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Only kings, presidents, editors, and people with tapeworms have the right to use the editorial “we.”
—Mark Twain
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Overview
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 these large, repulsive intestinal parasites. Ironically, when a human serves as the definitive host for tapeworms (when they harbor adult worms), 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 (harbor larvae), 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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Like all helminths, tapeworms lack vascular and respiratory systems. In addition, they are devoid of both gut and body cavities. Nutrients are absorbed across their surface 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 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.) ...