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Epidemiology and Pathology

Man acquires adult tapeworms (taeniasis) by eating undercooked or raw infected meat ("measly pork" or beef). The cysticercus in the ingested meat is released by the digestive juices and evaginates, and the larva attaches itself to the mucosa of the small bowel and develops into an adult tapeworm in 5-12 weeks (Fig. 7.1).


Fig. 7.1 A, B. Life cycle of Taenia solium.

The tapeworm inhabits the jejunum of man and obtains its nourishment from the food of the host. The head (scolex) of the worm is attached to the bowel by hooklets and suckers; there may be several hundred segments (or proglottids) liberating eggs as they mature (Figs. 7.2, 7.3). The pork tapeworm (T. solium) averages 6-20 feet in length and the beef tapeworm (T. saginata) 15-30 feet.


Fig. 7.2. (A) Morphology of adult tapeworms. (Courtesy of U. S. Naval Medical Corps) (B) Gravid proglottid segment of T. saginata showing branching uterus and genital pore (left center). x 7.5. AFIP 56-3320. (C) Similar proglottids of T. solium. x 7.5. AFIP 56-7108.


Fig. 7.3. (A) Adult T. saginata worm. Note the tiny head (scolex) and neck of the worm (center), with the worm gradually widening into hundreds of proglottid segments. (B) Magnified view of the scolex of T. saginata showing the suckers to good advantage. AMP 68-1462-6. (C) Adult T. solium worm showing the numerous proglottids. (Courtesy of Dr. Herman Zaiman).

The life cycle of both worms is similar except for the intermediate host. Each gravid segment of T. solium inhabiting the bowel of man contains 30,000-40,000 eggs. When the proglottids are released, they travel down the gut and, with the help of independent movement, pass through the anus. On the ground they disintegrate, releasing their eggs, which may survive for several weeks, during which time they must be ingested by the intermediate hosts, pigs (T. solium) or cattle (T. saginata), to complete their life cycle. The eggs hatch in the intestines of their host with the help of digestive juices, liberating their active embryos (oncospheres), which penetrate the wall of the gut with the aid of hooks, and probably also by means of lytic secretions. The oncospheres travel in the blood (and probably the lymphatics) to the muscles and elsewhere throughout the body. In the tissues, the hooks are shed and the oncospheres develop into larvae (cysticerci), completing the life cycle. Man is an accidental intermediate host and, like the pig, may develop larval taeniasis.

The life cycle of T. saginata is the same as for T. solium except that cattle are the intermediate hosts, and the eggs released are even more numerous (594 million per annum - an astronomical thought for a radiologist!); moreover, the worms can live nearly 30 years.

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