In this article we will discuss about Taenia Solium:- 1. Habit and Habitat of Taenia Solium 2. Structure of Taenia Solium 3. Body Wall 4. Nutrition and Respiration 5. Nervous System 6. Excretory System 7. Reproductive System 8. Life History 9. Parasitic Adaptations.
Contents:
- Habit and Habitat of Taenia Solium
- Structure of Taenia Solium
- Body Wall of Taenia Solium
- Nutrition and Respiration in Taenia Solium
- Nervous System of Taenia Solium
- Excretory System of Taenia Solium
- Reproductive System of Taenia Solium
- Life History of Taenia Solium
- Parasitic Adaptations of Taenia Solium
Contents
- 1. Habit and Habitat of Taenia Solium:
- 2. Structure of Taenia Solium:
- 3. Body Wall of Taenia Solium:
- 4. Nutrition and Respiration in Taenia Solium:
- 5. Nervous System of Taenia Solium:
- 6. Excretory System of Taenia Solium:
- 7. Reproductive System of Taenia Solium:
- 8. Life History of Taenia Solium:
- 9. Parasitic Adaptations of Taenia Solium:
1. Habit and Habitat of Taenia Solium:
Taenia solium, the pork tapeworm of man, as adult lives in the intestine of man leading an endoparasitic life. Its life history is completed in two hosts, i.e., digenetic; man being the primary host and pig as secondary host. Except the adult, various stages of its life history are passed in the body of secondary host.
Other animals like goat, cattle, horse and monkey may also serve as secondary host. T. solium is, however, reported from those parts of the world where pork is eaten, either raw or improperly cooked, especially in European countries.
Since, it lives in adult stage in the intestine of man, it causes injuries to mucous membrane lining the alimentary canal where it adheres by its scolex; it may even cause mechanical injury by obstructing the passage of alimentary canal; it causes abdominal pains, weakness, loss of weight and excessive appetite. However, disease caused by this worm is called taeniasis.
2. Structure of Taenia Solium:
The body of Taenia solium is long, dorsoventrally flattened, narrow, ribbon-like, reaching a length of two to three metres. The colour of the body is opaque-white. Body consists of scolex, neck and strobila or body segments.
(i) Scolex:
The anterior end of the body of Taenia has a knob-like scolex. The scolex is smaller than the head of a pin about 1 mm in diameter with 4 cup-like muscular suckers having radial muscles, and an anterior round prominence, the rostellum having about 22 to 32 curved, chitinous hooks in two circles, the inner circle with larger hooks and outer circle with smaller ones.
The long and short hooks alternate with each other. Each hook is made of a base by which it fixes, a handle directed towards the apex and a conical blade directed outwardly. The rostellum can be protruded slightly. The scolex with its suckers and hooks is an organ of attachment to the intestinal wall of the host, thus, working as an organ of adhesion or the holdfast.
The scolex is, sometimes, wrongly referred to as ‘head’ but it cannot be the head because it is neither related to food- perception nor an organ of catching the food.
Taenia solium proglottid.
(ii) Neck:
Behind the scolex is a thin, small, narrow, un-segmented neck which grows continuously and proliferates proglottids by transverse fission or asexual budding. Therefore, this region is variously called like growth zone, area of proliferation or budding zone, etc.
(iii) Strobila:
The neck is followed by the flattened, ribbon-like body called strobila. The strobila forms the main bulk of the body and consists of a series of proglottids arranged in a linear fashion.
The strobila of a mature tapeworm measures about three metres in length having 800 to 900 proglottids. A proglottid is one complete unit of the body having a complete set of genitalium and surrounding tissue. The linear arrangement or repetition of these units is called proglottisation. The proglottids, internally, remain connected together by muscles, excretory vessels and nerve cords.
However, proglottids are not metameric segments like those of annelids, arthropods and other animals because these are independent, self-contained units, each with a complete set of reproductive organs both male and female and a part of excretory and nervous systems; and they are formed at the zone of proliferation situated anteriorly behind the scolex.
Therefore, the youngest proglottid is next to the neck and the oldest at the posterior end of the strobila.
The proglottids in a mature tapeworm are, however, differentiated into three kinds:
(a) Immature:
The proglottids just behind the neck, devoid of reproductive organs, are broader than long and are called immature proglottids. These include nearly 200 anterior proglottids.
(b) Mature:
Nearly 100 to 150 proglottids, after immature proglottids, bear male reproductive organs only. As these proglottids are pushed back they develop female reproductive organs also. Thus, the mature proglottids are hermaphrodite and they number about 300 to 400 proglottids. The mature proglottids are squarish in shape.
(c) Gravid:
These proglottids are the oldest and towards the posterior side of the strobila and include nearly 150 to 200 proglottids. These segments are longer than their breadth and no reproductive organs are found in them. They contain only branched uterus packed with fertilised eggs.
In fact, the proglottids of strobila widen gradually along their length from anterior to posterior side. The proglottids bear genital papilla and pore, alternating once to right and then to left. In T. solium like other tapeworms, the gravid proglottids are regularly cut off either singly or in group of two to five proglottids from the posterior end of strobila; this is called apolysis.
These detached proglottids are passed out from the body of the host during defaecation along with the faeces. The phenomenon of apolysis is significant because it helps in transferring the developing embryos to the outside from the body of primary host so that they can find a secondary host, and it also keeps the size of strobila restricted within a limit due to continuous proliferation of new proglottids from the neck region.
3. Body Wall of Taenia Solium:
The body wall consists of cuticle, subcuticular muscles, subcuticular cells and parenchyma or mesenchyma.
1. Cuticle:
The cuticle forms the outermost, thick, waxy and enzyme resistant covering of the body. It is composed of protein impregnated with calcium carbonate, and it is traversed by many fine canals. However, it consists of three layers, an outer comidial layer having fine thread-like spines called microtriches, a thick middle homogeneous layer, and the innermost thin basement membrane.
As referred to, some of the fine canals in cuticle layer are connected by gland cells and some receive free nerve endings.
2. Sub-Cuticular Muscles:
Just below the basement membrane of cuticular layer, sub-cuticular musculature is situated. It consists of outer circular muscle layer and inner longitudinal muscle layer. Some longitudinal and circular or transverse muscle fibres are also found in the mesenchyme which constitute mesenchymal or parenchymal musculature.
3. Sub-Cuticular Cells:
A large number of long necked sub-cuticular cells are also found in the mesenchyma which open just below the basement membrane. These cells are said to secrete cuticle. The epidermis is absent; in fact, both ectoderm and endoderm are absent in the adult, only mesoderm is present which forms the various organs of the body.
4. Parenchyma or Mesenchyma:
It is a fluid-filled spongy network of branching mesenchyme cells forming a packing substance around the internal organs and sub-cuticular muscle layer. Therefore, there is no body cavity in it. The turgidity of fluid in the mesenchyme cells helps in maintaining the form of the body. It contains some calcareous lime cells whose secretion probably neutralizes the acid of the digestive juice of the host.
The parenchyma in the neck and young proglottids is thicker and contains some free cells which later differentiate to form the reproductive organs. A band of parenchymal musculature, which is incomplete at the edges, divides it into outer cortex or cortical region and inner medulla or medullary region; the reproductive organs lie in the medullary region.
Electron Structure of Body Wall:
Thread gold and others have studied the electron structure of the body wall (Fig. 42.4); their studies suggest the presence of mitochondria, endoplasmic reticulum and lysosomes in the cuticle. Therefore, it is considered to be living syncytial layer being continuous with the sub-cuticular cells.
Considering this Meglitsch (1972) has called it tegument and the sub-cuticular cells as the tegument secreting cells which continuously renew the tegument.
Functions of Body Wall:
It serves the function of protecting internal organs and absorbs nourishment from the host’s intestine. The microtriches help in increasing its surface area which facilitate absorption or nourishment from host’s intestine and also perform anchoring role with the intestinal wall of the host (Rothman, 1963).
The muscles of body wall enable it to perform movements and parenchyma provides a packing ground as well as it also helps in maintaining the form of the body.
4. Nutrition and Respiration in Taenia Solium:
The alimentary canal is altogether absent in Taenia solium, therefore, its mode of nutrition is saprozoic, i.e., it absorbs digested liquid food from the intestine of the host through its general body surface. In fact, the digested nutrients like glucose, amino acids, glycerol, etc., from the host’s intestine, particularly from ileum, diffuse directly through its general body surface.
It is also thought that as its scolex is deeply anchored into the intestinal mucosa, hence, it also absorbs tissue fluids from the host. The reserve food is stored as glycogen and lipoids in the parenchyma.
Respiration of Taenia Solium:
The mode of respiration is anaerobic, as the tapeworm lives in the intestinal contents which is oxygen free. The energy is derived from the breakdown of glycogen. During the breakdown process, carbon dioxide, fatty acids and other organic acids are produced as waste products.
The CO2 is diffused out through its general body surface, while fatty acids, etc., are removed through the excretory system. It is interesting to note that whenever free oxygen is available to tapeworm, it consumes the oxygen and the rate of consumption being maximum in the anterior proglottids which gradually decreases towards the posterior side of the body.
5. Nervous System of Taenia Solium:
Nervous system of T. solium (Fig. 42.5) consists of two small cerebral ganglia in the scolex connected together by a thick transverse nerve band and by the dorsal and ventral commissures. All these structures together are referred to as the brain complex.
The brain complex is connected with another rostellar nerve ring having a pair of rostellar ganglia in the rostellum by eight nerves. The suckers and rostellum are supplied by the nerve fibres from cerebral and rostellar ganglia. From the posterior side of the brain complex, five pairs of longitudinal nerve cords arise and travel in the strobila.
Out of these, two lateral longitudinal nerves are best developed, they run along the entire length of strobila lying just outside the longitudinal excretory canal. The longitudinal nerve cords are connected in each proglottid by a ring connective situated below the transverse excretory canal.
Nerves originating from ring connective and longitudinal nerve cords innervate the proglottids. Special sense organs are absent but free nerve endings are abundant in the scolex and rest of the body.
6. Excretory System of Taenia Solium:
There are four longitudinal excretory vessels, two on each side, along the margins. Two of these are dorsal in position and they exist only in the anterior part of the strobila; but the other two are ventral and run along the whole length. The four excretory vessels are joined in the scolex by the nephridial plexus. The dorsal excretory vessels carry fluid towards the scolex and the ventral vessels away from it.
The ventral excretory vessels are of unequal diameter and are joined by a transverse excretory canal in the posterior margin of each proglottid.
The excretory canals are lined by cuticle and are non-ciliated, but they are fed by small ciliated branches which ramify into fine capillaries terminating in characteristic flame cells found all over in the parenchyma. The long cilia of flame cells flicker and maintain a hydrostatic pressure by which waste is driven into the excretory canals.
The last proglottid has a bladder or caudal vesicle opening out by an excretory pore, but when this proglottid is shed, the caudal vesicle is lost and the ventral excretory canals open independently to exterior.
The metabolic waste products like fatty acids, organic acids, etc., are removed by the excretory system. This system is said to regulate the fluid contents of the body of Taenia, hence, also regarded to be osmoregulatory in function.
7. Reproductive System of Taenia Solium:
It has already been referred to, that nearly 300 to 400 proglottids of mature proglottids are hermaphrodite having complete sets of male and female reproductive organs. However, the structure of a mature proglottid clearly exhibits the reproductive organs in the following way.
Male Reproductive Organs:
Male reproductive organs consist of testes, vasa efferentia, vas deferens, cirrus or penis and cirrus sac.
Testes:
There are numerous small spherical bodies found scattered throughout the parenchyma in the greater part of the proglottid towards the dorsal side. Few workers are of the view that testis is single but it is branched so that numerous small rounded follicles are formed; however, in the present text the follicles have been referred to as testes.
Vasa Efferentia:
From each testis minute capillary originates, called vas efferens. The vasa efferentia from surrounding testes unite to form a common sperm duct approximately in the middle of the proglottid.
Vas Deferens:
The common sperm duct or vas deferens runs as a convoluted duct transversely to open in the cirrus.
Cirrus and Cirrus Sac:
The cirrus or penis is a muscular intermittent organ in which vas deferens continues. The cirrus is protrusible and is surrounded by the cirrus sac. The cirrus opens by male gonopore into a cup-shaped genital atrium, situated more or less in the middle of the proglottid and at its lateral margin.
The genital atrium opens by the common gonopore on a swollen genital papilla. The genital papilla lie alternately, more or less regularly, in the successive proglottids on the right and left sides.
Female Reproductive Organs:
Female reproductive organs consist of a bilobed ovary, oviduct, ootype, vagina, uterus, Mehlis’s glands and vitelline gland.
Ovary:
It is bilobed situated ventrally in the posterior part of the proglottid, and also called germarium. Both the lobes are connected by a transverse ovarian bridge or isthmus. Each lobe of ovary is formed of a number of radially arranged follicles.
Oviduct:
Nearly from the middle of ovarian bridge a median short but wide oviduct arises which opens into the ootype.
Ootype:
It is a small, spherical bulb-like structure situated at the junction of oviduct, uterus and vitelline duct.
Vagina:
It is a narrow tubular structure that runs slightly obliquely inwards from the female gonopore in the genital atrium and dilates to form seminal receptacle. The seminal receptacle stores the sperms temporarily and continues as a narrow short fertilising duct to join the oviduct.
Uterus:
It is a blind club-shaped, sac-like structure; it originates from the ootype and extends towards the anterior side of the proglottid. The fertilised eggs are packed in it and in gravid proglottids (Fig. 42.12), it becomes highly branched having 7 to 10 branches.
Vitelline Gland:
It is a large lobulated glandular mass situated at the posterior margin of the proglottid. A median vitelline duct originates from it which joins the ootype.
Mehlis’s Glands:
The ootype is surrounded by a large number of unicellular glands called Mehlis’s glands.
8. Life History of Taenia Solium:
Copulation and Fertilization:
Self-fertilisation occurs by the insertion of the cirrus of one proglottid into the vagina of the same proglottid and sperms are deposited there. From the vagina the sperms come to lie in the seminal receptacle from where they fertilize the eggs in oviduct. But cross-fertilisation between different proglottids of the same tapeworm is very common. Actually, T. solium is protandrous, i.e., the testes mature first.
Hence, after copulation the sperms are stored temporarily in the seminal receptacle waiting for the maturity of the eggs and when such eggs come in the oviduct, fertilisation occurs. After fertilisation, the eggs are transformed into capsules and packed in the uterus. Later, the various reproductive organs degenerate and the uterus becomes distended and branched having more than 30,000-40,000 egg capsules.
Formation of Egg Capsules:
Just after fertilisation, the zygote gets surrounded by yolk in the ootype received from the vitelline glands through the vitelline duct. The zygote and the yolk then become enclosed in a thin shell or chorionic membrane which is formed from the yolk material. The structure, thus, formed, is called capsule which passes into the uterus.
The secretion of Mehils’s gland facilitates the passage of capsule in the uterus. However, the gravid proglottid gets separated from the strobila and passes out with the faeces of the host.
Development of Taenia:
It starts when the eggs enter the uterus. The zygote first divides unequally to give rise a larger megamere and a smaller embryonic cell. The megamere divides a number of times to give rise several similar megameres. Similarly, the embryonic cell divides repeatedly to give rise two types of cells, larger mesomeres and smaller micromeres.
Hence, from zygote, three types of cells are resulted. These are larger megameres, medium-sized mesomeres and smaller micromeres which are arranged in a characteristic sequence. In fact, the smaller micromeres form an inner ball of cell mass called morula; the mesomeres are placed as an envelope around the morula, while the megameres as an outer envelope around the mesomeres.
The megameres fuse to form the outer embryonic membrane which finally disappears; the mesomeres form the inner embryonic membrane or embryophore which is thick, hard, cuticularised and striated. Below the embryophore, a thin basement membrane is also formed. The inner cell mass of morula forms an embryo which develops six chitinous hooks at its posterior side.
The hooks are secreted by some differentiated cells of morula called onchoblasts.
This six-hooked embryo is called hexacanth which possesses a pair of penetration glands and is surrounded by two hexacanth membranes. The hexacanth embryo, hexacanth membranes, basement membrane, embryophore and the egg shell or chorionic membrane together is known as onchosphere. The gravid proglottids which pass out from the host body contain embryos in onchosphere stage.
Transmission to Secondary Host:
The gravid proglottids or sometimes when they disintegrate, the onchospheres are eaten up by the pigs with human faeces due to their coprophagous habit. After reaching in the stomach of a pig, the capsule shell and other membranes around the hexacanths are dissolved resulting into the liberation of hexacanths. Sometimes dogs, camels and monkeys may also become infected by these onchospheres.
However, the hexacanth now starts boring through the intestinal wall with the help of a pair of unicellular penetration glands found in it between the hooks. The hooks do not play any role in boring the interstinal tissue but they help in anchoring it.
Thus, the hexacanth enters the blood vessels of the intestine and passes through the heart and finally comes to lie in the striated muscles in any part of the body. But, they usually settle in the muscles of the tongue, neck, heart and shoulder.
After reaching in the muscles, they lose their hooks, increase in size and acquire a fluid filled central cavity then they become encysted in a cuticular covering to become cysticerci or bladderworms. The cysticercus of Taenia solium is called cysticercus cellulosae.
The flesh of pig or pork containing these cysticerci appears white spotted resembling something like that of measles, hence, it is characteristically called measly pork. Thus, the pig becomes infected.
Cysticercus or Bladder-Worm:
It is the larval stage of Taenia Solium which has been formed by the transformation or modification of hexacanth stage. It is a bladder-like sac filled with a clear watery fluid having mostly blood plasma of the host. The wall of the bladder consists of an outer cuticle and inner mesenchyme. A thickening arises gradually on one side of the bladder which marks the anterior end of the larva.
The thickened area invaginates as a hollow knob. The invaginated knob develops suckers on its inner surface and hooks are developed at its bottom. Now, this inverted knob is called proscolex which bears suckers, hooks and rostellum. In fact, the embryo at this stage is called cysticercus or bladderworm whose further development does not take place unless it reaches to the main host, the man.
Transmission to Primary Host:
When man, the main or primary host ingests raw or improperly cooked pork containing cysticerci or measly pork, the cysticerci become active in the intestine. Actually their bladder is digested in the stomach of the host and the proscolex gets evaginated or turned inside out, so that the suckers and rostellum come to lie on the outer surface as in the adult. Thus, a scolex and a small neck is formed.
The scolex anchors itself to the mucous membrane of the intestine and the neck proliferates a series of proglottids to form the strobila. It takes nearly ten to twelve weeks to a proscolex to be converted into an adult Taenia and the adult Taenia, thus, formed, starts producing gravid proglottids with onchospheres within eight to ten weeks.
The life history of Taenia solium is not so complicated because it does not involve any asexual generation.
However, the complete life history of Taenia solium may be represented with the help of the following flow-chart:
Adult tapeworm in human gut → Fertilised eggs in mature proglottids → Egg capsules in gravid proglottids → Onchospheres in gravid proglottids → Gravid proglottids or onchospheres in human faeces → Out from the human body with faeces → Onchosphere in the gut of pig due to coprophagy → Hexacanths in the gut → Hexacanth in the intestinal blood vessels → Hexacanth in the heart Hexacanth in the muscles → Cysticercus in the striped muscles Measly pork → Cysticercus in the gut of human beings → Adult tapeworm in human gut.
Effect of Parasite on the Host:
The infection of Taenia Solium causes a disease called taeniasis in human beings. The taeniasis is characterised by abdominal discomforts like pain, indigestion, vomiting, constipation, loss of appetite, diarrhoea and nervous disorder like nervousness, insomnia, nausea and epileptic fits, etc. Its infection may cause eosinophilia up to 13 per cent and obstruction in the passage of alimentary canal.
Taeniasis is comparatively lesser dangerous than cysticercosis caused by the infection of bladder worm or cysticercus larva. Sometimes, it has been seen that if somehow man ingests onchospheres with contaminated food and drink or due to antiperistaltic movements of the intestine, then bladder worm enters through the intestinal circulation in different parts of the body.
After reaching in the vital organs like liver, eyes and brain, the cysticerci get encysted and cause serious even fatal diseases. Encystment in eyes may cause blindness and in brain develops epilepsy.
Treatment and Control Measures:
Antihelminthic drugs like carbon tetrachloride, oil of chenopodium and oil of male fern Dryopteris may be used to remove the adult tapeworm from the human intestine. These drugs usually remove the strobila only, while scolices can only be removed by surgical operations.
The mode of infection can be checked by the destruction of developmental stages like onchospheres and cysticerci. The onchospheres may be destroyed through proper disposal of human faeces by sewage system or else by preventing the pigs from visiting human faeces.
9. Parasitic Adaptations of Taenia Solium:
The tapeworm exhibits a number of adaptive features to live comfortably in the intestine of the human beings.
Some of these include the following:
1. It has well developed four suckers and hooks to anchor with the intestinal wall of the host, which prevents it from being pushed out with food due to peristaltic movements of the intestine.
2. Its body is covered externally by tegument which protects it form hosts’ digestive juice.
3. Loss of alimentary canal is compensated by freely permeable tegument for water and nutrients from the digested food of the host intestine.
4. The power of anaerobic respiration enables it to live in an environment of intestinal contents which is oxygen-free.
5. The long flattened body provides larger surface area for its saprozoic mode of nutrition.
6. The sense organs are altogether absent due to its sheltered habit.
7. Huge power of reproduction makes it able to ensure for the transference of at least a few embryos to pig and of larvae from pig to man, to maintain the continuity of the race.
8. A tapeworm can survive for more than thirty years and every year it sheds nearly 2500 gravid proglottids containing nearly 30,000-40,000 onchospheres in each of them.
9. The simplicity of its life cycle lowers the chances of hazards it has to face in transfer from man to pig and pig to man. The pigs being coprophagous in habit automatically approach the human faeces containing onchospheres and ingest them to become infective. Man also feeds on pork and, hence, by ingesting measly pork it becomes infected.