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INTRODUCTION

Trematoda (flukes) and Cestoda (tapeworms) are the two large classes of parasites in the phylum Platyhelminthes. The most important trematodes are Schistosoma species (blood flukes), Clonorchis sinensis (liver fluke), and Paragonimus westermani (lung fluke). Schistosomes have by far the greatest impact in terms of the number of people infected, morbidity, and mortality. Features of the medically important trematodes are summarized in Table 55–1, and the medically important stages in the life cycle of these organisms are described in Table 55–2. Three trematodes of lesser importance, Fasciola hepatica, Fasciolopsis buski, and Heterophyes heterophyes, are described at the end of this chapter.

TABLE 55–1Features of Medically Important Trematodes (Flukes)
TABLE 55–2Medically Important Stages in Life Cycle of Trematodes (Flukes)

The life cycle of the medically important trematodes involves a sexual cycle in humans (definitive host) and asexual reproduction in freshwater snails (intermediate hosts) (Figure 55–1). Transmission to humans takes place either via penetration of the skin by the free-swimming cercariae of the schistosomes (Figures 55–2D and 55–3) or via ingestion of cysts in undercooked (raw) fish or crabs in Clonorchis and Paragonimus infection, respectively.

FIGURE 55–1

Schistosoma species. Life cycle. Right side of figure describes the stages within the human (blue arrows). Humans are infected at step 2 when free-swimming cercariae penetrate human skin. Cercariae differentiate into adult worms (two sexes) that migrate to the mesenteric veins (Schistosoma mansoni and Schistosoma japonicum) or the venous plexus of the urinary bladder (Schistosoma haematobium). The adult worms lay eggs, which ...

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