The 3rd Lecture
CLASS CESTOIDEA
SUBCLASS CESTODA
These are called the tapeworms (cestos = tape). Sexually mature forms live in the intestine of all classes of vertebrates.
General characters:
As platyhelmitnths, they are dorsoventrally flattened, and have no body cavity.
They are ribbon-like, and of variable lengths, ranging from few mms. up to several meters.
The body is formed of three main parts:
The scolex (head) is the anterior end of the worm and usually it is equipped with fixation organs to maintain its position in the gut. The organs of fixation are in the form of suckers (acetabula), grooves or bothria, and protrusible leaf-like organs (bothridia). Scolices having acetabula may carry hooks for anchoring them to the gut.
The neck: it lies behind the scolex, usually thinner in size. It is an undifferentiated zone containing the stem or germinal cells that give rise to the following part of the body.
The strobila: it is a unique structure for cestodes. It is a linear series of sets of reproductive organs; each set is called proglottis or proglottid. Immature recently formed proglottids are near the neck and they are more mature when we go posteriorly. Mature proglottids carry reproductive organs of both sexes. When the uterus becomes full of eggs, which cannot pass outside, it compresses other organs causing their disappearance and the gravid proglottid is formed. There are constrictions between proglottids, but there is no true segmentation because the tegument, muscle, and some organs are continuous between them.
Alimentary canal is absent. Cestodes get the nutrients through active transport of readily digested food.
The tissues of most cestodes contain cells that form calcareous corpuscles. These are highly refractile bodies of 12-32 μm. in diameter
and are formed of inorganic elements.
The muscular system in the cestodes is composed of contractile longitudinal and circular bundles in the tegument. If any part of the strobila is cut, it can move spontaneously and independently, so groups of gravid proglottids can pass independently from the anus.
The nervous system is centered in the scolex.
The excretory or osmoregulatory system is composed of main canals that run longitudinally from the scolex down to the posterior end, usually in two lateral pairs one ventral and anotherdorsal.
The reproductive system of cestodes is represented in each proglottid. The male system is composed of many testes distributed dorsally ending in the genital pore with the vagina. The female system is formed of an ovary, tubular uterus connected to a vagina opening in the genital pore, vitelline glands differ in shape according to the species, and open with the shell glands into the ootype between the ovary and uterus.
The development of cestodes differs according to the orders. In common, they are heterozygous, having a definitive host in which they live in the intestine, and an intermediate host that harbors the larval stages in its tissues. Table showing the comparison between the two orders:
There are two orders of medical importance that infect man and domestic animals; these are Pseudophyllidea and Cyclophyllidea.
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Pseudophyllidea |
Cyclophyllidea |
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Scolex Organ of fixation |
Elongate (almond-shape) Bothria (longitudinal grooves) |
Globular Suckers (acetabula) Rostellum with hooks |
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Mature segments: Genital pore:
Uterus:
Vitelline glands: |
Ventral
Opens ventrally passing eggs
Scattered in the lateral fields |
Lateral
Blind-ended, becomes full of eggs forming gravid segment Single compact mass.
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|
Gravid segments |
Absent. |
Present. |
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Eggs: |
Operculated Immature embryo Covered by egg-shell |
Non-operculated Mature embryo Covered by 2 embryophores |
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Onchospere: |
Ciliated (coracidium) |
Non-ciliated onchosphere |
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Intermediate host: |
Two Solid larval stages: * Procercoid. * Plerocercoid (sparganum) |
One, or sometimes none Cystic larval stages: * Cysticercus * Cysticercoid * Coenurus * Hydatid. |
|
Members: |
Genus Diphyllobothrium |
Family Taeniidae Family Hymenolepididae Family Dilepididae |
ORDER PSEUDOPHYLLIDEA
Diphyllobothrium latum
(The broad fish tapeworm)
Hosts:
-Man and fish eating mammals
-First intermediate host is crustacean of gnus Cyclops.
-Second intermediate host is fresh water salmon or trout fish.
Distribution:
Widely distributed in the world. It is present in Northern Europe, Siberia, North America, Japan, South Africa, South America, and Israel.
Morphology:
Size: 3-10 meter in length
Scolex:
Club-like, 2-3mm in length, with ventral and dorsal bothria. (Groove).
Mature segment :
Broader than long (nearly 3x15 mm)
containing both male and female genitalia.
Male genitalia: consists of numerous testes scattered in the dorsal part of the proglottis.
ovary bilobed, lies in the posterior third of the segment
Uterus is convoluted, rosette-shaped and situated in the middle of the segment and uterine pore lying ventrally posterior to the genital atrium.
Vagina opens by female genital pore in the genital atrium.
Egg
Ellipsoid shaped, about 70x45 m,yellowish brown in colour.
Operculated and thin- shelled.
Contains one cell stage embryo surrounded by yolk cells (immature content).
Life cycle:
Adults inhabit the small intestine of the final host.
An adult worm sheds about million eggs / day. Immature eggs must reach fresh water to complete their development. Development of eggs takes about 2 weeks then they hatch and the ciliated coracidium swims in water.
The coracidium is a spherical ciliated hexacanth embryo. It swims in water and is swallowed by the Cyclops or Diaptomus (the first intermediate host). Inside the host’s gut the coracidium loses its cilia, becomes an onchosphere and penetrates the gut wall into the body cavity to transform into the first larval stage.
The 1st. larval stage or procercoid forms within 2-3 weeks. It is a solid elongated organism, about 4mm. long, and has a spherical caudal end
(the cercomer) the 6 hooks of the onchosphere are present.
When the infected Cyclops is swallowed by the 2nd. Intermediate host, the salmon or trout fish, the procercoid becomes free after digestion of its host in the fish intestine. The procercoid penetrates the intestine and reaches various tissues and muscles where it grows to the 2nd. Larval stage, the plerocercoid or sparganum within 2-3 weeks. This can happen also in paratenic hosts like pigs. The plerocercoid is solid, elongated, falsely segmented, 1-2cm. long and has small bothria on the anterior narrow end.
The plerocercoid infects the final host on eating the infected fish fresh, improperly cooked or smoked. In the gut of the final host it attaches itself by bothria and grows to an adult worm producing eggs within 1-2 months.
Pathogenesis and clinical picture :
Many cases of diphyllobothriasis are asymptomatic, or have poorly defined symptoms associated with other tapeworms, and discover the infection accidentally when they pass a series of old proglottids. But infected people may have:
Intestinal disturbances as nausea, vomiting, abdominal pain and diarrhea.
Neurological manifestations as irritability, insomnia and rarely convulsions.
Megaloblastic anemia or bothriocephalus anemia. This is caused by the affinity of the worm to absorb vitamin B12 formed in the small intestine. The anemia does not differ from pernicious anemia of vitamin B12 deficiency except in that it is rapidly cured after treatment of diphyllobothriasis. Infection is easily diagnosed by finding the eggs or proglottids on stool examination.The drug used effectively in the past 20 years is niclosamide (Yomesan).
Prevention and control:
Praziquantel is used recently with the same efficiency.
Treatment:
Diagnosis:
Proper cooking fish or prior freezing at -10ºC for at least 72 hours, or prolonging salting before smoking.
Sanitary sewage disposal to cut off the life cycle.
Trial to get rid of reservoir hosts like stray dogs or cats, but the parasite has a wide range of wild reservoirs like bears, raccoons, wild felines and other fish eating mammals.Sparganosis
Group discussion
Taenia saginata
(Beef tapeworm, Bald tapeworm)
Disease:
Taeniasis saginata.
Distribution:
Cosmopolitan.
Host:
Final host: Man.
Intermediate host: cattle.
Morphology:
Scolex: globular in shape with 4 cup shaped suckers ,no rostellum, no hooks.
Adult: Size:4-10 meters
Mature segment:
Slightly broader than long (about 1cm long), containing the genital organs.
The genital pores open laterally in an irregular alternating manner.
Male genital organs consist of numerous testes scattered throughout the segment.
Ovary: bilobed in the posterior part of the segment.
Uterus: simple tube in the median plane.
Vitelline gland: compact and posterior to the ovary.Gravid segment:
Longer than broader (about2×1cm in size).
Filled with the branching uterus engorged with eggs.
Having 15-30 lateral branches on each side.
Other genitalia disappeared or atrophied except genital pores.Egg:
Spheroid in shape, 30-40 m in diameter
Yellowish brown in colour.
Containing a hexacanth embryo (onchosphere).
The embryophore is thick, radially striated and covered with a mucoid egg-shell with two polar filaments. Life cycle:
The adult worm lives in the upper part of the small intestine of man.
Separate gravid segments full of mature eggs pass in faeces, and then they disintegrate liberating eggs. Gravid segments can also creep outside the anus by their own movements.
If the eggs are swallowed by the intermediate host, the onchosphere hatches, penetrates through the intestine reaching the lymphatics, then the blood stream to the right side of the heart, the lung, then left side of the heart to be distributed by the systemic circulation to the various organs specially the active muscles like tongue, heart and limb muscles, …etc.
The infective cysticercus bovis is fully developed in about 12 weeks and remains viable for about one year.
The final host is infected by swallowing the raw or undercooked beef containing the viable cysticercus bovis. In the intestine, the scolex evaginates, attaches to the mucosa and the neck begins to form strobila, which matures in about 10 weeks.
Pathogenesis and clinical manifestations:
The worm is of low pathogenicity. It competes with the host for the nutrients in the intestine.
Intestinal disturbances as hunger-pains, indigestion, abdominal discomfort, diarrhea or constipation can occur due to the presence of a large worm in the intestine.
Loss of weight and multiple vitamin deficiencies.
Intestinal obstruction rarely occurs.
Diagnosis:
Finding gravid proglottids in the faeces.
Finding the eggs from deranged proglottids in feces.
Gravid segments can pass out of anus by their own movement without defecation. The drugs of choice are niclosamide (Yomesan) and praziquantelPrevention and control:
Treatment:
Sanitary sewage disposal.
Treatment of infected cases.
Inspection of meat at the slaughterhouses.
Thorough cooking of meat (at least 56°C for 5 min.) or freezing at -10°C for 5-10 days.
Taenia solium
(Pork tapeworm)
The difference between Taenia saginata and Taenia solium:
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Taenia saginata |
Taenia solium
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Distribution |
Cosmopolitan |
Cosmopolitan |
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Adult: Size Scolex
Mature segment
Gravid segment |
4-10 m. 2mm in diameter. No rostellum or hooks.
testes: 400 Ovary: bilobed. Vaginal opening: with a sphincter. 20x7 mm Uterus: with 15-30 lateral branches
Segments detach singly and can creep outside the anus without defecation. |
2-4m. 1mm in diameter. Having a rostellum with double crownof 36 hooks. testes:200 ovary: trilobed without a sphincter.
12x6mm 7-12 lateral rather simple branches.
Segments detach in-groups and do not creep outside the anus without defecation. |
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Egg |
Similar (not infective to man) |
Similar, but infective to man. (cysticercosis) |
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Definitive host Intermediate host |
Man Cattle. |
Man Pig and man. |
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Larval stage |
Cysticercus bovis |
Cysticercus cellulosae (scolex with hooks) |
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Human infection |
Ingestion of undercooked beef containing viable Cysticercus bovis. |
1-Ingestion of undercooked pork containing viable Cysticercus cellulosae infects with the adult worm. 2- Ingestion of the eggs, external and internal autoinfection lead to cysticercosis. |
Pathogenesis:
Adult worms: as in T.saginata.
Cysticercosis:
Man can be infected with T. solium eggs either by
external autoinfection (hand to mouth), or
from soiled vegetables), or
internal autoinfection when gravid segments pass back to the stomach with antiperistalsis during vomiting or anesthesia.
Gastric juice breaks the embryophore and the onchosphere completely becomes free in the duodenum.
The onchosphere penetrates intestinal wall and passes with circulation to the liver, lungs and then systemic circulation to the brain, kidneys, and muscles.
The cyst elicits an immunological inflammatory reaction in the surrounding tissue. This begins by aggregation of lymphocytes, monocytes, then aggregation of fibroblasts, transformation into fibrocytes and the cysticercus becomes surrounded by a fibrous capsule. Eventually calcification occurs in the cysticercus.
The inflammatory reaction can pass unnoticed in organs like liver or gives mild symptoms. In case of muscles it causes myositis and limitation of movement. Cerebral cysticercosis causes neurological symptoms.
Diagnosis:
The adult worm: as in T.saginata.
Cysticercosis:
Biopsy whenever possible.
X-ray (may show calcification present in old cases).
Ultrasonography, C.T., MRI can show the lesions in liver or brain.
Serological
Immunological tests, these are available in endemic areas even to pig-raising farms.Treatment:
The adult: as in T.saginata, but anti-emetic drugs should precede treatment to prevent vomiting and reverse peristalsis which gives the eggs returning to the stomach a chance to hatch and produce Cysticercosis.
Cysticercosis: Surgical treatment (if available) in association with anti-inflammatory drugs and steroids to prevent immune reaction.
Prevention and control:
Sanitary sewage disposal.
Avoiding plant fertilization with human excreta.
Proper inspection of restaurants and food handlers, as well as slaughterhouses.
Cooking meat for enough time to kill cysticerci, or freezing at -10°C for 5-10 days. Echinococcus ganulosus
Distribution:
Worldwide, where dogs are used as pets or as guards or for herding.
Hosts:
Definitive hosts are dogs, wolfs, wild canines.
Intermediate hosts are grazing herbivorous animals, and accidentally man.Small intestine of dogsSize: 3-6 mmWith 4 suckers and Rostellum with double crown of hooks. Gravid segment: similar to T.saginata but differs in:
Mature segment similar to that of T.saginata
Scolex
Morphology:
Habitat:
Smaller in size.
Uterine branches are 7-1
2 on each side of the uterine tube. Thick-walled cyst 1- 5, up to 15 cm. in diameter.Contains the hydatid fluid, this is turbid fluid filled with microscopic scolices, brood capsules (collections of scolices) and fragmented scolices releasing hooks and calcarious corpuscles (hydatid sand).Invagination of germinal layer only inside the cyst results in formation of brood capsules (which may separate in the hydatid fluid) while invagination of the whole (3) wall layers form an endogenous daughter cyst, its evagination outside the cyst wall causes the formation of exogenous daughter cyst. Adults inhabit the small intestine of dogs; their gravid segments disintegrate releasing eggs in dog’s feces, or may come out of the dog’s anus by their own movement.The embryophore is lysed in the stomach, and the onchosphere is released in the upper part of small intestine. It penetrates the intestinal wall and goes with the circulation to the liver, then lungs then distributes systematically everywhere in the body.The scolices in the hydatid fluid attach themselves to the intestinal wall and grow to form adults.Pathogenesis:Swallowing of eggs results in formation of hydatid cysts (hydatid disease or hydatidosis).The enlarging cyst causes pressure necrosis in the surrounding tissues by mechanical pressure. It can cause also abscess formation by the pathogens carried along with the onchosphere.
Hydatid cysts form in liver (65%), lung (23%), muscles (5%), kidney (2%) brain, bone (3%), spleen (1%), and other organs (1%). They are found in children and young adults more than other age groups.
The adult does not infect humans or exist in their intestine.
Dogs get the infection from slaughtered animals, as the butcher always throws out the hydatid cysts during meat preparation. One cyst can infect the dog with 3-6 thousand scolices, i.e. adult worms.
Eggs can reach the digestive tract of herbivorous animals (the intermediate hosts) or accidentally man either by contamination of food, or by inhalation. Children can get the infection by soiling their hands during playing and fondling of dogs, as the eggs from gravid segments can stick to the hairs around the anus and the tail.
Life cycle:
Osseous hydatid cyst is the cyst that forms in bones, it is usually small, of limited growth, usually present in the epiphyses and causes thinning of surrounding bone.
Some cysts are sterile, the germinal layer does not form scolices so they are full of serous clear fluid.
The wall is formed of three layers: outer fibrous (may be calcified), a laminated layer followed by innermost germinal layer.
Hydatid cyst (larval stage):
Egg: similar to T.saginata.
Radiographic examination:
Calcification of the cyst shows a sharp outline
In the liver it may cause hepatomegaly, epigastric discomfort and abdominal enlargement.
In the lungs an enlarging cyst may compress a bronchus and rupture into it, thus causing lung abscess, empyema or pneumothorax.
Neurological manifestations occur according to the site of settlement.
Osseous hydatid can be discovered accidentally during X-ray examination or presented with pathological fracture.
Rupture of the cyst e.g. by abdominal trauma causes an intense immediate hypersensitivy reaction (acute anaphylactic reaction), shock and may be fatal.Diagnosis:
Plain X-ray, sonography and CT scanning can reveal the presence of the cyst.
Blood film examination shows high eosinophilia, can be up to 20%.
Sputum examination in suspected pulmonary hydatid can show hooks and sometimes scolices.
Sonographic-guided aspiration can differentiate between liver abscess and hydatid disease.Casoni test: this is the first test used to diagnose hydatid disease. It is intradermal injection of 0.5 ml. of 1/1000 solution of purified hydatid fluid in one forearm and saline in the other. A wheel forms within 15-30 minutes in positive cases. False positive tests can occur in tuberculous patients, these are re-tested by injection of 0.5 ml of 1/1000 solution of dried Taenia powder.ELISA and counter-immunoelectrophoresis are more specific and can be used for confirmation. Treatment:
Indirect haemagglutination, and latex agglutination tests, these are available for screening, because they are also simple and sensitive.
5- Serological tests:
Surgical removal of cysts is still the method of choice, especially in single, accessible cysts. Care should be taken to remove the cyst intact to avoid anaphylactic reaction and dissemination by microscopic fragments of germinal layer or scolices which may form new cysts.
Albendazole can be given in cases with multiple cysts, inaccessible cysts like those in bone or brain. It can be given prior to surgical removal to avoid dissemination.
sonographic-guided aspiration, then intracystic injection of 10% formalin or 70% alcohol and its re-aspiration is an alternative for surgery in case of large or inaccessible cysts.
Prevention and control:
Elimination of stray dogs.
Sanitary disposal of slaughterhouse refuge.
Avoiding slaughter outside slaughterhouses.
Measurements of personal hygiene and cleanliness.
Proper handling of raw vegetables and fruits. Larval cestodes in humans are:
Pseudophyllidian larvae:
Sparganum mansoni, larval stage of D. mansoni.
Sparganum proliferum.
Cyclophyllidian larvae:
Cysticercus cellulosae, larval stage of Taenia solium.
Hydatid cyst, larval stage of Echinococcus granulosus.
Alveolar hydatid, larval stage of Echinococcus multilocularis.
Coenurus cerebralis larval stage of Multiceps multiceps.
Coenurus serialis, larval stage of M. serialis.
Cercocystic cysticercoid, larval stage of Hymenolepis nana.
Hymenolepis nana
(Dwarf tapeworm)
Disease:
Hymenolepiasis nana.
Host:
Humans especially children, mice and rats.
Intermediate hosts are grain beetles and rat fleas.
Distribution:
All over the world, but most prevalent in temperate areas with poor sanitary conditions.
Morphology:
Size: 5-50mm in length.
Scolex: as 4 suckers and rostellum which armed with single row of hooks
Mature segments:
broader than long
testes are 3 in number, close to each other in the middle part of the segment.
Ovary is single, globular and found in the middle.
Genital pore is unilateral.Egg:
Spheroid, translucent, 40-50μ in diameter.
Each has 2 envelopes: an inner embryophore with 2 polar thickenings from each 4-8 filament run through the gelatinous material filling the space between the two envelopes, and an outer eggshell.
Each contains a mature hexacanth embryo.Life cycle: When the final host swallows the egg by autoinfection, polluted hands, food or water, the onchosphere hatches in the small intestine. It buries itself in the lymph channels of the villi to become a cysticercoid. After about one week it returns to the lumen and matures into an adult worm Larvae of fleas and beetles can also serve as intermediate host. They eat up the eggs during feeding on organic excrements of mice and rats. The eggs hatch in their intestine and the onchosphere penetrates to the hemocele and stays there till it transforms to cysticercoid while the larva maturates into an adult flea, which must be ingested to complete the life cycle. Pathogenesis and clinical picture: Neurotoxic products of the worms also tend to cause dizziness and convulsions especially in children.Diagnosis:
Treatment:
By recognition of the characteristic eggs in the fecal sample.
Allergic manifestations due to tissue phases as flectins or flectinular conjunctivitis (around the cornea) was also reported.
They are rare, and usually occur in massive infections, where enteritis, abdominal discomfort, loss of appetite, colic and diarrhea with passage of mucus may occur.
Internal autoinfection could also occur, the eggs hatch in the intestine before passing out, this occurs most frequently in immunoco- mpromised persons.
in about 2 weeks. So man can act as an intermediate and as a definitive host.
Adults live in the small intestine and produce mature eggs, which are already infective. The intermediate host is optional both in case of humans and rodents.
Praziquantel acts very rapidly against Hymenolepis nana, both for luminal and tissue phases.
Yomesan is effective for luminal phases only, so must be given at Proper personal hygiene.Rodent control.
Treatment of infected persons.
Prevention and control:
weekly intervals for two or three times.
Hymenolepis diminuta
Dipylidium caninum
(Dog tapeworm)
Group discussion
Reference:
Roberts, LS. & Janovy, JJ. (eds.), 2000: Foundations of Parasitology. 6th edition McGraw hill.
http://www.parasitesonline.net.
http://www.livjm.ac.uk/parasitology/