Tapeworms of man and other human cestode, Biology tutorial

Introduction:

Two species from the genus Taenia are very common parasites of man, these being Taenia solium (that is, the Pork tapeworm) and Taenia saginata (that is, the Beef tapeworm).

Taenia saginata consists of a cosmopolitan distribution having estimates of around 50 million cases of infection world-wide yearly. As with T. saginata and T. solium this parasite consists of a cosmopolitan distribution having estimates of around 50 million cases of infection world-wide yearly. Though the incidence of infection might differ considerably and might be influenced through a number of factors like religious inhibitions on eating pork, as in most of the Islamic countries or in other countries by high degrees of cleanliness, limiting exposure of the intermediate hosts to the human faeces. This parasite has pigs as the major intermediate host; however man might as well act as an intermediate host for this parasite and also being infected by the adult tapeworms. This feature of the parasites lifecycle consists of significant implications for the pathology related with infection with this parasite.

Echinococcus granulosus is one of the three species of Echinococcus which is usually accepted as parasites of man. It is the causative agent of Hydatid disease in man and most of the other mammals. It takes place in Europe and Artic area of the North America.

Taenia spp:

Life cycle of Taenia spp:

This parasite has cattle or associated animals as its major intermediate hosts, however other animals like camels, llamas and some antelopes might as well occasionally be infected. The larval form in such animals is a cysticercus in the heart and muscles.

These are infected through 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 discharge the hexacanth larvae that migrate via the intestinal wall to reach the blood or lymphatic systems, from where it is taken to the tissues, specifically the heart and other muscles to build up into the cysticercus. Man is infected through ingestion of undercooked or raw meat, the bladder wall of the cysticercus being digested in the intestine to discharge the scolex of the parasite. This joins to the intestinal wall and grows into the mature adult tapeworm.

Morphology of Taenia spp:

Taenia saginata:

Larvae: These cysticerci are around 7.5-10mm wide by 4-6mm in length. 

Adults: The adult tapeworms encompass an average length of around 5 meters, comprising of around 1000 proglottids, however might grow up to 17 meters in length occasionally and are thus longer than the adult forms of Taenia solium. The mature proglottids have around double the number of testes that T. saginata consists and are larger. The gravid proglottids are as well bigger, measuring around 20mm long by 6mm wide having a uterus with more lateral branches than T. solium. These gravid proglottids when separated from the strobila might be much active, not only crawling away from the faeces when passed, however often actively emerging from the anus to deposit the eggs from ruptured uterus around the perianal area. The scolex in this tapeworm might as well be differentiated from T. solium as it is slightly bigger, at around 2mm in diameter and is unarmed, devoid of any hooks; however the 4 acetabular suckers are still present.

Taenia solium:

Larvae: These small cysticerci (termed to as Cysticercus cellulosae) are around 6- 18mm wide by 4-6mm in length when found in the muscles or subcutaneous tissues. The cysticerci might though be found in other tissues like those of the central nervous system where they might grow much larger, up to a few centimeters in diameter. 

Adults: The adult tapeworms encompass an average length of around 3 meters, however might grow up to 8 meters in length occasionally and follow the typical morphology of the cestode tapeworms. The strobila comprises of between 800 and 1000 proglottids.

The mature proglottids with trilobed ovaries having a small central lobe in addition to the two lateral lobes and only around half the number of testes that T. saginata has. The gravid proglottids, measuring around 12 mm long by 6 mm wide, encompass a uterus with between 8 to 12 lateral branches, less than T. saginata. The scolex in this tapeworm might as well be differentiated from T. saginata as it is equipped by a low rostellum having a double crown of around 30 hooks. 

Pathology of Infection in Taenia spp:

T. saginata:

The pathology of infection having adult T. saginata is highly variable. Often infections are fully asymptomatic, however in other cases some degree of pathology might be seen, most seriously intestinal blockage. In certain cases vitamin deficiency might be the outcome of excessive absorption of nutrients by the parasite, however this feature of tapeworm pathology is more a feature of infection by the fish tapeworm D. latum.

T. solium:

Infection by Taenia solium adults is generally asymptomatic. The armed scolex might cause a few inflammation of the intestinal wall and seldom might penetrate the intestine, causing the peritonitis.

The most significant health problem is infection by the cysticerci, cysticercosis. Eggs which pass via the stomach (either from the ingestion or reverse peristalsis) hatch in the small intestine penetrate the intestinal wall and are distributed all through the body in the circulatory system.  The most general sites of infection are the brain and skeletal muscle.

Diagnosis of Infection due to Taenia spp:

1) Explanation of scolex and proglottids in the faeces. Though, scolex is hardly ever excreted in faeces.

2) The eggs of T. saginata and T. solium are identical. Though, most of the laboratory diagnosis is via the observation of Taenia spp eggs in the faecal sample.

3) Examination of gravid uterus exhibits 15-25 lateral branches in the T. Saginata and 7-13 lateral branches in T. Solium when short chains of 5 to 8 proglottids passed out in the faeces are pressed via glass slides.

4) The scolex of T. saginata is simply differentiated from that of T. solium in that it consists of just 4 suckers however no hooks. 

5) Radiological assessment of the intestinal tract might reveal tapeworm infection.

Other Taenia cestodes:

Infection by Adult tapeworms:

Taenia taeniformis: This parasite consists of a cosmopolitan distribution, the adult parasites are generally found in cats and associated carnivores, and however it has been reported from an Argentinean child. The adult tapeworms are around 60cm long, and are unusual in that they deficient a neck.

Taenia bremneri (Syn. T. confusa): reported from man in Japan, Africa and the United State of America. This parasite might be a synonym of T. saginata. 

Taenia africanus: reported a several times in East Africa. This tapeworm has wide segments and an unarmed scolex having a small apical sucker. 

Infection by Larvae (Metacestode Infections):

Taenia multiceps: The adult tapeworms of this species are mainly found in dogs and associated canids. The larva is a fluid having cyst 5 cm or more in diameter, having several hundred protoscolices and is termed as a coenurus. It is generally found in the brain or spinal cord of goats and sheep where it is a significant pathogen.

Taenia serialis: A similar parasite to T. multiceps, the coenurus larvae, measuring 4 cm in diameter or larger, is generally found in the subcutaneous and intramuscular tissues of lagomorphs. The adult tapeworms are found in foxes and dogs having a cosmopolitan distribution.

Taenia glomerulatus: The larvae generally infect rodents; however the coenurus larvae have as well been reported as hardly ever infecting man in Africa.

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