An adult tapeworm consists of a knoblike head, or scolex, equipped with hooks for attaching to the intestinal wall of the host (which may be a human), a neck region, and a series of flat, rectangular body segments, or proglottids, generated by the neck. The chain of proglottids may reach a length of 15 or 20 ft (4.6-6.1 m). Terminal proglottids break off and are excreted in the feces of the host, but new ones are constantly formed at the anterior end of the worm. As long as the scolex and neck are intact the worm is alive and capable of growth. A rudimentary nervous system and excretory system run the length of the worm, through the proglottids. However, there is no digestive tract; the worm absorbs the host's digested food through its cuticle, or outer covering.
Each proglottid contains a complete set of male and female reproductive organs that produce the sex cells. Fertilization is internal; in most species cross fertilization between two adjacent worms is necessary, but in a few species self-fertilization may occur between two proglottids of the same worm, or within the same proglottid. In some species the fertilized eggs are shed continuously and leave the host's body in the feces; in others the fertilized eggs are stored until the proglottid is filled with them and the entire proglottid is then shed. The eggs develop into embryos with a hard outer shell; these do not hatch until they are eaten by a suitable intermediate host.
Human tapeworm infestations are most common in regions where there is fecal contamination of soil and water and where meat and fish are eaten raw or lightly cooked. In the case of the human tapeworm most common in the United States (the beef tapeworm, Taenia saginata) the usual intermediate host is a cow, which ingests the proglottid while drinking or grazing. The round-bodied embryos, equipped with sharp hooks, hatch and bore through the cow's intestinal wall into the bloodstream, where they are carried to the muscles. Here each embryo encloses itself in a cyst, or bladder; at this stage it is called a bladder worm. During the bladder worm stage the embryo develops into a miniature scolex; it remains encysted until the muscle is eaten by a primary host, in this case a human. If the scolex has not been killed by sufficient cooking of the meat, it sheds its covering and attaches to the intestinal wall, where it begins producing proglottids.
A human tapeworm common in Mexico, the pork tapeworm (T. solium), has a similar life cycle, with a pig as the usual intermediate host. The fish tapeworm, Diphyllobothrium latum, transmitted to humans from fish, especially pike, is common in Asia and in Canada and the northern lake regions of the United States. This tapeworm has a more elaborate life cycle, involving both a fish and a crustacean as intermediate hosts. The dwarf tapeworm, Hymenolepsis nana, is transmitted through fecal contamination and is common in children in the southeastern United States. There are also several tapeworms for whom humans the usual intermediate host; among these, the dog tapeworm, Echinococcus granulosis, spends its adult phase in the intestines of dogs.
Intestinal tapeworm infestation frequently occurs without symptoms; occasionally there is abdominal discomfort, diarrhea, constipation, or weight loss. The presence of tapeworm proglottids in clothing, bedding, or feces is the usual sign of infestation. Treatment is with quinacrine hydrochloride (Atabrine) or niclosamide, which kill the worm.
The most serious tapeworm infestation in humans is caused by the ingestion of T. solanum eggs through fecal contamination, which results in the person serving as the intermediate, rather than the primary, host. The embryos migrate throughout the body, producing serious illness if they lodge in the central nervous system. The embryos of the dog tapeworm encyst in various internal organs of humans, most commonly in the liver. The cysts produced by these embryos are called hydatid cysts, and the infestation of the liver is called hydatid disease.
Tapeworms are classified in the phylum Platyhelminthes, class Cestoda.
Principal features of the beef tapeworm (Taenia saginata). The head, elipsis
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Taenia is a genus of tapeworm that includes some important parasites of livestock. Not all members of the Genus Taenia have an armed scolex (hooks located in the "head" region), for example, Taenia saginata has an unarmed scolex, while Taenia solium has an armed scolex.3
Proglottids have central ovary, with a vitellarium (yolk gland) posterior to it. As in all cyclophyllid cestodes, there is genital pore on the side of the proglottid. Eggs are released when proglottid deteriorates, and so a uterine pore is unnecessary.
Important species include:
The life cycle begins with either the eggs or the gravid proglottids being passed in the feces, which can last for days to months in the environment (1). Then, cattle or pigs ingest the contaminated vegetation with eggs or proglottids (2). The oncospheres hatch in the small intestine of the cattle or pig(3) and invade the intestinal wall to travel to the striated muscles to develop into cysticerci. Humans can become infected when eating raw beef or pork meat (4). In the human, the cysticercus develop into adults in two months in the intestines. Using their scolex, they attach to the small intestine(5) where they reside(6). Taenia saginata are about 1,000-2,000 proglottids long with each gravid proglottid containing 100,000 eggs, while Taenia solium contain about 1,000 proglottids with each gravid proglottid containing 50,000 eggs.2
The taenia has caused discussion among creationist theologians since its description by Charles Bonnet : had it been created before Adam's fall or after? If before, what was he created for and how did he live ? If after, did that mean humans were not the last created species ?
3. Roberts, L.S. and Janovy, John Jr. Foundations of Parasitology 7th Edition. McGraw-Hill. 2005.
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