出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/02/24 09:47:37」(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 (synonym Taeniarhynchus saginata), commonly known as the beef tapeworm, is a zoonotic tapeworm belonging to the order Cyclophyllidea. It is an intestinal parasite of cattle and humans, causing taeniasis in humans. Cattle are the intermediate hosts where larval development occurs, while humans are definitive hosts harbouring the adult worm. It is found globally and most prevalent where cattle are raised, and beef is consumed. It is relatively common in Africa, some parts of Eastern Europe, Southeast Asia, and Latin America.[1] Humans are generally infected as a result of poor hygiene. The infective larvae called cysticerci are transmitted from undercooked beef. As hermaphrodite each body segment called proglottid contains complete sets of both male and female reproductive systems. Thus reproduction is by self-fertilisation. From humans, embryonated eggs called oncospheres are released along faeces and are transmitted to cattle through contaminated fodder. Oncospheres develop inside muscle, liver and lungs of cattle into infective cysticerci.[2]
T. saginata has a striking resemblance to the other human tapeworms, such as Taenia asiatica and Taenia solium, in structure and biology, except for few details. It is typically larger and longer, with more proglottids, more testes and higher branching of uterus. It also lacks armed scolex unlike other Taenia. Like the other tapeworms it causes taeniasis inside the human intestine, but does not cause cysticercosis. Its infection is relatively harmless and clinically asymptomatic.[3][4]
T. saginata is the largest of species belonging to the genus Taenia. Adult worm is normally 4 to 10 m in length, but can become very large, over 22 m long specimens are reported. Typical of cestodes, its body is flattened dorso-ventrally and heavily segmented. It is entirely covered by a tegument. The body is white in colour and consists of three portions: scolex, neck and strobila. The scolex has are four suckers, but suckers have no hooks. Lack of hooks and rostellum is an identifying character from other Taenia species.[4] The rest of the body proper is called strobila, which is basically a chain of numerous body segments called proglottids. The neck is the shortest part of the body, and consists of immature proglottids. The mid-strobila is made of mature proglottids that eventually lead to the gravid proglottids, which are at the posterior end. There can be as many as 1000 to 2000 proglottids in an individual.
T. saginata does not have a digestive system, mouth, anus, or digestive tract. It derives nutrients from the host through its tegument, as the tegument is completely covered with absorptive hair-like microtriches. It is also an acoelomate, having no body cavity. The inside of each mature proglottid is filled with muscular layers and complete male and female reproductive systems, including the tubular unbranched uterus, ovary, genital pore, testes, and vitelline gland. In the gravid proglottid, the uterus contains up to 15 side branches filled with eggs.[3][5]
The life cycle of T. saginata is indirect and digenetic, involving cattle and humans with an interim of living in the environment. Humans as the definitive host harbour adult worms which release infective eggs into the environment. Cattle as the intermediate host pick up the viable eggs from contaminated vegetation.
Cattle acquire the embryonated eggs called the oncospheres when they eat contaminated food. Oncospheres enter duodenum, the anterior portion of small intestine. The oncospheres hatch in the duodenum under the influence of gastric juices. The embryonic membranes are removed, liberating free hexacanth ("six hooked") larvae. With their hooks they get attached to the intestinal wall and penetrate the intestinal mucosa into the blood vessels. The larvae can move to all parts of the body via general circulatory system, and finally settle in skeletal muscles within 70 days. Inside the tissue they cast off their hooks and instead develop a protective cuticular shell, called cyst. Thus it becomes a fluid-filled cysticercus. Cystercus can also form in lungs and liver. The inner membrane of the cysticercus soon develops numerous protoscolices (small scolices) that are invertedly attached to the inner surface. The cysticercus of T. saginata is specifically named cysticercus bovis to differentiate from that of T, solium, cysticercus cellulosae.[3][5]
Humans contract infective cysticercus by eating raw or undercooked meat. Once reaching the jejunum the inverted scolex becomes evaginated to the exterior under stimuli from the digestive enzymes of the host. Using the scolex it gets attached on the intestinal wall. It takes about 5 to 12 weeks for the larva to mature into adulthood. Adult worm can live to about 25 years in the host.[3] Usually only a single worm is present at time. However, multiple worms are also reported. In each mature proglottid self-fertilisation produces zygotes, which divide and differentiate into embryonated eggs called oncospheres. With thousands of oncospheres, the oldest gravid proglottid detach. Unlike in other Taenia gravid proglottids are shed individually. In some cases the proglottid ruptures inside the intestine, and the eggs are released. The free proglottids and liberated eggs are removed by peristalsis into the environment. On the ground the proglottids are motile and will shed eggs as they move. These oncospheres in external environment can remain viable for several days to weeks in sewage, rivers, and pastures.[3][5][6]
The disease is relatively common in Africa, some parts of Eastern Europe, the Philippines, and Latin America.[2] 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.[5] The total global infection is estimated to be between 40 and 60 billion.[1] It is most prevalent in Sub-Saharan Africa and Middle East.[6]
T. saginata infection is usually asymptomatic. However heavy infection often results in weight loss, dizziness, abdominal pain, diarrhoea, 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.[7] It is an also rare cause of ileus, pancreatitis, cholecystitis and cholangitis.[8]
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.[5] 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).[2]
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.[9] 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.[10]
Eosinophilia and elevated IgE levels are chief hematological findings. Also Ziehl–Neelsen stain can be used to differentiate between mature Taenia saginata and Taenia soium, in most cases Taenia saginata will stain while Taenia solium will not, however the method is not strict.[11]
Taenaisis is easily treated with praziquantel (5–10 mg/kg, single-administration) or niclosamide (adults and children over 6 years: 2 g, single-administration after a light breakfast, followed after 2 hours by a laxative; children aged 2–6 years: 1 g; children under 2 years: 500 mg).[7] Albendazole is also highly effective for treatment of cattle infection.[12]
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.[7]
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| リンク元 | 「Taenia saginata」「無鉤条虫」 |
| 関連記事 | 「bee」「tapeworms」「be」 |