Volume 4. The Cestodes




Co- Authors:  M. Arcari 1, A. Baxendine 1 and C. E. Bennett

1. Intersep Ltd    2. University of Southampton


More information can be obtained on Return to Diasys Ltd and www.soton.ac.uk/~ceb/, Ectoparasites and Endoparasites.




        Helminth Parasites                                                             1


4.               Infections through the Gastrointestinal Tract

Part Four                                                                                              

The Cestodes                                                                                1

         Taenia species                                                                                2

         Hymenolepis nana                                                                          6

          Hymenolepis diminuta                                                                   9

          Dihyllobothrium latum                                                                  10                                                                            



4.1           Identifying Intestinal Helminths                                                    12




Helminth Parasites


The word ‘worm’ is used loosely to describe organisms with elongated bodies and a more or less creeping habit.


The word ‘Helminth’ does mean ‘worm’, but in zoological terms it is more restricted to members of the phyla Platyhelminths , Nematoda and Acanthocephala.


There are three groups of medically important helminths; Cestodes (tapeworms), Nematodes (roundworms) and Trematodes (flukes).


These parasites live in both the body spaces (gut lumen, bile ducts, lungs, oral cavity, etc.) and in tissues (blood, muscles and skin).


4. Infections through the Gastrointestinal Tract

Part Four - The Cestodes


The cestodes (or tapeworms) form a group of worms, exhibiting two unmistakable morphological features; they all possess flat, ribbon like bodies and lack an alimentary canal.


Adult tapeworms usually inhabit the alimentary canal of their hosts (though they occasionally are found in the bile or pancreatic ducts) and attach themselves to the mucosa by means of a scolex. Despite the lack of a digestive system they do absorb food from the hosts intestine; thereby providing the tapeworms a habitat that is associated with high nutritional levels, feeding the tapeworms high growth rate.


Larvae on the other hand show a wide range of habitat preferences, being found in almost any organ of both vertebrate and invertebrate hosts. Though most larval species show a preference for a particular site.


This lack of an alimentary canal markedly separates tapeworms from nematodes and trematodes. The outer tegument of the body must serve not only as a protective coating but also as a metabolically active layer through which nutritive material can be absorbed, along with secretions and waste material to be transported out of the body.


The body consists of a chain of segments or proglottids, which can be immature, mature or gravid; the latter of which contain a fully developed uterus packed with eggs.  Therefore, each tapeworm is made up of a ‘string of individuals’ having a complete set of reproductive organs in progressive degrees of sexual maturity and budding off from a body attached to the host tissue by a head or scolex.

Except for Hymenolepis nana, which can develop directly in the same host, the lifecycle of tapeworms involves both an intermediate and definitive host. 


Taenia species



Taenia species are the most common cestode parasites of humans.  More than 60 million people are infected with T. saginata (‘beef’ tapeworm) world wide and about 4 million are infected with T. solium (‘pork’ tapeworm). The life cycle of a Taenia species can be seen in Diag 1. T. saginata has a comsmopolitan distribution, but is more common in developing countries where hygiene is poor and the inhabitants have a tendency of eating raw or insufficiently cooked meat. T saginata is the most common adult tapeworm found in man. T solium is virtually extinct in Europe and the USA.





Diagram 1. Diagram illustrating the life cycle of a Taenia species.


The adults of both species live in the small intestine of man (1), the definitive host. The gravid segments (2) are very active and escape through the anus, releasing large numbers of eggs (3) in the perianal region or on the ground where they can survive for long periods. When ingested by pigs or catle (4), the eggs hatch, each releasing an oncosphere which migrates through the intestinal wall and blood vessels to reach striated muscle within which it encysts, forming cysticerci (5). When inadequately cooked meat containing the cysts is eaten by man, the oncospheres excyst (6), settle in the small intestine and develop there into adult cestodes (1) over the next 3 months or so. The segments of T. solium are somewhat less active than those of the beef tapeworm but its eggs, if released in the upper intestine, can invade the host (auto-infection) (7), setting up the potentially dangerous larval infection known as cysticercosis in muscle of any other site. (Peters & Gilles, 1995)

Both humans and cattle or pigs are necessary to the complete life cycle of Taenia species. (In Europe and the USA cattle are the normal intermediate hosts, but in the tropics several other ruminants, e.g. goat, sheep llama and giraffe, may serve as the intermediate hosts.) Eggs ingested by the intermediate hosts usually contain oncospheres.  The oncospheres then hatch out in the duodenum, pass into the intestine where they penetrate the intestinal wall and are then carried by the circulation to be deposited in tissues (usually muscle).  There they develop into cysticerci larva which are white and ovoid, measuring approximately 8 x 5mm. (Fig 5)



Humans become infected by ingesting inadequately, cooked beef or pork with cysticerci, containing an invaginated protoscolex. The protoscolexes evaginate and pass into the small intestine where they attach themselves to the mucosa and develop into adult worms.


Eggs and proglottids are passed out in the faeces, and are then eaten by the intermediate host, thus, perpetuating the life cycle.




Ova of Taenia species are spherical, yellowish brown and measure 31 - 34mm in diameter.  The shell is thick and radially striated. (Fig 1) Within the shell, the onchosphere has 3 pairs of hooklets.  However, the microscopical appearance of the ova of T. saginata and T. solium are identical. (Table 1 highlights some of the differences between the two species)


The length of the adult T. saginata is 4 - 8 meters long and that of T. solium is 3 - 5metres long. (Fig 2)  The proglottids of Taenia species can be identified by the number of uterine branches;  7 - 13 for T. solium and 15 - 20 for T. saginata. (Fig 3 & 4) If the scolex is recovered, the 4 suckers and rostellum of hooklets of T. solium will distinguish it from T. saginata, which has 4 suckers but no hooklets.



Fig 1.Taenia spp. eggs of the two species are        Fig 2. Segments of an adult

identical, spherical (31 - 34 µm), brown to               tapeworm. Adults of T. solium may

dark-yellow  with a thick shell wall and contain        reach 2 – 8m in length and

an onchosphere.                                                       multiple infections can occur. (Peters &

                                                                                  Gilles, 1995)






Clinical Disease

The presence of the adult worm rarely causes symptoms apart from slight abdominal irritation with diarrhoea, constipation or indigestion. The accidental ingestion of the embryonated ova of T. solium may result in cysticercosis in man. An infection due to an adult Taenia, in man or animals, is referred to as taeniasis.


T. saginata (the ‘beef’ tapeworm) does not cause human cysticercosis. 


When the embryonated eggs are ingested, the embryos hatch out, migrate through the intestinal wall and are carried around the body in the circulation and deposited in various tissues.  Muscle and subcutaneous tissues are usually infected, but cysticerci can infect most organs and tissues.  Human cysticercosis is usually asymptomatic unless the infection is particularly heavy or cysticerci are formed in some vital area e.g. the brain, resulting in neurological sequelae.


Fig 3. A gravid proglottid of T. saginata                        Fig 4. Illustrates a gravid proglottid

with more than 14 lateral uterine branches                   of T. solium with less than 14 lateral

(on one side). (India ink technique)                               uterine branches (on one side). (www.medicine.cmu.ac.th)                                             (India ink technique)




Taenia Saginata

Taenia solium

Intermediate Host

Cattle, reindeer

Pig, wild boar

Site of Development

Muscle, viscera

Brain, skin, muscle

Scolex: adult worm

No hooks


Scolex: cysticercus

No rostellum

Rostellum & hooks

Proglottis: uterine branches

23 (14 – 32) *

8 (7 –11) *

Passing of proglottids

Single, spontaneous

In groups, passively


2 lobes

3 lobes

Vagina: sphincter muscle



* No universal agreement to the number of uterine branches in these 2 species. As a rough guide, specimens with more than 16 branches are likely to be those of T. saginata and those with less than 10 branches are ikely to be of T. solium.


Table 1. Some characteristics differentiating T. saginata from T. solium. (Smyth, 1994)


Fig 5. Cysticercus in pork. The larval stage occurs in the pig, they are usually white and ovoid, measuring approximately 8 x 5mm. Man becomes infected by eating undercooked meat which is infected with cysticerci containing an invaginated protscolex. (x20)(Peters & Gilles, 1995)



Laboratory diagnosis

Diagnosis of intestinal taeniasis can be made by recovery of the characteristic ova in the stool.  However, the ova of T. solium and T. saginata are identical and diagnosis is made by the recovery of the segments or scolex.


The diagnosis of cysticercosis depends upon serology.  MRI scans may reveal the presence of lesions in the brain. Calcified cysticerci are less often seen in the brain: in about one-third of cases, 10 years or more after infection. Occasionally, the diagnosis is made histologically on surgical specimens. Calcification in muscles usually appears 3 – 5 years after initial infection, and are most typically seen as spindle-shaped calcifications, most numerous in the thighs.



Western Blots

Various Immunodiagnostic tests appear to give good results on serum or CSF.


Diagnosis using an immunodiagnostic test can be achieved using an in vitro qualitative assay for the detection of IgG antibodies in serum reactive with T. solium antigens present on a membrane.


Infected individuals develop a predominately IgG response to the parasite. ELISA has been used as a screening test, but low sensitivity and frequent artifactual crossreactions, or crossreactions with antibodies from other parasitic infections, limit its usefulness as a confirmatory diagnostic test. The Western Blot assay (U.S Patent No. 5,354,660) developed by Tsang et al, at the U.S. Centers for Disease Control has been shown to provide a reliable method for evaluation of sera from patients with clinically diagnosed active cysticercosis. Field studies support a sensitivity of 98% and specificity of 100% for this assay.

This assay is known as the QualiCodeTM Cysticercosis Kit, the principle behind the test is that it is a qualitative membrane-based immunoassay manufactured from T. solium proteins. The T. solium proteins are fractionated according to molecular weight by electrophoresis on a polyacrylamide slab gel (PAGE) in the presence of sodium dodecyl sulfate (SDS). The separated T. solium proteins are then transferred via electrophoretic blotting from the gel to a nitrocellulose membrane. This antigen-bearing membrane has been cut into strips for testing of individual samples. Sera are tested at 100X dilution.


During the procedure, the strips containing the T. solium proteins are incubated with serum specimens and washed to remove unbound antibodies. Visualisation of human immunoglobulins specifically bound to T. solium proteins is performed by sequential reaction with goat anti-human immunoglobulin-alkaline phosphatase conjugate and BCIP/NBT substrate. Bands corresponding to the positions of the resoled T. solium proteins will be visualised on the strip, indicating the presence in the serum sample of IgG antibodies direct against Taenia antigens. Band positions are compared to those on a reference strip developed using the cysticersosis positive control.



Hymenolepis nana



Hymenolepis nana, the dwarf tapeworm, is the smallest tapeworm to infect humans. This cestode belongs to a large family known as Hymenolepididae. The diagnostic features of this family are: scolex armed with one circlet of five hooks; 1 – 3 large testes and sacciform uterus.  In addition to the H.nana, three other species, H. diminuta, H. microstoma and H. citelli have been used extensively for studies on cestodes.


Hymenolepis nana has a cosmopolitan distribution and is thought to be the most common tapeworm throughout the world. The infection is more frequently seen in children although adults are also infected, causing hymenolepiasis.


Life cycle

The lifecycle of H. nana does not require an intermediate host, complete development occurring within the villi of a single host, resulting in a ‘direct’ life cycle. Though it can also utilise an insect as an intermediate host. (Diag 2)




Diagram 2. Diagram illustrating the life cycle of the ‘dwarf’ tapeworm, Hymenolepis nana.



The eggs that are released from mature proglottids in the upper ileum are usually passed out in the faeces. (Fig 6) If swallowed by another human they develop into hexacanth oncospheres and burrow into the villi of the small intestine. This is where they develop into tailless cysticercoids and then migrate towards the ileum and attach to commence the formation of proglottids. The eggs which are ingested by insects, such as fleas, beetles or cockroaches hatch to form tailed cysticercoids which remain unmodified as long as they are inside the insect. If they are accidentally swallowed by a human they pass down to the ileum and establish themselves. (Peters & Gilles, 1995)





The egg containing the oncosphere bears three pairs of hooklets and is surrounded by a membrane. (Fig 6 & 7) This membrane has 2 polar thickenings from which arise threadlike filaments extending into the space between the membrane and the colourless hyaline shell, unlike those of H. diminuta which do not possess any filaments.


The adult tapeworm is normally 2.5 - 4cm long.  The scolex is knob like in shape, has a rostellum with hooklets and 4 suckers. (Fig 8)  The segments are wider than they are long.   Ova are spherical or ovoid measuring 30 - 47mm in diameter. This is what distinguishes it morphologically from H. diminuta.



  Fig 6. Egg of a Hymenolepis nana found in an         Fig 7. Oncosphere of a Hymenolepis

  Iodine stained wet mount. Eggs are usually              nana. After humans become infected the

  spherical , 30 - 47mm in diameter. They contain       eggs hatch into an oncosphere. (Phase               

  a hyaline shell and a 6 hooked oncosphere.             contrast x 1,150) (Peters & Gilles, 1995)



Fig 8. Hymenolepis nana scolex. Stained to show the solex with a knob like rostellum bearing a ring of hookelts. They possess 4 suckers, 2 of which can be seen just below the protruding rostellum. (www.medicine.cmu.ac.th)


Clinical Disease

Infections due to H. nana may cause no symptoms even with heavy worm burdens.  However, symptoms of restlessness, irritability, anorexia, abdominal pain and diarrhoea have been reported.  Heavy worm burdens may be caused by auto-infection which can be a problem in the immunocompromised.


Laboratory Diagnosis

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Diagnosis is based on recovery and identification of the characteristic ova in a formol-ether concentrate of faeces.  Adult worms and proglottids are rarely seen in stool samples.


Hymenolepis diminuta



Hymenolepis diminuta is a small tapeworm commonly found in rats and mice.  It has a world wide distribution in these hosts but is infrequently found in humans, with only sporadic cases being reported. 


Life cycle and transmission

The life cycle of H. diminuta requires an intermediate arthropod host e.g. earwigs, larval fleas and various beetles. (Diag 3) Human infection occurs by the accidental ingestion of an infected arthropod, which contains the cysticercoids.



Diagram 3. Diagram illustrating the life cycle of the rat tapeworm, Hymenolepis diminuta.


The ova are large, ovoid and yellowish with a moderately thick shell.  They contain an onchosphere with 6 hooklets and a clear area between the oncosphere and the shell. They measure 70 - 85mm by 60 - 80mm. (Fig 9 & 10)


The adult worm is 20 – 60cm long.  It has a knob like scolex with a rostellum but no hooklets and 4 suckers (in contrast to H. nana). (Fig 11) The rostellum can be withdrawn into a rostellar sac.  The tapeworm contains about 1000 proglottids, each of which is wider than long.


Clinical Disease

The symptoms associated with H. diminuta infections are few if any.





Fig 9 & 10. Eggs of H. diminuta are larger (70 - 85mm by 60 - 80µm) than H. nana and lack the polar filaments. Typical eggs, measure between 70 x 80µm; spherical in shape with a yellow-transparent cover. The internal membrane (that surrounds the oncosphere) has polar salients without filaments and six hooks on its interior. (www.cdfound.to.it)



Fig 11.  Hymenolepis diminuta scolex (permanently stained) bearing a knob-like rostellum and 4 suckers, but has no hooklets. The rostellum can be withdrawn into a rostellar sac.




Laboratory Diagnosis

Diagnosis is based on recovery and identification of the characteristic ova in a formol-ether concentrate of faeces.  Adult worms and proglottids are rarely seen in stool samples.




Diphyllobothrium latum



Members of this order, commonly known as pseudophyllids, are chiefly parasites of fish-eating mammals, birds and fish. They typically are found with a scolex which is characterised by two shallow elongated bothria situated with one dorsally and one ventrally. The proglottids are flattened dorsoventrally.


Diphyllobothrium latum is an intestinal tapeworm, known as the human ‘broad’ tapeworm. It is the largest tapeworm found in man. The term ‘broad’ relates to the fact that the proglottids are generally wider than they are long. It is an important human parasite. The adult worms of two other species of the genus, D. dendriticum and D. ditremum are chiefly parasite of fish-eating birds and mammals.


The tapeworm, D. latum has a wide distribution, occurring especially in countries bordering the Baltic Sea (Finland, Sweden etc.): and also in Russia, Switzerland and North America. It is in these countries where the populations are known to eat uncooked or partly cooked (i.e. smoked) fish.


Apart from man they are found in many other hosts, especially the dog, cat and pig. This is due to the host countries allowing the domestic animals access to the offal from the infected fish.



Life cycle and transmission

The life cycle of this tapeworm requires two intermediate hosts.


The eggs are passed out in human faeces, once in water they hatch out into small ciliates coracidium larvaem which swim until ingested by Copepods. It is in these intermediate hosts that growth and development of the 1st larval stage are completed (They are now known as procercoids). When these crustaceans (fresh water) are eaten by fish, the procercoid larvae continue to develop in the flesh of the fish and become known as plerocercoid larvae. It is this stage of the larvae which develops in man when they eat undercooked fish and they grow into adult worms in the small intestine. (Diag 4)


Diagram 4. Diagram illustrating the life cycle of the broad tapeworm, Diphyllobothrium latum.



The egg is usually ovoid and has a small knob at the opercular end and is yellowish-brown in colour with a smooth shell, of moderate thickness. (Fig 12 & 13) They measure 58 - 75mm by 40 - 50mm in size.


Text Box: Egg Text Box: Egg with operculum open Text Box: Coricidium

Adult worms can reach up to a length of 10 metres or more and may contain up to 3,000 proglottids. (Fig 14) The scolex is spatulate with no rostellum or hooklets.  It has 2 shallow grooves or bothria, which are unlike the typical 4 suckers seen on the Taenia species. The proglottids measure 3mm long and 11mm wide and have a rosette shaped central uterus. 

Fig 12.  Illustrations showing the morphology of the separate stages of the Diphyllobothrium latum life cycle. (www.aisr.lib.tju.edu)


Fig 13. Diphyllobothrium latum egg. They are              Fig 14. The proglottids measure 3mm

Ovoid with a small knob at the opercular end.               long and 11mm wide and have a

They are usually yellowish brown in colour                   rosette shaped central uterus. These

and 58 - 75mm by  40 - 50mm in size.                            proglottids tend to be passed in

                                                                     strands of variable length in the 

                                                                     stool. (www.dpd.cdc.gov)



Clinical Disease

The infection caused by D. latum is due to the ingestion of raw, poorly cooked or pickled fresh water fish.  The symptoms associated with D. latum infection may be absent or minimal with eosinophilia. There may be occasional intestinal obstruction, diarrhoea, abdominal pain. The most serious symptom is the onset of pernicious anaemia. This is due to a vitamin B12 deficiency, caused by excessive absorption of the vitamin by the adult worm and the absorption of cobalamins from the host intestine (occurring only in a small percentage of people).


Laboratory diagnosis

Laboratory diagnosis depends on the recovery of characteristic eggs from a formol ether concentrate of faeces. Proglottids may also be seen in faecal samples usually in a chain of segments from a few centimetres to about 0.5 metres in length.


4.1 Identifying Intestinal Helminths


The usual diagnostic stages for identifying medically important helminths are the eggs and larvae. Occasionally, adult worms like Ascaris and Enterobius may be seen and segments or proglottids are used for diagnosing certain tapeworms.


If an egg, is found the following features as described below and in Fig 15, should be carefully observed in order to make a specific identification.


1.     Size: The length and width are measured and are generally within a specific range.

2.     Shape: Each species has its own particular shape.

3.     Stage of development when passed: In some species, the eggs consist of a single cell; in some, there may be several cells; and some species are usually embryonated (i.e., they contain a larva) when passed in the faeces.


Occasionally, if the stool specimens are several hours or 1 – 2 days old, eggs may develop to more advanced stages. Ascaris eggs usually have only 1 cell when passed in the faeces; however, the single cell may divide and, in old specimens, eggs with 2 or 4 cells may be seen. Hookworm eggs in specimens that are several hours old may contain 16, 32 or more cells. In 12 – 24 hours, the egg may be embryonated and later still the larvae may hatch. Therefore, when observing the stage of development of helminth eggs, be sure that the stool specimen is freshly passed. If it is several hours or a day old, expect to see changes in the stage of development of some species. Ideally only fresh samples should be accepted for diagnosis.


4.     Thickness of the egg shell: Some species, like Ascaris, have thick shells; others, like hookworm, have thin shells.

5.     Colour: Some eggs are colourless (e.g., hookworm, Enterobius), others are yellow or brown (Ascaris, Trichuris).

6.     Presence of characteristic like opercula (lids), spines, plugs, hooklets, or mammillated outer coats.


Fig 15. Key to the relative sizes of Helminth eggs.


 1 Metagonimus yokogawai

 2 Heterophyes heyterophyes

 3 Opisthorchis felineus

 4 Clonorchis sinensis

 5 Taenia

 6 Hymenolepis nana

 7 Enterobius vermicularis

 8 Trichuris trichuria

 9 Ascaris lumbricides (fertile)

10 Hookworm

11 Diphyllobothrium latum

12 Hymenolepis diminuta

13 Paragonimus westermani

14 Trichostrongylus

15 Ascaris lumbricoides (infertile)

16 Schistosoma japonicum

17 Schistosoma haematobium

18 Schistosoma mansoni

19 Fasciola hepatica

20 Fasciolopsis buski                                                                                                                                                                                                                                     









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During the test the sample is exposed to protein A-colloidal gold conjugate. The serum migrates chromatographically along the membrane binding with the test line in the presence of T. pallidum antibodies. Independent of a positive reaction the sample continues to migrate to the control line where the conjugate binds to the membrane bound anti-protein A antibody to demonstrate correct device function.

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