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Platyhelminthes (platy means flat; helminth means worm) are divided into two classes: Cestoda (tapeworms) and Trematoda (flukes). The trematodes are described in Chapter 55.
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Tapeworms consist of two main parts: a rounded head called a scolex and a flat body consisting of multiple segments. Each segment is called a proglottid. The scolex has specialized means of attaching to the intestinal wall, namely, suckers, hooks, or sucking grooves. The worm grows by adding new proglottids from its germinal center next to the scolex. The oldest proglottids at the distal end are gravid and produce many eggs, which are excreted in the feces and transmitted to various intermediate hosts such as cattle, pigs, and fish.
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Humans usually acquire the infection when undercooked meat or fish containing the larvae is ingested. However, in two important human diseases, cysticercosis and hydatid disease, it is the eggs that are ingested and the resulting larvae cause the disease.
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There are four medically important cestodes: Taenia solium, Taenia saginata, Diphyllobothrium latum, and Echinococcus granulosus. Their features are summarized in Table 54–1, and the medically important stages in the life cycle of these organisms are described in Table 54–2. Three cestodes of lesser importance, Echinococcus multilocularis, Hymenolepis nana, and Dipylidium caninum, are described at the end of this chapter.
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