Introduction to the Cestodes
Adult Bodyplan
Cestode Tegument
Larval Metacercaria
Reproductive System
Infections of Man
Specific Cestodes
Examples of Common Cestodes
1) Diphylobothrium latum (The Broad Fish Tapeworm)
D. latum Egg
Diphyllobothrium latum is the fish tapeworm of man. It has a fairly cosmopolitan distribution, but is particularly common in the Baltic region, Russia and the Great Lakes region of the U.S.A.
Morphology of the adult Tapeworm
The adult parasites are typically between 2 and 12 m in length by up to 2 cm in width, but may grow even longer in some cases. The anterior organ of attachment is a bothria, a pair of shallow, elongated muscular grooves, typical of tapeworms of the order Pseudophyllidea. The body is divided into proglottids, as is the case of all pseudophyllidean tapeworms. These proglottids are more broad than they are long, except at the terminal end, where they a approximately square in shape. Internally the proglottids are typical of pseudophyllidean tapeworms, with numerous testes and vitellaria arranged on the lateral margins of the proglottid, with a central bilobed ovary. An important difference between this parasite and the other tapeworms of man is that the uterus open to the exterior (cyclophyllidean tapeworms have closed uteruses). Eggs are therefore actively deposited by the parasite, in contrast to the disintegration of the proglottids seen in the other human tapeworms.
Lifecycle
Diphylobothrium latum has a typical Pseudophyllidean tapeworm lifecycle, with in addition to the adult parasites in the definitive host, (i.e. man), there are two intermediate hosts containing larval stages. Eggs are passed from man in the faeces and hatch in water to release a small motile embryonic parasite, the Coracidium. This is internally similar to the hexacanth larvae of the Cyclophyllidean tapeworms, being equipped with 6 hooks, but this hexacanth larvae is covered in a ciliated embryophore. The coracidium is a free swimming stage, but cannot survive long. For further development it must be ingested by the first intermediate host, a copepod. On ingestion the embryonic larvae penetrates the arthropods gut wall, entering the haemocoel to develop into the first larval stage, the procercoid, measuring 500µm in length. This larvae, (as well as the next larval stage, the plerocercoid described below) are very different from the cyclophyllidean parasite larvae in that they have elongated and solid bodies. In addition the procercoid bears the embryonic hooklets on a posterior bulb like rounded growth, the cercomer. To continue the lifecycle the copepod must be ingested by the next intermediate host, a fish. The procercoid penetrates the gut wall of the fish, and develops into the next larval stage, the plerocercoid, measuring 4 - 5 mm in length, in the viscera or musculature of the fish. These plerocercoids are again elongated solid bodied parasites, but differ from the procercoids in the absence of the cercomer and hooklets, and at the anterior end having a developed attachment organ, the bothridium, similar to the adult parasite. A number of different species of fish may act as intermediate hosts for the plerocercoids of D. latum, but the highest densities of plerocercoids are found in carnivorous fish such as the pike. These high parasite loads are because, in addition to infection by ingestion of the copepod plus procercoid, if another infected fish is eaten the plerocercoids within the body tissues of this predated fish are released in the intestine of the carnivorous fish. These then migrate through the intestinal wall, to invade the new host, which is then acting as a Paratenic host for these secondary plerocercoids. The plerocercoids are, in addition, very long lived, and very high parasite densities may be achieved.
Man is infected by ingestion of raw or undercooked fish, the plerocercoids emerging in the intestine to grow into the adult parasite. In addition to man a number of other fish eating mammals may also be infected, including cats, dogs, pigs, bears. Therefore D. latum in addition to being a parasite of man, is also a zoonotic infection. In man multiple infections may occur, sometimes of very high numbers (up to 143 worms have been reported from a single individual). In these cases the parasites length is considerably reduced.
To avoid infection in man, fish should be properly cooked, killing the infective plerocercoids.
Pathology of Infection
Infection, as is often the case with adult tapeworms, presents a variable range of pathology, but again is not commonly the cause of serious disease in man. Symptoms, when they occur, include a variety of non-specific abdominal signs, including abdominal pain and loss of weight, and are often very similar to the symptoms displayed during infection with adult Taenia. However D. latum differs from Taenia in absorbing much more vitamin B12, (between ten and fifty times more) than other tapeworms. Infection may therefore result in a macrocytic hypochromic anaemia in some cases, vitamin B12 having an important role in formation of blood cells. This feature of the disease is much more common in the Baltic region, particularly in Finland. This tapeworm derived anaemia may be due to host derived genetic factors. It is also more commonly seen when the tapeworm is situated higher in the intestine.
2) Dipylidium caninum - (The Dog Tapeworm)
This Tapeworm is primarily a parasite of the Dog and the Cat. However man, and in particular children, may also be infected, and so a description of this parasite is included in these pages.
Lifecycle
Similarly to Taenia saginata the proglottids of this tapeworm are actively motile, and are able to crawl out of the anus of the definitive host as well as being passed in the faeces. The eggs of this species of tapeworm are contained in egg-capsules, each containing up to twenty eggs. These eggs are ingested by the parasites intermediate host, in this case an invertebrate arthropod such as fleas (only the larval flea can be infected) or the dog louse Trichodectes canis. The onchosphere larvae is released in the arthropods gut and penetrates the through the gut wall, developing into a cysticercoid, similar to the hymenolepid larval tapeworms. Infection of the Definitive host, whether dog, cat or man, occurs on ingestion of the larval parasite, either when the intermediate host is ingested, or ingestion of the crushed bodies of these hosts, for example if the dog licks the face of the child just after it has bitten a flea or louse. On ingestion the cysticercoid larvae develops into the adult parasite in the small intestine in about twenty days.
Morphology
Larvae - The larvae are roughly pear-shaped, and follow the typical cysticercoid body pattern.
Adults - These are relatively short tapeworms, measuring between 15 and 17cm in length and consisting of up to 170 proglottids. These are elongated in form, the gravid proglottids, measuring approximately 12 x 3mm and packed full of egg-capsules, having the appearance of grains of rice. The scolex by which the parasite attaches to the wall of the small intestine has four large acetabulate suckers, a retractile rostellum and six rows of 30 to 150 rose-thorn shaped hooks. The eggs which are typical Cyclophyllidean tapeworm eggs, are round in shape and measure up to 60µm, and are held within egg-capsules as described above.
Pathology of Infection.
The infection appears to be asymptomatic and generally non-pathogenic, although there may be some degree of mild pruritis, or itching, around the perineum due to the presence of emerging proglottids.
3) Tapeworms of the Genus Hymenolepis
There are a number of species in this genus, two of which are common parasites of man.
H. nana - The Dwarf Tapeworm
This tapeworm is relatively small, growing up to 4cm in length, the size of the parasite being inversely proportional to the number of worms present in the infection. Infections, which are more commonly seen in children in warmer climates, are characterised by the presence of numerous parasites (both cysticercoid larvae and adults) in the small intestine. Infection is by ingestion of soil contaminated with faeces containing eggs (see lifecycles below), and may give rise to abdominal discomfort.
H. diminuta - The Rat Tapeworm
This tapeworm is much longer than H. nana, growing up to 60cm or more in length. This is primarily a parasite of the rat, humans only being infected by accidental ingestion of the insect intermediate host (see lifecycles below). This species is of more importance as a research model for the study of the biochemistry, physiology, chemotherapy and immunology of tapeworm infections.
In addition there are a number of species found in animals, including;
H. carioca - A common non-pathogenic parasite of fowl in the USA.
H. microstoma - A parasite of rodents.
H. lanceolata - A pathogenic parasite of ducks, geese and other anseriform domestic fowl.
H. coronula- A parasite of anseriform domestic fowl.
H. cantaniana - A parasite of chickens and other galliform domestic fowl.
Morphology
Apart from their relative sizes the two parasites of man are very similar, H. nana being up to 4cm in size, the strobila consisting of up to 200 proglottids, whilst H. diminuta grows up to 60cm or more in length and the strobila consists of up to 1000 proglottids. These proglottids are trapezoidal in shape, and are approximately four times as wide as they are long. Each proglottid contains three round testes, a bi-lobed ovary, a compact vitelline gland and a large uterus opening to a lateral genital pore (as does the cirrus). The scolex in both parasites have four suckers and a retractile rostellum which in H. nana is equipped with 20 - 30 hooks (the rostellum is unarmed in H. diminuta). Finally the eggs of the two species both have the characteristic thickened walls of all cestode eggs, but may easily be differentiated. Those of the yellowish brown H. diminuta eggs are much rounder than colourless H. nana eggs and are larger at 60 - 80µm in diameter. In H. nana the eggs are oval in shape, measuring ~ 40 by 50µm and contains an oncosphere equipped with 3 pairs of embryonic hooks (i.e. a "hexacanth" larvae) and long wavy filaments (absent in H. diminuta ) which lie in the space between the larvae and the egg shell wall.
Lifecycle
The two species infecting man have rather different lifecycles which will be considered separately here.
i) H. nana
This parasite has rats and mice as well as man as the definitive host, and differs from H. diminuta and almost all other tapeworm in that an intermediate host is not required, although fleas and beetles may be used. The embryonated eggs are passed in the faeces where they contaminate soil. If the eggs are ingested by the definitive host the oncosphere is activated and breaks out of the egg and penetrates the gut villus. Here it develops as a cysticoid larvae in ~ 4 days before rupturing into the gut lumen. Once rupture the scolex attaches to the gut mucosa and the parasite develops into the adult tapeworm after ~ 15 to 20 days. If the insect intermediate hosts are utilised the lifecycle is similar to that of H. diminuta below.
In heavy infections eggs liberated by adult worms in the intestine may hatch here rather than passing out of the body, to give auto infections.
ii) H. diminuta
This parasite in common with most tapeworms does require an intermediate host. Embryonated eggs pass out of the body of the definitive host in the faeces and are ingested by the insect intermediate hosts. Many insects may act as intermediate hosts for this parasite, the most common being fleas and beetles such as the flour beetle. When ingested by the intermediate host the oncosphere larvae becomes activated, breaks out of the egg shell and penetrates into the insects body cavity where it develops into a cysticercoid larvae. For completion of the lifecycle the infected intermediate host must be eaten by the definitive host. On ingestion the cysticercoid larvae becomes activated, the scolex becomes attached to the gut mucosal wall, and the parasite develops into the adult tapeworm.
An interesting feature of Hymenolepis tapeworms is that they undergo a diurnal migration within the gut, which is associated with the feeding patterns of the host. From ~ 4pm to 4am few parasites are seen in the lower part of the small intestine, whilst from ~ 4am to 4pm many parasites are seen in the upper part of the small intestine. This was first observed in H. diminuta and subsequently in other species, and is indicative of a nocturnal feeding pattern by the parasite.
Pathology of Infection.
These parasites are not very pathogenic, usually with asymptomatic infections. In man infected with H. nana there may be a slight irritation of the gut mucosa and slight abdominal pain, and with very heavy infections (>2000 worms) there may also be some diarrhoea. In the bird species there may be enteritis and intestinal obstruction with some species.
4) Tapeworms of Man of the Genus Taenia (The Pork and Beef Tapeworms)
Two species from the genus Taenia are common parasites of man, these being Taenia solium (the Pork tapeworm) and Taenia saginata (the Beef tapeworm).
i) Taenia saginata - The Beef Tapeworm
This parasite has a cosmopolitan distribution, with estimates of approximately 50 million cases of infection world-wide annually.
Lifecycle
This parasite has cattle or related animals as its main intermediate hosts, although other animals such as camels, llamas and some antelope may also occasionally be infected. The larval form in these animals is a cysticercus in the muscles and heart. These are infected by ingestion of the eggs of the tapeworm, shed from the faeces of the carnivorous definitive host, in this case man. Once ingested the eggs hatch to release the hexacanth larvae, which migrates through the intestinal wall to reach the blood or lymphatic systems, from where it is carried to the tissues, particularly the heart and other muscles to develop into the cysticercus. Man is infected by ingestion of undercooked or raw meat, the bladder wall of the cysticercus being digested in the intestine to release the scolex of the parasite. This attaches to the intestinal wall and grows into the mature adult tapeworm.
Morphology
Larvae - These cysticerci are approximately 7.5 - 10mm wide by 4 - 6mm in length.
Adults - The adult tapeworms have an average length of ~ 5 meters , consisting of approximately 1000 proglottids, but may grow up to 17 metres in length occasionally, and are therefore longer than the adult forms of Taenia solium. The mature proglottids have approximately double the number of testes that T. saginata has and are larger. The gravid proglottids are also larger, measuring approximately 20mm long by 6mm wide, have a uterus with more lateral branches, less than T. solium. These gravid proglottids when detached from the strobila may be very active, not only crawling away from the faeces when passed, but often actively emerging from the anus to deposit eggs from the ruptured uterus around the perianal region. The scolex in this tapeworm may also be differentiated from T. solium as it is slightly larger, at approximately 2mm in diameter and is unarmed, without any hooks, although the 4 acetabular suckers are still present.
Pathology of Infection.
Larvae - Unlike T. solium, T. saginata does not utilise man as an intermediate host, and therefore pathology due to the larval form is not a feature in human disease. In cattle the cysticercus, refered to as Cysticercus bovis (named before the parasite lifecycle had been determined, and the connection between the two forms had been established) is completely asymptomatic.
Adults - The pathology of infection with adult T. saginata is highly variable. Often infections are completely asymptomatic, but in other cases some degree of pathology may be seen, most seriously intestinal blockage. In some cases vitamin deficiency may be the result of excessive absorption of nutrients by the parasite, (although this aspect of tapeworm pathology is more a feature of infection with the fish tapeworm D. latum. In addition infection may be accompanied by a broad range of non-specific symptoms, including more commonly, (if seen at all), abdominal pain, digestive disturbances, excessive appetite or loss of appetite, weakness and loss of weight.
ii) Taenia solium - The Pork Tapeworm
As with T. saginata this parasite has a cosmopolitan distribution, with estimates of approximately 50 million cases of infection world-wide annually. However the incidence of infection may vary considerably, and may be influenced by a number of factors such as religious inhibitions on eating pork, as in many Islamic countries, or in other countries by high degrees of sanitation, limiting exposure of the intermediate hosts to human faeces.
Lifecycle
This parasite has pigs as the main intermediate host, but man may also act as an intermediate host for this parasite as well as being infected with the adult tapeworms. This aspect of the parasites lifecycle has important implications for the pathology associated with infection with this parasite, as detailed below.
Morphology
Larvae - These small cysticerci (refered to as Cysticercus cellulosae) are approximately 6- 18mm wide by 4 - 6mm in length when found in the muscles or subcutaneous tissues (the normal sites for the larval of this parasite). The cysticerci may however be found in other tissues such as those of the central nervous system where they may grow much larger, up to several cm in diameter.
Adults - The adult tapeworms have an average length of ~ 3 meters, but may grow up to 8 metres in length occasionally, and follow the typical morphology of cestode tapeworms. The strobila consists of between 800 and 1000 proglottids. The mature proglottids having trilobed ovaries with a small central lobe in addition to the two lateral lobes and only approximately half the number of testes that T. saginata has. The gravid proglottids, measuring approximately 12mm long by 6mm wide, have a uterus with between 8 to 12 lateral branches, less than T. saginata. The scolex in this tapeworm may also be differentiated from T. saginata as it is equipped with a low rostellum with a double crown of approximately 30 hooks.
Pathology of Infection.
Larvae - Infection with the larval form of T. solium Cysticercus cellulosae, (called "Cysticercosis") may have severe consequences, the annual world-wide mortality due to cysticercosis having been estimated at approximately 50 000 cases. In man the cysticerci mainly develop in the subcutaneous tissues, but infections in both the Central Nervous System (C.N.S.) and ocular tissues are also very common. Infection of the C.N.S. may cause severe pain, paralysis, optical and/or psychic disturbances and epileptic convulsions, mainly due to mechanical pressure as the larvae develop. Later there may be loss of consciousness and even death. infections involving the eye may give rise to discomfort, and can cause detachment of the retina.
Adults - Usually only a single adult specimen is present, which may cause a slight degree of mucosal inflammation. The actual effects on the host may vary considerably, often there are few symptoms, but in some cases a variety of non-specific symptoms such as constipation, epigastric pain and diarrhoea, are present. Very rarely there may be perforation of the intestinal wall, with subsequent peritonitis may occur. However, more seriously, as detailed above, the presence of adult worms carries the risk of autoinfection due to reverse-peristalsis resulting in cysticercosis, it being estimated that approximately 25% of cases of Cysticercus cellulosae infections in man being aquired by this route.
iii) Other Taenia Cestodes Reported as Accidental Infections in Man
Infection by Adult tapeworms
Taenia taeniformis - This parasite has a cosmopolitan distribution, the adult parasites are normally found in cats and related carnivores, but it has been reported from an Argentinean child. The adult tapeworms are ~ 60cm long, and are unusual in that they lack a neck. The scolex is large and equipped with two rows of hooks, whilst the posterior gravid proglottids have a characteristic bell shape. The larvae, which is found in wild rodents, is a strobilocercus, a development of a cysticercus where the scolex has evaginated, but is still attached to the bladder of the cysticercus by a short segmented strobila.
Taenia bremneri (Syn. T. confusa) - reported from man in Africa, Japan and USA. This parasite may be a synonym of T. saginata.
Taenia africanus - reported a few times in East Africa. This tapeworm has broad segments and an unarmed scolex with a small apical sucker.
Taenia multiceps - The adult tapeworms of this species are found in dogs and related canids. The larvae is a fluid containing cyst 5 cm or more in diameter, containing several hundred protoscolices, and is called a coenurus. It is normally found in the brain or spinal cord of sheep and goats where it is an important pathogen. In these animals it causes a condition known as 'gid' or 'staggers' as the coenurus develops along with an associated destruction of nervous tissue. The larval form may rarely infect man, where it causes a condition called coenurus cerebralis, on accidental ingestion of tapeworm eggs from the faeces of dogs.
Taenia serialis - A similar parasite to T. multiceps, the coenurus larvae, measuring 4cm in diameter or larger, is usually found in the subcutaneous and intramuscular tissues of lagomorphs. The adult tapeworms are found in dogs and foxes with a cosmopolitan distribution. they measure ~ 70cm in length and have a scolex with two rows of ~30 hooks. The larvae have been reported very rarely in man.
Taenia glomerulatus - The larvae normally infect rodents, but the coenurus larvae has also been reported as rarely infecting man in Africa.
5) Three species of Echinococcus have been generally accepted as parasites of man
i) Echinococcus granulosus
The causative agent of Hydatid disease in man and many other mammals. The dog acts as the definitive host for this species. A number of sub-species of this parasite have also been described, the most universally accepted being E. g. granulosus (thought to be the original species found in Europe, although now more widespread) and E. g. canadensis ( the indigenous species of the Arctic region of North America, for more details see below). In addition there is considerable strain variation within this parasite, with differing preferences for intermediate hosts. For example in Ireland a strain exists whose larvae only infect the horse, man being resistant to infection.
Lifecycle
Dogs and other canids are parasitised by the adult tapeworm. When shed by the tapeworm the gravid proglottids disintegrate in the dogs intestine, and eggs which are passed in the faeces, are highly resistant, being able to survive freezing and drying on the ground for up to a year. Many mammals apart from man may act as intermediate hosts, in particular sheep and horses. The situation is highly complex as at least 9 subspecies have been identified, all with different host specificity's, for example;
E. g. granulosus - Adult form in most canids apart from the red fox, hydatids in sheep, pigs, cattle, man and many wild ruminants.
E. g. equinus - Adults in canids, hydatids in horses and other Equidae, but probably not man.
E. g. canadensis - Adults in canids, hydatids in caribou, reindeer and man.
E. g. borealis - Adults in canids, hydatids in many cervids and man.
In addition, in parts of Kenya there is a strain or subspecies that is particularly adapted to transmission between man and domesticated dogs. Other uncharacterised strains or subspecies may weel also exist.
The egg enters the host by ingestion, either from contaminated grass (as is the case in infections of herbivorous ruminants), or in the case of man, by contamination, (for example by the dog licking face after it has been cleaning itself) or other examples of bad hygiene, followed by transfer to the mouth. The egg then hatches in the intestine, penetrates the gut wall, and travels via the lymphatic or blood system throughout the body, from where they lodge within the bodies tissues. The cysts may develop anywhere within its intermediate hosts body, but as the circulatory blood stream passes from the mesenteric blood vessels to the liver, it is in the liver that the majority of the cysts (in about 65% of cases) are found. Next in frequency of infection is the lungs (~20%), brain (1%), peritoneal cavity (8%), kidneys (3%) and bone marrow or other organs. Development of the cysts to produce infective protoscolices takes approximately 1 to 2 years. On the death of the intermediate host, either directly by predation on the part of the dog, or by the scavenging of the dead cadaver, (the protosocialises are also highly resistant, being able to survive in carrion for several weeks), the cyst is ingested along with the offal. The cyst wall is then digested, liberating the protoscolices which quickly evaginate, penetrating deeply into the crypts of Lieberkuhn, and developing to adult worms in approximately 7 to 9 weeks. Due to the presence of many protoscolices in each hydatid cyst, dogs may be infected with many E. granulosus tapeworms.
Morphology
Larvae - These Metacestodes (called 'Hydatids') are large, roughly spherical, fluid filled hollow bladders, containing numerous protoscolices (forming the so-called hydatid sand), brood capsules, and daughter cysts which are identical in form to their parent cyst. The cyst wall itself consists of an outer laminated hyaline wall, supporting the whole cyst. Beneath this there is a nucleated germinal layer, studded with developing brood capsules, which may eventually break off to float freely in the fluid filled cyst. The protoscolices are formed within the brood capsules, which may rupture to give the free protoscolices in the hydatid fluid.
They vary considerably in size depending on where in the body they form, which may be almost any organ of the body. Those found in the liver (the most common organ affected) may be approximately 20cm in diameter, but those found in the peritoneal cavity may sometimes be very much larger, containing several litres of fluid. For example one case has been reported of a cyst 50cm in diameter, containing 16 litre of fluid.
Adults - The adult parasites in the dog represent one of the smallest of the tapeworms. They measure between 3 and 9mm in length, and usually consist of only 3 proglottids, an immature, a mature, and a gravid proglottid. The scolex is globular in shape, and has a prominent rostellum, armed with a double row of between 30 and 36 hooks. The eggs are very similar to those of the genus Taenia, and measure between 30 and 40µm in diameter
Pathology of Infection.
Larvae - In domesticated animals clinical signs appear to be uncommon, whilst in man they will vary in their seriousness depending on where in the body the hydatid develops, and how large it grows. Sometimes the infection is asymptomatic, the only evidence of infection being the presence of calcified cysts on autopsy after death due to an unrelated cause. The major pathology is due to the size of the cyst, giving rise to pressure related injury. A complication may arise if the cyst is ruptured, possibly due to blows to the body, muscular strain, or during operations. In this case the contents of the hydatid is released into the body's circulatory system, and the liberated protoscolices may give rise to numerous secondary cysts throughout the body. In addition the hydatid cyst fluid is highly allergenic and cyst rupture may result in anaphylactic shock and rapid death.
Adults - The adult tapeworm is usually non-pathogenic to its canine hosts, although sometimes in very heavy infections there may be some inflammation of the intestinal wall.
ii) Echinococcus multilocularis
Causative agent of highly pathogenic Alveolar Hydatid disease in man and other mammals. The fox is the most important definitive host, although dogs, and occasionally cats, may also be infected with the adult parasite. Again there appears to be a number of sub-species of this organism, E. m. multilocularis in Europe and E. m. sibiricencis in North America.
Lifecycle
This is very similar to that of E. granulosus, but with more adaptations for colder climates. For example the eggs are highly resistant to cold temperatures, being able to survive at -20° for more than 2 weeks. In addition the pre patent period in the definitive host is much shorter, at around 4 to 5 weeks.
Morphology
Larvae - The larval E. multilocularis is very different from that of E. granulosus. In this case the 'cyst' grows invasively by external budding, forming a diffuse growth through the infected organ, replacing that organs tissues. The growth itself, (it cannot truly be called a cyst as there is no real cyst wall), is composed of numerous cavities containing a gelatinous matrix within which protoscolices and numerous brood capsules are produced, and which in its behaviour, most closely resembles a malignant neoplasm. In contrast to E. granulosus this growth is also very rapid, infective protosocialises being present after only 2 to 3 months, as compared to the 1 to 2 years in the related metacestode.
Adults - The adult parasite is very similar to E. granulosus, being slightly smaller, with a maximum length of approximately 4mm, and consisting of 4 to 5 proglottids.
Pathology of Infection.
Larvae - The multilocular cyst is highly pathogenic due to its fast growth rate and invasive nature, in extreme cases completely replacing liver tissue. As the cyst lacks the tough laminated layer seen in E. granulosus, and by its nature grows by budding, metastases of grow may also be seen, colonising other body organs. Due this aspect of the parasite it may also be transferred by transplantation. This parasite must be considered one of the most pathogenic of the parasitic helminths.
Adults - As with E. granulolsus the adult tapeworm is usually non-pathogenic to its canine hosts.
iii) Echinococcus vogeli
This parasite is found in Central and Northern South America, with bush dogs as the definitive hosts, and pacas and other rodents as the normal definitive host. The cysts resemble E. granulosus, but often become septate, forming multichambered cysts (i.e. polycystic hydatids).
NB. This species is often thought of as a variant of E. granulosus.
Lifecycle
Similar to that of E. granulosus.
Morphology
Larvae - Similar to that of E. granulosus, but as they develop they become septate, forming multichambered cysts, (a 'polycystic' hydatid).
Adults - Similar to E. granulosus, but rather smaller measuring 4 and 6mm in length.
Pathology of Infection.
Larvae - Similar to that of E. granulosus.
Adults - Non-pathogenic.