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INTRODUCTION

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Cestodes are long, ribbon-like helminths that have gained the common appellation of “tapeworm” from their superficial resemblance to sewing tape. Their appearance, number, and exaggerated reputation for inducing weight loss have made them the best known of the intestinal worms. Although improvements in sanitation have dramatically reduced their prevalence in the United States, they continue to inhabit the bowels of many of its citizens. In some parts of the world, indigenous populations take purgatives monthly to rid themselves of this, the largest and most repulsive of the intestinal parasites. Ironically, when a human serves as the definitive host for tapeworms, it is of little consequence to that person; the intestinal form of these creatures rarely causes serious harm. In contrast, clinical disease is a greater concern when people serve as intermediate hosts, because it is the presence of cysts in tissue that is most dangerous. Life cycles and characteristics of the six most important tapeworms infecting humans are summarized in Table 56–1.

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Table Graphic Jump Location
TABLE 56–1Intestinal and Tissue Tapeworms
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Clinical effects depend on whether humans are definitive hosts or intermediate hosts

Tissue cysts, not intestinal worms, cause serious disease

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PARASITOLOGY

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MORPHOLOGY

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Like all helminths, tapeworms lack vascular and respiratory systems. In addition, they are devoid of both gut and body cavity. Food is absorbed across a complex cuticle, and the internal organs are embedded in solid parenchyma. The adult is divided into three distinct parts: The “head” or scolex; a generative “neck”; and a long, segmented body called the strobila. The scolex typically measures less than 2 mm in diameter and is equipped with four muscular sucking disks used to attach the worm to the intestinal mucosa of its host. (In one genus, Diphyllobothrium, the disks are replaced by two grooves called bothria.) As a further aid in attachment, the scolex of some species possesses a retractable protuberance, or rostellum, ...

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