Taenia saginata
| Taenia saginata | |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Platyhelminthes |
| Class: | Cestoda |
| Order: | Cyclophyllidea |
| Family: | Taeniidae |
| Genus: | Taenia |
| Species: | T. saginata |
| Binomial name | |
| Taenia saginata Goeze, 1782 |
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Taenia saginata, also known as Taeniarhynchus saginata or the beef tapeworm, is a parasite of both cattle and humans, causing taeniasis in humans. Taenia saginata occurs where cattle are raised by infected humans maintaining poor hygiene, human feces are improperly disposed of, meat inspection programs are poor, and where meat is eaten without proper cooking. The disease is relatively common in Africa, some parts of Eastern Europe, Southeast Asia, and Latin America.[1]
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[edit] Description
T. saginata is normally 4 m to 10 m in length, but can become very large, over 12 m long in some situations. The body is whitish in colour, divided into the anterior scolex, followed by a short neck and a highly extended body proper called strobila. Unlike other tapeworms the scolex does not have a rostellum or scolex armature. It is composed of 4 powerful suckers. The strobila is composed a series of ribbon-like segments called proglottids. The segments are made up of mature and gravid proglottids. T. saginata is the largest of genus Taenia consisting between 1000 to 2000 proglottids and can also have a lifespan of 25 years in a host's intestine.[2] The mature proglottid contains the uterus (unbranched), ovary, genital pore, testes, and vitelline gland. It does not have a digestive system, no mouth, no anus, or digestive tract. It is also an acoelomate, meaning that it does not have a body cavity. In the gravid proglottid, the uterus is branched and is filled with eggs. The gravid segments detach and are passed in the feces. Each of these segments can act like a worm. When they dry up, the proglottid ruptures, and the eggs are released. The egg can only infect cattle, the intermediate host. Inside the cow's duodenum the oncosphere hatches with the help of the gastric and intestinal secretions and migrates through the blood to the muscle. There it develops into infective cysticercoid cysticerci.[3]
[edit] Life cycle
The life cycle is indirect and complicated, and is completed in humans as the definitive host and cattle as the intermediate host. The adult worm inhabits the small intestine of humans. Fertilized eggs are released through the faeces along with the gravid proglottid which gets detached from the strobila. Cattle ingest the infective embryo while grazing. The digestive enzymes will break the thick shell of the egg and allow formation of the zygotes called "oncospheres". These zygotes then penetrate the mucous layer of the digestive tract and enter the circulation of the host. This is where the young larval stages form a pea-sized, fluid filled cyst, also known as “Cysticercus” and these cysts seem to form in the muscular fibers and are sometimes seen in specific organs like the lungs and liver. Humans acquire the infective larvae from eating undercooked meat. The digestive enzymes break down the cysticercus and the larval cyst is released and the inverted scolex is able to come out and attach to the host’s intestine. Adult tapeworm soon develop, and within three months it can reach 5 m long.[2]
[edit] Epidemiology
The disease is relatively common in Africa, some parts of Eastern Europe, the Philippines, and Latin America.[1] Humans become infected when they eat beef that is not cooked fully. Prevention is easy. Cook beef until it is no longer pink inside because cysticerci die at 56 degrees Celsius. Also, if beef is frozen at -5 degrees Celsius it is considered to be safe to consume.[3]
This parasite is found anywhere where beef is eaten, even in countries like the United States where there are strict federal sanitation policies. In the U.S. the incidence of becoming infected is low, however, 25% of infected cattle are still sold.[3]
[edit] Symptoms
Tapeworms are usually asymptomatic. However heavy infection often results in weight loss, dizziness, abdominal pain, diarrhea, headaches, nausea, constipation, or chronic indigestion, and loss of appetite. There can be intestinal obstruction in humans and this can be alleviated by surgery. The tapeworm can also expel antigens that can cause an allergic reaction in the individual.[3]
[edit] Diagnosis
The basic diagnosis is done from a stool sample. Feces are examined to find parasite eggs. The eggs look like other eggs from the family Taeniidae, so it is only possible to identify the eggs to the family, not to the species level. Since it is difficult to diagnose using eggs alone, looking at the scolex or the gravid proglottids can help identify it as Taenia saginata.[3] Proglottids sometimes trickle down the thighs of infected humans and are visible with unaided eye and aid with identification. When the uterus is injected with India ink, its branches become visible. Counting the uterine branches enables some identification (Taenia saginata uteri have twelve or more branches on each side, while other species like Taenia solium only have five to ten).[1]
It is notoriously difficult to differentiate the species from other species of Taenia such as T. solium and T. asiatica because of their close morphological resemblance, and their eggs are more or less identical. Identification often requires histological observation of the uterine branches and PCR detection of ribosomal 5.8S gene.[4] T. saginata’s uterus stems out from its center forming 12 to 20 branches, but in contrast to its closely related Taenia species, the branches are much less in number and comparatively thicker; in addition the ovaries are bilobed and testes are twice as many.[5]
[edit] Treatment
Treatment for cestode infection can be done with the drug praziquantel. Praziquantel opens membrane calcium channels causing paralysis of the worm, aiding the body in expelling the parasite through peristalsis. Niclosamide, used to treat many different kinds of infections with trematodes and adult tapeworms, is quite effective.
[edit] See also
[edit] References
- ^ a b c Lange Microbiology, Chapter 46. Medical Parasitology.
- ^ a b Bogitsh BJ, Carter CE (2005). Human Parasitology, 3rd Edition. Academic Press, pp. 273-277. ISBN 0120884682
- ^ a b c d e Roberts L, Janovy JrJ, Schmidt GD (2005). Foundations of Parasitology (8th edn). McGraw-Hill Companies, Inc., New York. ISBN 0071284583
- ^ González LM, Montero E, Harrison LJ, Parkhouse RM, Garate T. (2000). "Differential diagnosis of Taenia saginata and Taenia solium infection by PCR.". J Clin Microbiol. 38 (2): 737–744. PMC 86191. PMID 10655377. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=86191.
- ^ Zarlenga DS. (1991). "The differentiation of a newly described Asian taeniid from Taenia saginata using enzymatically amplified non-transcribed ribosomal DNA repeat sequences.". Southeast Asian J Trop Med Public Health. 22 (suppl): 251–255. PMID 1822899. http://content.nejm.org/cgi/content/full/350/3/249.
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