The male repro­ductive system is composed of testes, vasa effe­rentia, vas deferens, cirrus and cirrus sac.

Testes:

These are small rounded or follicular lobes and are distributed throughout the mesen­chyme in each proglottid dorsally. In the older proglottids the testes disappear. The testes pro­duce spermatozoa.

Vas efferens:

From each testis a fine ductule arises known as vas efferens, which frequently gets interconnected with similar ductules from the surrounding testes. All the vasa efferentia finally unite, approximately in the middle of proglottid to form a common duct or vas defe­rens.

Vas deferens:

The vas deferens is a convolu­ted transverse duct which extends towards the late­ral margin of the proglottid. The end of the vas deferens is provided with a cirrus which opens in the genital atrium. Spermatozoa are carried through the vas efferens and vas deferens to the cirrus.

Cirrus:

The terminal part of the vas deferens becomes thick, muscular and retractile which is known as cirrus. It is a copulatory organ which opens through an aperture termed as male gonopore. The cirrus is enclosed in a firm sheath called the cirrus sheath.

Genital atrium:

The genital atrium swells outside and this swelling at the lateral margin of the proglottid is called genital papilla. The genital atrium opens to the outside by male gonopore. The genital papilla with gonopore is housed at the middle region of the proglottid and it lies more or less regularly on the right or on the left margin of the proglottids.

Female reproductive system:

It consists of the following organs—ovary, oviduct, uterus, ootype, vagina, seminal receptacle, vitelline gland and Mehli’s gland.

i. Ovary:

There is a bilobed ovary where two lobes joined by an isthmus. This bilobed ovary is situated ventrally in the posterior part of each mature proglottid. Each lobe of ovary is dorso­ventrally flattened and is made up of number of follicles which produce numerous eggs.

ii. Oviduct:

A single median oviduct arises from about the middle of the ovarian isthmus and joins with another slender tube posteriorly, the vagina.

iii. Ootype:

It is a swollen chamber formed by the union of oviduct, uterus and vitelline duct. Many unicellular glands known as Mehli’s glands surround the ootype and open into it.

iv Vagina:

It is a bent narrow tube which origi­nates from the female genital pore situated behind the male genital pore in genital atrium. The vagina carries the sperms from the genital atrium to the ootype.

v. Vitelline gland:

There is a large lobulated gland, known as vitelline gland which is situated just behind the ovary. This gland is also termed as yolk gland. It is connected with ootype by a short median vitelline duct. The vitelline gland secretes yolk materials that surround the fertilised egg.

vi. Uterus:

It is a finger like blind cylindrical tube that arises from the ootype and extends up to the anterior region of the mature proglottid. In gravid proglottid it accommodates thousands of ferti­lised eggs by forming 5 to 10 lateral branches on each side.

vii. Mehli’s glands:

Many unicellular glands are present surrounding the ootype. These are called Mehli’s glands or Shell glands. They open into the ootype. The eggs are fertilised in the ootype. A portion of the vagina is dilated to form the spermatheca which stores the sperms.

What is Protandrous Condition in Taenia solium?

Taenia solium is a hermaphroditic animal yet in the first hundred mature proglottids only the male reproductive organs are formed before the development of female reproductive organs. The phenomenon of this parasitic nature is called protandrous condition.

Primary and secondary hosts of Taenia solium and the life history of T. solium along with transmission from the intermediate host to final host.

Hosts:

Taenia solium passes its life cycle in two hosts (digenic). The sexual cycle of pork tape­worm is passed in man, hence man is the primary or definitive host. The intermediate host is the pig who harbours the various larval stages as well as the asexual cycle of pork tapeworm.

Life history:

Taenia solium occurs in the small intestine of man.

The adult tapeworm is hermaphrodite and the mature proglottids bear both male and female reproductive organs producing sperms and eggs respectively (Fig. 7.6).

Copulation and fertilization:

Usually self-fertilization occurs in Taenia, it may be within the same proglottid or between two proglottids of the same individual. Generally single tapeworm resi­des in a primary host (man), so self-fertilization is the usual occurrence.

During copulation the cirrus of male reproductive organ is introduced into vagina of the same or other proglottid to release spermatozoa. It becomes possible when the com­mon gonopores of two mature proglottids come in contact due to folding of strobila. Fertilization, following copulation between two different proglottids, is sometime called cross-fertilization.

Development as well as formation of onco­sphere:

Fertilization occurs within the ootype and the fertilized eggs develop in the uterus. In the meantime the fertilized egg is now covered by yolk-cells containing yolk that are produced from the vitelline gland and now this fertilized egg (zygote) is again covered by a thin membrane which is formed by the secretion of shell glands. All these events occur within the ootype.

So the zygote with two envelopes is called capsule. The capsule so formed passes into uterus, where fur­ther development takes place. As more and more capsules enter into the uterus, so that it develops lateral branches to accommodate the capsules.

The zygote undergoes cleavage and the clea­vage is holoblastic and unequal. Hence the zygote forms three kinds of cells, such as mega-meres, mesomeres and micromeres. Micromeres ulti­mately form the embryo. In the first stage micromeres divide repeatedly to form a mass of cells known as morula which is surrounded by two coverings, first envelope surrounding morula is made by mesomeres and the second envelope outside the mesomeres is formed by megameres.

The yolk cells which are situated outside the megameres gradually transfer the yolk material to the megameres. After transferring the yolk to the megameres, the yolk cells gradually disintegrate. Large yolky megameres fuse to form an outer syn­cytial nutritive envelope which nourishes the embryonic cells and finally disappears.

Now the mesomeres that form an outer covering of an embryo form a thick hard radially striated shell and this shell or membrane is called embryophore. Beneath the embryophore is a thin basement membrane. Inside the embryophore there is a morula. This morula develops an embryo which acquires six chitinous hooks posteriorly. This six- hooked embryo is known hexacanth embryo and the whole structure is called an onchosphere.

Infection to secondary host (pig):

The gravid proglottids, bearing onchospheres, are detached from the terminal part of the strobila which is known as apolysis and pass out of the body of the host along with faeces. These proglottids even­tually disintegrate but the onchospheres remain viable for a long time.

Further development of tapeworm occurs only when the onchospheres are taken by the pig with its food. The secondary host (pig) acquires infection by ingesting the onchospheres. The pig, which regularly feeds on human excreta, is the usual intermediate host.

The dog, camel, monkey, sheep or even human beings are also known to get the infection. Man himself may also serve as the intermediate host by ingesting onchospheres with raw-vegetables or improperly cooked vegetables.

Migration of hexacanth within the secon­dary host:

Onchospheres enter the stomach of secondary host along with the human faecal mat­ter. On coming to the stomach the onchospheres loose their embryophore and basement membrane by the action of an acidic part of gastric juice of secondary host. The hexacanth is now released and enter into the intestine. From one to three days after ingestion, the hexacanth embryos are set free.

Now the hexacanth remains attached on the inner epithelial lining of small intestine of pig by their six hooks and the penetration glands that remain between the hooks secrete a substance that assists to dissolve the epithelial lining. This is accomplished, perhaps jointly by the six hooks and pen­etration glands.

The hooks of hexacanth merely help to anchor with the intestinal wall and the secretion of penetration glands dissolves the intestinal tissues. Now the hexacanths easily pene­trate the intestinal wall to enter into the intestinal blood vessel which carry the hexacanth embryos to the hepatic portal vein.

From the hepatic portal vein, the embryos are transported to the heart. From the heart they are drained (passed) into arte­rial circulation and finally the embryos come to lie in the voluntary (striated) muscles of pig (Fig. 7.7).

Formation of cysticercus or bladder worm:

On reaching their destination the hexacanth embryos loose their hooks, absorb nourishment from host’s tissue and increase in size attaining a diameter of about 18 mm. Now a cavity is developed at the central part of hexacanth and this cavity is filled with a fluid consisting mainly of blood plasma of the host.

The hexacanth embryo now looks like a fluid filled vesicle or bladder. The outer thin wall of vesicle is made up of cuticle (thick syncytial mass) and the inner layer is known as mesenchymal or germinal layer. At one end of the vesicle, the wall becomes thick (oppo­site end of hooks) and invaginates in the vesicle.

The invagination part becomes hollow knob like and the terminal part of the knob differentiates into an inverted scolex possessing rostellum, hooks and suckers. It is called proscolex and the embryo at this stage is called a bladder worm or cysticercus. Generally it is known as bladder worm.

In T. solium bladder worm is of cysticercus type which is characterised by a large vesicle and one scolex. That is why the bladder worm is also termed as cysticerus. It is thought that there is a wall of cellulose in cysticercus and on the basis of this idea the embryo is also referred to as cysticer­cus cellulosae. Cysticercae are oval and whitish in colour. These cysticerci look like small spots in the muscles of pig. Owing to this spotted appearance, the pork meat containing these cysticerci is com­monly called the measly pork.

It is to be noted that cysticerci remain alive in the muscles of pig for eight months in dormant condition, after which they die and become calci­fied in the muscles. In T. solium it takes about 60 to 70 days for the oncospheres to metamorphose into the cysticercus stage.

Infection to the definitive or primary host (man):

Further development does not take place in pig. Cysticercus is the infective stage to man. Human beings are infected through the eating of half cooked, undercooked or raw measly pork. Because in undercooked measly pork, the cysticerci are not being destroyed. When the measly pork enters into the stomach of man, the cysticerci are released from the muscles and the blad­der walls of cysticerci are digested by the action of gastric juice of man. Cysticercus becomes active on reaching the small intestine.

Here the proscolex gets evaginated or turned inside out, so that the hooks, suckers and rostellum come to lie on the outer surface as in the adult. The scolex anchors itself to the wall of the intestinal mucosa with the help of hooks and suckers and develops into an adult worm by gradual strobilisation. The tape­worm grows to sexual maturity in 10 to 12 weeks and starts producing eggs which in turn are passed in the faeces along with gravid proglottids, thereby repeating the cycle.

Distinction between T. Solium and T. Saginata:

The morphology of the genitalia in a proglot­tid follows closely that of the typical taeniid type. It is important—especially in clinical situations— to be able to distinguish specimens (passed by patients) of T. saginata from those of T. solium, since the eggs of T. solium are infective to pig as well as to man and in case of T. saginata the eggs are only infective to cattle.

A much-used criterion is the number of lateral branches of the uterus in gravid proglottids. In T. solium these number are 7-11 and in T. saginata these number are 14-32. A proglottid with 10 branches or fewer can be con­sidered likely to be T. solium and one with more than 16 branches to be T. saginata (WHO, 1983), other morphological or biological differences are listed below (Table 7.1).

Different adaptations of tapeworm for its parasitic way of life:

The parasitic adaptations of tapeworm may be of four types, viz.: morphological, anatomical, physiological and adaptations in life cycle.

(a) Morphological adaptations:

i. Body ribbon-like and consists of many segments of proglottids.

ii. The digestive system is absent so that it absorbs the nourishment from the host body through its body surface.

iii. The head is pear shaped and bears hooks and suckers for anchoring to the intestinal wall.

iv. The reproductive organs are highly deve­loped and reproduction is profile.

v. The hexacanth embryo bears six chitinoid hooks to bore through the gut wall of the host to reach the voluntary muscle.

vi. Larval stages can withstand heat consi­derably and are not killed at the boiling temperature of water.

vii. Strobilisation of the neck region provides life-long growth of tapeworm.

(b) Anatomical adaptations:

i. As digested food can pass on from one cell to other by the process of diffusion, there is no circulatory system.

ii. Reproductive system is highly compli­cated. Every proglottid acts as a single individual bearing a full set of reproduc­tive organs. The ovary is highly developed for producing large number of eggs.

iii. Nervous system is poorly developed and there is no sense organ.

iv. Devoid of mouth and the entire digestive system; the parasite absorbs pre-digested fluid food from the host body.

v. There are no special respiratory organs, as there is low oxygen content in the lumen of the gut of the host, it respires anaerobically.

vi. The locomotory organs are not present as it remains attached to the host body.

vii. Well-developed excretory system is provi­ded with longitudinal excretory canals and flame cells which collect the excretory mate­rials and pour in the lumen of the longi­tudinal canals. This is needed for keeping the body free from nitrogenous wastes.

Physiological adaptations:

i. The osmotic pressure of tapeworm is almost the same as that of the host.

ii. The tapeworm respires anaerobically as it resides in a low oxygen content.

iii. They secrete antienzyme and lime to coun­teract the gastric juice as well as digestive enzymes of the host.

iv. Tapeworm excites the cells of the intestinal wall of host which secrete profuse mucous that provides protective covering over the body of the parasite.

v. Taenia is physiologically adapted to take the food in liquid form from the intestine of the host.

Adaptations in the life cycle:

i. In Taenia large number of eggs are pro­duced to compensate the loss during passage to outside.

ii. The eggs are well-developed and protec­ted by egg-shells. These prevent desication.

iii. Bladder-worm or cysticercus can protect itself in the flesh of the host as it encysts itself.

iv. Gravid proglottids with much branched uterus provide space for a large number of eggs and developing embryos.

v. Hexacanth embryo with six chitinoid hooks ensures a firm hold of the host-tissue. This embryo is also provided with pene­tration glands which assists the hexacanth to penetrate the intestinal wall of interme­diate host (pig) to reach blood circulation.

vi. The presence of an intermediate host ensures its dispersal which is needed for its existence.

vii. Production of huge number of eggs and embryos ensures continuity of its life.

So it is seen that parasitism does not always lead to degeneracy of organs but it is special mode of life that leads to specialisation of organs due to parasitic mode of life or differential livings.

Symptoms of Taeniasis Disease:

Taeniasis:

The disease caused by infection of cestode tapeworm, Taenia solium or Taenia saginatain man, clinically manifested by abdominal dis­comforts, chronic indigestion etc. is called Taeniasis.

Symptoms of this disease:

i. Abdominal pain is a persisting problem.

ii. The patient suffers from nausea.

iii. Indigestion also takes place.

iv. Nervous disorders like epilepsy may be present owing to Cerebral cysticercosis.

v. Anaemia is followed by an increase in the number of eosinophils.

vi. Reduction and occlusion of the lumen of intestine also take place, so the free move­ment of food stuff is disturbed.

vii. The hooks and sucker of tapeworm may cause mechanical irritation in the intestine of the host.

The parasite has more pronounced effect on children. The infection by cysticercus or bladder worm is more harmful as they may reach in impor­tant organs of the host such as brain, eyes, etc.

Taeniasis and cysticercosis:

An infection due to an adult Taenia in man is known as Taeniasis and that due to larval stage is referred to as cys­ticercosis.

Prophylaxis and Treatment of Taenia solium:

Prophylaxis (Prevention):

Following mea­sures should be adopted to prevent the infection of Taenia:

i. Consumption of half cooked or under­cooked measly pork or measly beef should be avoided.

ii. Faeces of tapeworm infected persons should be properly disposed off and destroyed so that vegetation does not get contaminated.

iii. Careful disposal of garbage and arrange­ment of proper sanitation.

iv. Proper care of intermediate host of tape­worm (i.e. pig) should be taken so that they cannot get chance to consume the infected human faecal matters.

v. Personal sense of hygiene should be deve­loped.

vi. Pigs or cattle should be fed with uncontaminated food stuff.

vii. Examination of the flesh or meat before cooking and to avoid measly pork.

viii. Proper treatment of the infected person.

ix. Pigs should not run on free range where human excreta may be found and all per­sons concerned in pig-raising should be regularly examined.

x. Adequate meat inspection should be done in the slaughter house.

Treatment:

i. tapeworm infected person will have to take the advice of a doctor. They should be treated with anti-helminth drugs, such as Mepacrine, Camoquin, Hetrajen, Preziquantal etc.

ii. Kousso flower extract is used to remove the tapeworm.

iii. The most satisfactory compound for human infection is Yomesan (5-chloro-N; 2-chloro-4-nitro-phenyl).

iv. Praziquantel and niclosamide are the drugs of choice for adult worm (T. solium) eradication.

Under the action of drugs, strobila is removed but the embedded scolex persists which again buds off a new strobila in Taenia.

(a) How does infection of tapeworm occurs in human beings?

(b) How the hexacanth embryo of tapeworm enters into the blood vessels of inter­mediate host (pig)?

(c) Mention the route of migration of hexacanth within this host.

(a) Infection to man: The tapeworm is finally transferred to definitive host (man) by ill-cooked measly pork contai­ning the bladder worms (In T. solium— pork containing cysticercus cellulosae). Within the intestine of man, the bladder of cysticercus is thrown off and the scolex gets attached to the intestinal mucosa. The animal starts growing and new seg­ments or proglottids are developed.

(b) When the free hexacanth embryo passes into the small intestine of intermediate host (pig), it is attached to the inner lining of small intestinal epithelium of pig by its six hooks and suckers. Now the penetration glands which are present between the hooks secrete a substance and this secretion assists to dissolve the intestinal tissues and the dissolving pro­cess is accomplished jointly by the six hooks and penetration glands, after that the hexacanth easily penetrate the intes­tinal wall and enters into the intestinal blood vessel.

Entire process occurs within about 10 minutes, after which the hooks, are of no further use, and are shed off.

(c) The intestinal or submucosal blood ves­sel carries hexacanth to liver via hepatic portal vein. From the liver it is trans­ported to the heart via hepatic vein and inferior vena cava. From the heart it is drained into the arterial circulation and ultimately this hexacanth embryo rea­ches the striped (voluntary) muscle of pig. Generally the hexacanth embryo resides specially in the muscles of tongue, neck, shoulder, thigh of pig where it settles to develop into a bladder- worm or cysticercus.