出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2013/10/10 12:49:10」(JST)
| Taenia saginata | |
|---|---|
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Platyhelminthes |
| Class: | Cestoda |
| Order: | Cyclophyllidea |
| Family: | Taeniidae |
| Genus: | Taenia |
| Species: | T. saginata |
| Binomial name | |
| Taenia saginata Goeze, 1782 |
|
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]
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 the strobila. Unlike other tapeworms, the scolex does not have a rostellum or scolex armature. It is composed of four 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, mouth, anus, or digestive tract. It is also an acoelomate, meaning it does not have a body cavity. In the gravid proglottid, the uterus is branched and filled with eggs. The gravid segments detach and are passed in the feces. Each of these segments can act as 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] http://strobilasoft.wix.com/strobila
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 bovis” 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 i.e.,measley beef. 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 take about 2 months to develop, and within three months it can reach 5 m long.[2]
The disease is relatively common in Africa, some parts of Eastern Europe, the Philippines, and Latin America.[1] Humans become infected when they eat undercooked beef. Prevention is easy; cook beef until it is no longer pink inside and 56°C in the center, because this kills the cysticerci. Also, beef frozen at -5°C is considered to be safe to consume.[3]
This parasite is found anywhere where beef is eaten, even in countries such as the United States, with strict federal sanitation policies. In the US, the incidence of infection is low, but 25% of infected cattle are still sold.[3]
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]
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, so can aid with identification. Observation of scolex help distinguish between T. saginata, T. solium and T. asiatica. When the uterus is injected with India ink, its branches become visible. Counting the uterine branches enables some identification (Taenia saginata uteri have 12 or more branches on each side, while other species such as Taenia solium only have five to 10).[1]
Differentiation of the species from other species of Taenia, such as T. solium and T. asiatica, is notoriously difficult 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 to form 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]
Eosinophilia and elevated IgE levels are chief hematological findings.
Treatment for cestode infection with the drug praziquantel opens membrane calcium channels, causing paralysis of the worm, thus aiding the body in expelling the parasite through peristalsis. Niclosamide, used to treat many different kinds of infections with trematodes and adult tapeworms, is also quite effective.
Adequate cooking (56°C for 5 minutes) of beef viscera destroys cysticerci. Refrigeration, freezing (-10°C for 9 days) or long period salting is lethal to cysticerci. Inspection of beef and proper disposal of human excreta are also important measures.
| Wikimedia Commons has media related to Taeniarhynchus saginatus. |
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| リンク元 | 「無鉤条虫」「beef tapeworm」 |
| 関連記事 | 「Taenia」 |