Life Cycle and Epidemiology

The adult stage of Taenia saginata is a flat tapeworm that resides in the human small intestine (3). The tapeworm is composed of a chain (strobila) of proglottids or segments each of which contains male and female reproduction systems. As the proglottid matures, it produces a large number of eggs. Gavid proglottids detach from the strobila and pass out of the intestine through the host's anus. Generally, an infected person harbors one tapeworm, although in some regions of the world, such as, Asia Minor, multiple infections are frequent. The adult tapeworm normally localizes in the jejunum, about 50 cm below the duodenojejunal flexure, and rarely enters the gallbladder, appendix, or nasopharynx. The life span of T. saginata in humans may be as long as 30-40 years.

Taenia saginata is capable of producing a very large number of eggs, and an individual proglottid may contain about 80,000 eggs. Because each strobila can shed 6-9 gravid proglottids/day, a single infected individual may contaminate the pasture, lot, or barn with one-half million eggs or more each day. The mature egg contains the larval stage termed the onchosphere; immature eggs that are passed may complete their development outside the host. Ingestion of the egg by cattle is followed by disintegration of the outer embryophore and activation of oncosphere activity by gastric juice. Aided by its hooks and penetration glands, the onchosphere penetrates the bovine intestinal mucosa and within a couple of hours enters the blood or lymphatic system. The onchosphere utilizes the host's circulatory system to migrate throughout the host's body; however, successful development to the cyst or cysticercus stage usually occurs in skeletal muscle or the heart. The cysticercus, when fully developed, is composed of an invag-inated scolex within a fluid-filled vesicle or bladder. When improperly cooked beef is eaten, the larvae are digested free in the gut and the scolex evaginates to attach by its suckers to the intestinal wall. The development of the stobila then commences, including proglottids with reproductive organs and eventually, eggs.

The life cycle of T. solium is similar but differs in several important respects (3). The gravid proglottids contain fewer eggs than do those of T. saginata. Another distinction is the lack of motility of T. solium proglottids, which results in their being shed in the feces as connected segments. The major difference from T. saginata, however, is the suitability of humans to serve as an intermediate host for T. solium cysticerci. As in pigs, the cysticerci are fairly evenly distributed throughout the liver, brain, central nervous system, skeletal muscle, and myocardium. The cysticercus of T. solium is bigger than that of T. saginata (5-20 mm in diameter). After 2-3 months of development, the cysticercus appears as a glistening pearly white cyst, with the scolex or head deeply invaginated into the fluid-filled bladder; the scolex bears four suckers and an apical crown of hook-lets. When humans ingest infected pork, the larvae are digested out of the meat or tissue, the scolex evaginates from the bladder, attaches to the wall of the proximal portion of the ileum, and develops into a complete worm or strobila in about 3 months. The tapeworm begins to excrete proglottids after about 2 months and may live for many years.

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