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Home , Taenia solium

Platyhelminthes

Introduction : Phylum Platyhelminthes belongs to kingdom Animalia. This phylum includes 13,000 species. The organisms are also known as . These are acoelomates and they include many free-living and parasitic life forms.Members of this phylum range in size from a single-celled organism to around 2-3 feet long. The simplest that are bilaterally symmetrical and triploblastic (composed of three fundamental cell layers) are the Platyhelminthes, the flatworms. Flatworms have no body cavity other than the gut (and the smallest free-living forms may even lack that!) and lack an anus; the same pharyngeal opening both takes in food and expels waste. Because of the lack of any other body cavity, in larger flatworms the gut is often very highly branched in order to transport food to all parts of the body. The lack of a cavity also constrains flatworms to be flat; they must respire by diffusion, and no cell can be too far from the outside, making a flattened shape necessary. Flatworms were once divided into three groups. The mostly free-living Turbellaria include the planarian, Dugesia, shown above; these are found in the oceans, in fresh water, and in moist terrestrial habitats, and a few are parasitic. The , or flukes, are all parasitic, and have complex life cycles specialized for in tissues. Members of one major taxon of flukes, the Digenea -- which includes the human lung fluke depicted at right -- pass through a number of juvenile stages that are parasitic in one, two, or more intermediate hosts before reaching adulthood, at which time they parasitize a definitive . The , or tapeworms, are intestinal parasites in vertebrates, and they also show anatomical and life history modifications for parasitism. Platyhelminths have practically no fossil record. A few trace fossils have been reported that were probably made by platyhelminths (Alessandrello et al., 1988), and fossil trematode eggs have been found in Egyptian mummies and in the dried dung of Pleistocene ground sloth. Trematode larvae that parasitize molluscs may leave pits or thin spots on the inside of the shell, and these pits may be recognized on fossil shells. If the mollusc is irritated by the presence of trematode larvae, it may be able to surround them with layers of shelly material - and thus do parasites become natural pearls.

Characteristics of Platyhelminthes Platyhelminthes have the following important characteristics: 1. They are triploblastic, acoelomate, and bilaterally symmetrical. 2. They may be free-living or parasites. 3. The body has a soft covering with or without cilia. 4. Their body is dorsoventrally flattened without any segments and appears like a leaf. 5. They are devoid of the anus and but has a mouth. 6. They respire by simple diffusion through the body surface. 7. They have an organ level of organization. 8. They do not have a digestive tract. 9. The space between the body wall and organs is filled with connective tissue parenchyma which helps in transporting the food material. 10. They are , i.e., both male and female organs are present in the same body. 11. They reproduce sexually by fusion of and asexually by regeneration by fission and regeneration. Fertilization is internal. 12. The life cycle is complicated with one or more larval stages. 13. They possess the quality of regeneration. 14. The flame cells help in excretion and osmoregulation. 15. The nervous system comprises the and two longitudinal nerve cords arranged in a ladder-like fashion.

Unique Characteristics of Platyhelminthes Some of the characteristics that distinguish the organisms belonging to phylum Platyhelminthes from others are:

• Presence of flame cells.

• Ladder-like nervous system.

• Presence of parenchyma in the body cavity.

• Self-fertilization

Physiological Processes of Flatworms

The free-living species of flatworms are predators or scavengers. Parasitic forms feed on the tissues of their hosts. Most flatworms, such as the planarian shown in Figure 1, have a gastrovascular cavity rather than a complete digestive system. In such animals, the “mouth” is also used to expel waste materials from the digestive system. Some species also have an anal opening. The gut may be a simple sac or highly branched. Digestion is extracellular, with digested materials taken in to the cells of the gut lining by phagocytosis. One group, the cestodes, lacks a digestive system. Flatworms have an excretory system with a network of tubules throughout the body with openings to the environment and nearby flame cells, whose cilia beat to direct waste fluids concentrated in the tubules out of the body. The system is responsible for the regulation of dissolved salts and the excretion of nitrogenous wastes. The nervous system consists of a pair of nerve cords running the length of the body with connections between them and a large ganglion or concentration of nerves at the anterior end of the worm, where there may also be a concentration of photosensory and chemosensory cells.

There is neither a circulatory nor respiratory system, with gas and nutrient exchange dependent on diffusion and cell-cell junctions. This necessarily limits the thickness of the body in these organisms, constraining them to be “flat” worms. Most species are monoecious, and fertilization is typically internal. Asexual reproduction is common in some groups.

Diversity of Flatworms

Platyhelminthes are traditionally divided into four classes: Turbellaria, Monogenea, Trematoda, and Cestoda (Figure 2). As discussed above, the relationships among members of these classes is being reassessed, with the turbellarians in particular now viewed as a paraphyletic group, a group that does not have a single common ancestor.

The class Turbellaria includes mainly free-living, marine species, although some species live in freshwater or moist terrestrial environments. The ventral epidermis of turbellarians is ciliated and facilitates their locomotion. Some turbellarians are capable of remarkable feats of regeneration in which they may regrow the body, even from a small fragment.

The monogeneans are ectoparasites, mostly of fish, with simple lifecycles that consist of a free-swimming that attaches to a fish to begin transformation to the parasitic adult form. The parasite has only one host and that host is usually only one species. The worms may produce enzymes that digest the host tissues or simply graze on surface mucus and skin particles. Most monogeneans are hermaphroditic, but the male gametes develop first and so cross-fertilization is quite common.

The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex lifecycles that involve a primary host in which sexual reproduction occurs, and one or more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk. Trematodes are responsible for serious human diseases including , a blood fluke. The disease infects an estimated 200 million people in the tropics, leading to organ damage and chronic symptoms like fatigue. Infection occurs when the human enters the water and a larva, released from the primary snail host, locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cells before reproducing. Many of the eggs are released in feces and find their way into a waterway, where they are able to reinfect the primary snail host.

The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates (Figure 3). Tapeworms live in the intestinal tract of the primary host and remain fixed using a sucker on the anterior end, or scolex, of the tapeworm body. The remaining body of the tapeworm is made up of a long series of units called proglottids, each of which may contain an excretory system with flame cells, but contain reproductive structures, both male and female. Tapeworms do not possess a digestive system; instead, they absorb nutrients from the food matter passing them in the host’s intestine. Proglottids are produced at the scolex and gradually migrate to the end of the tapeworm; at this point, they are “mature” and all structures except fertilized eggs have degenerated. Most reproduction occurs by cross-fertilization. The proglottid detaches from the body of the worm and is released into the feces of the organism. The eggs are eaten by an intermediate host. The juvenile worm infects the intermediate host and takes up residence, usually in muscle tissue. When the muscle tissue is eaten by the primary host, the cycle is completed. There are several tapeworm parasites of humans that are transmitted by eating uncooked or poorly cooked , beef, and fish.

Classification of Phylum Platyhelminthes: In the evolutionary process, the evolving of mesoderm, a third layer between ectoderm and endoderm has further resulted in structural complexities, coupled with increased in size. Animals built on this three layered foundation have been designated as triploblastic. Platyhelminthes is considered to be one of the primitive triploblastic groups.

Platyhelminthes or flatworms are a diverse group comprising of 25000 living species that show evolutionary achievements over the diplo-blasts in having a structural body plan that is based on bilateral symmetry and in having definite organs or system of organs.

They, however, lack (the space between the various organs being filled with a special connective tissue called parenchyma), the body being compact (acoelomate) and the absence of blood vascular system.

According to the classification scheme followed of Ruppert and Barnes, 1994 (6th edn.) - the phylum Platyhelminthes is divided into four classes. Three are entirely parasitic and include Trematoda, Monogenea and Cestoidea (tapeworms). The fourth class, Turbelaria, is free-living and from this group arose the ancestors of the three parasitic classes.

Systematic Resume of Phylum Platyhelminthes:

Class Turbellaria: i. Turbellarians are mainly free-living and aquatic (majority marine, few fresh water), but there are some terrestrial species also confined to humid areas. Aquatic forms are benthic. ii. Body size ranges from a few millimeters to 50 centimeters. iii. An acoelomate grade of body construction present with a low level of cephalization. iv. Most turbellarians move by cilia (lining the epidermis), which provide the propulsive force; larger forms (poly- clads) are markedly flattened and move by combine ciliary motion with muscular undulations. v. Mucus produced from the gland cells, lubricates and protects the body surface, provides leverage for the cilia and is used in the capture of food by entangling the prey. vi. The mouth, which is present on the ventral surface, is located at the end of an eversible pharynx and leads into a sac-like, lobed or much branched intestine. vii. Digestion is initially extracellular and then intracellular. Most turbellarians are predators and scavengers while a few are herbivores, commensals or parasites. viii. Excretion by protonephridia. ix. Several pairs of longitudinal nerve cord are radially arranged and associated with peripheral nerve nets. Numerous pigment cup, ocelli and statocysts are the principal sensory organs. x. , with internal fertilization. Cleavage is spiral and development is direct in most species. Many turbellarians also reproduce asexually by means of budding or transverse fission and show high power of regeneration.

Examples: Ectocotyla (commensal on hermit crab), Planeria (fresh water flat worm), Anaperus (marine flat worm), Gyratrix (marine interstitial species), Stenostomum (fresh water species), Bipalium (land planarian), Bdelloura (horse-shoe crab, commensal), Dugesia (fresh water).

Class Trematoda: i. All are parasites, occurring especially in vertebrates. ii. Body cylindrical or leaf shaped. iii. Rhabdites or cilia are absent. iv. Body is covered by cuticle, which provides protection against hosts enzyme- action and defense mechanism. v. Suckers and acetabulum present for attachment. vi. Mouth is anteriorly situated. vii. A well-developed gut is present. viii. In most cases the testes are two but always one is present. Reproduction is always sexual.

Class Monogenea: i. Monogeneans are mostly ectoparasites of aquatic vertebrates, particularly fishes, but amphibians, reptiles and cephalopod molluscs are also hosts. ii. Body dorsoventrally flattened and have a large, posterior attachment organ, the haptor, which bears hooks and suckers. iii. Anterior end has also adhesive glands. iv. A gut is present -but mouth lacks a sucker. The pharynx secretes a protease that digests the host’s skin. v. They respire aerobically, being ectoparasites. vi. Monogeneans have inconspicuous proto- nephridia for excretion. vii. All members are hermaphrodite. viii. Life cycle simple and having no intermediate host. ix. One egg by way of a ciliate larva (Oncomiracidium) gives rise to only one adult worm and hence the name monogenea meaning “one generation”.

Examples: Poly stoma (bladder of frogs), Polystomoidella ( of turtles), Dactylogyrus (gills of freshwater fishes).

Class Cestoidea: i. Endoparasitic helminth, whose body is covered by a syncytial . ii. Cestoideans have special tegumental modifications associated with nutrient uptake as they lack a mouth and digestive tract. iii. Organs of attachment in the form of hooks and suckers are present. iv. Absence of sense organs. v. Each segment excepting the head and neck is provided with one or two sets of complete sex organs. vi. Hermaphrodite. Life cycle is complicated and involves a larval stage.

Fasciola hepatica -

Lifecycle :

Liver fluke disease () is caused by the trematode parasite . Disease can result from the migration of large numbers of immature flukes through the , or from the presence of adult flukes in the ducts, or both. can infect all grazing animals (and man) but mainly affects sheep and cattle. It is most pathogenic in sheep. Compared to other helminths, the lifecycle is complex and involves an intermediate host, the mud snail Galba (Lymnaea) truncatula and several free-living stages. The role of the snail, which prefers muddy, slightly acidic conditions, particularly areas associated with poor drainage, means the incidence of liver fluke is far greater in the wetter areas of the country and in years when there is high summer rainfall. With the capacity of the snail to multiply rapidly (100,000 offspring in three to four months) along with the multiplication of the parasite within the snail, there is potential for very large numbers of parasites.

Adult fluke lay eggs that are passed out onto pasture in the faeces. At suitable temperatures, a miracidium develops within the egg, hatches and migrates in thin films of moisture, actively seeking the snail host. Miracidia can only survive for a few hours outside the snail. Within the snail they undergo two further developmental stages, including multiplication, eventually becoming infective cercariae, which emerge from the snail when the temperature and moisture levels are suitable. The cercariae migrate onto wet herbage, encysting as metacercariae, the highly resilient infective stage of the liver fluke. Following ingestion, the young flukes migrate to the liver, through which they tunnel, causing considerable tissue damage. The infection is patent about 10-12 weeks after the metacercariae are ingested. The whole cycle takes 18-20 weeks.

Epidemiology The hatching of fluke eggs and the multiplication of snails depend on adequate moisture and temperatures greater than 10ºC. Such conditions usually occur from May to October in the UK although patterns have been changing in recent years. The incidence of fasciolosis is highest in years when rainfall is above average during May, June and July. The epidemiology of liver fluke is often viewed as the result of two distinct cycles of snail infection and pasture contamination.

• Summer infection of snails: In wet summers, snail populations multiply rapidly and snails are invaded by hatching miracidia from May to July. If wet weather continues, the snails shed massive numbers of cercariae onto pasture during July to October. Conversely, if the climate in May and July is dry or cold, fewer snails appear, fewer fluke eggs hatch and levels of contamination in the autumn are much lower. Clinical fasciolosis resulting from summer infection of snails arises usually from ingestion of large numbers of metacercariae over a short period of time in July to October. • Winter infection of snails: Less commonly, snails can become infected in late summer or early autumn and development within infected snails is delayed as the snails become dormant and hibernate. The cercariae are then not shed onto the pasture until the following spring. This can produce an initial and significant infection in herds or flocks in the spring.

Fasciolosis

• Liver fluke disease in sheep occurs in three main clinical forms – acute, subacute and chronic fasciolosis. Which form occurs depends on the numbers of infective metaceriae ingested and the period of time over which they are ingested. Recent milder winters and wetter summers have seen changes patterns in parasite epidemiology and reported disease with earlier seasonal reports of acute disease.

Pathogenicity : Infections have been associated with two types of liver disease in domestic animals: acute or subacute necrotic disease due to juvenile flukes; and chronic fibrotic disease due to adult flukes. Penetration of the liver capsule by immature flukes generally does not cause much damage, but their subsequent migration through the liver parenchyma may cause significant necrosis (liver rot). Mass migration of juveniles may produce extensive traumatic tissue damage, coagulative necrosis, haemorrhage, urticaria, eosinophilia, leukocytosis, pallor, anaemia, and can be fatal. Acute infections in sheep can also be complicated by secondary bacterial infection causing clostridial necrotic hepatitis (‘black disease’). Chronic infections by the long-lived adults feeding on the lining of the bile ducts may result in progressive loss of condition, biliary epithelial hyperplasia, duct fibrosis, biliary obstruction and cholangitis, jaundice, and eventually a fibrotic hardened liver. Sheep may become anaemic and emaciated, developing submandibular oedema (bottle-jaw) and ascites. In cattle, the bile ducts often become calcified producing a ‘clay-pipe’ or ‘pipe- stem’ liver. Chronic fascioliasis causes significant economic losses to many animal industries through mortality, reduced meat, milk and fibre production, condemned , secondary infections and expensive treatments. Control measures : Subacute and chronic infections may be treated with triclabendazole or bithionol, which show excellent trematocidal activity with few side-effects. A range of other anthelmintics show variable activity, including carbon tetrachloride, rafoxanide, niclofolan, closantel and oxyclozanide, but their use may be contra-indicated under certain conditions in certain animals. Preventive measures are based on breaking the cycle of by reducing faecal contamination of water bodies, reducing snail populations using molluscicides (usually copper sulphate) or draining swampy fields, restricting access of livestock to aquatic vegetation, and avoiding watercress. Snail control is often difficult, particularly in high rainfall areas where even temporary pools may harbour large snail populations (they aestivate in the ground during dry conditions). Feral or wild animals (such as rabbits) may also continue to act as reservoirs of infection for domestic livestock.

Taenia solium : Lifecycle :

Taenia solium is one of the many species of cestodes (tapeworms) that can infect humans. It belongs to the phylum Platyhelminthes, class Cestoidea, order , family . In general terms, tapeworms are complex organisms having complex life cycles that require at least two hosts for their completion. In the case of Taenia solium, humans are the most important definitive hosts, whereas both and humans are the main intermediate hosts. The list of other animals that may act as definitive or intermediate hosts of Taenia solium is large but clinically irrelevant.

Tapeworms are internal parasites that attack vertebrates like human beings, cats, dogs and fish. The disease caused by tapeworm is known as cestodiasis. Physically the tapeworm has an elongated body segment, head and suckers or hooks for attaching themselves to the host. Tapeworm bodies are covered with hard cuticle covering, through which they absorb food.

▪ The life cycle is completed in two hosts. ▪ Definitive host: Human ▪ Intermediate Hosts: , occasionally human. ▪ Humans acquire infection by ingestion of inadequately or improperly cooked pork infected with cysticerci. ▪ Inside the alimentary canal of man the scolex on coming incontact with bile exvaginates and anchor to the gut wall with its hooks and suckers. ▪ The larvae develops into an adult worm by gradual strobilisation. ▪ The worm grows to sexual maturity in 2-3 months and start producing eggs which are then passes in the faeces along with the gravid segments. ▪ The pig gets infection by ingestion of eggs or gravid proglottids passed in human faeces. ▪ In the intestine of pig, the oncospheres hatch out of eggs. ▪ They attach to the intestinal mucosa by hooks, penetrate the gut- wall and gain entrance into the portal vessels or mesenteric lymphatic, finally reaching the systematic circulation. ▪ Usually they travel via the portal vein and successively reach the liver, right side of heart, lungs, left side of heart, brain or other tissue with high blood flow. ▪ The naked onchospheres are filtered out from the circulating blood into the muscular tissue where they ultimately settle down and undergo further development. ▪ They lose their hooklets, enlarge, and develops into a fluid-filled within a period of 9-10 weeks. ▪ They remain viable for up to 8 weeks in muscle of pig during which they remain infective for human. ▪ The new host gets infection by ingestion of the infected meat of pig and the cycle is repeated. ▪ Occasionally humans get infection by eating food or drinking water contaminated with eggs. ▪ On ingestion, the onchospheres are released from the eggs in the intestine. These larva invade the intestinal mucosa and are then carried by the circulation to different tissue where they develops into . ▪ In human most cysts are produced in the CNS, skeletal muscles, eye and subcutaneous tissue giving rise to a condition called .

Mode of transmission:

▪ Ingestion of uncooked pork infected with tape worm ▪ Ingestion of food and water contaminated by the eggs present in the infective faeces of a Taenia carrier. ▪ Endogenous auto infection: Anus-hand-mouth transfer of eggs by contaminated hands of person with poor personal hygiene. ▪ Autoinfection: Reverse peristalsis in which eggs produced by T. solium are thrown back to the , where they hatch and cause tissue infection

Majority of tapeworms are hermaphrodite. A hermaphrodite is an organism with both male and female reproductive organs. They produce gametes of both male and female sexes.

In the life cycle of Taenia solium, the adults (length 2 to 7 m; less than 1,000 proglottids; longevity up to 25 years) develop not only in humans but also some other animal species (monkeys, hamsters). Humans develop when they ingest undercooked pork meat containing cysticerci. They may become infected by ingesting T. solium eggs, either by ingestion of fecally contaminated food, or by autoinfection. In the latter case, a human infected with adult T. solium ingests eggs produced by that tapeworm, either through fecal contamination or, more arguably, from proglottids carried into the stomach by reverse peristalsis. The cysticercus develops not only in striated muscle, but also in the brain, liver, and other tissues of pigs and other animals, including humans. Taenia solium- pork tapeworm Humans are definitive host Pigs are the most common intermediate host Many animals including humans can harbor cysticerciusually in striated muscle, but often in other tissues/organs The eggs of T. solium are morphologically rounded or subspherical, diameter 31 - 43 µm, with a thick radially striated brown shell. Inside each shell is an embryonated oncosphere with 6 hooks. Adult tapeworms complete the sexual stages of life cycle in humans as definitive hosts. Pigs serve as an intermediate host where metacestodes (larva stage) survive in the muscles and other tissues as cysticerci. However, T. solium is unable to mature to adult stages in pigs to complete the reproductive cycle. While pigs acquire T. solium from ingesting Taenia eggs present in the , humans acquire the infection directly from eating viable cysticerci present in uncooked pork or through ingestion of contaminated food or water containing eggs or proglottids.

EPIDEMIOLOGY: Risk factors – Poor hygiene and limited use or absence of latrines • Low economic status, low level of household sanitation and low personal hygiene standards • Open field defecation • Those households that have latrines don’t always use them • Tapeworm carriers disseminate the parasite eggs in their environment leading to the contamination of – soil, – water (many homes lack piped water) – and food resources • Eggs may stay alive in the effluent from sewage treatment (shown for ) spreading the disease to other areas or villages

Pathogenicity :

▪ Both adult worm and cyst are pathogenic.The adult worms are less pathogenic. They occasionally cause mild irritation or inflammation of the intestinal mucosa by their armed scolex. ▪ The cyst, (Cysticercus cellulosae) are more pathogenic. They cause a serious disease cysticercosis in human, mostly cyst are produced in the skin, skeletal muscles, eye and CNS. ▪ The cyst can remain viable for few years. ▪ In the brain the cyst survives by overcoming the host defenses. It secretes the prostaglandins and other substances that inhibit activation of the complement and production of cytokines. This result in minimal host inflammation around the live cysticercus. The live cyst is surrounded by a local minimal cellular reaction that consists of few eosinophiles and macrophages. ▪ The dead cyst is surrounded by a dense infiltration that consists entire spectrum of inflammatory cells, including leucocytes and multinucleated giant macrophages, inflammatory cells and less frequently foreign body giant cells. Outside this area a zone of fibrosis and chronic inflammatory infiltration are present.

Control measures : • Most effective: General improvement of the economic situation of the endemic areas and improvements in public sanitation and pig husbandry – Largely eradicated in high income areas without special interventions. • Confinement of the pigs to avoiding access to human faeces. • Meat inspection - however lack of slaughterhouse facilities and inspection of pork is poorly implemented. Inspection of pork for cysticerci. • Intensive educational intervention. • Treatment of infected persons. Chemotherapy against human taeniosis – but effect is partial; cannot eliminate taeniosis/cysticercosis complex if the entire targeted population is not treated repeatedly. • Avoidance of food contaminated with eggs of T. solium