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Trematodes, or flatworms, are a group of helminths that belong to the phylum Platyhelminthes. The adult flatworms share some common characteristics, such as macroscopic size (from one to several centimeters); dorsoventrally flattened, bilaterally symmetric bodies; and two suckers—oral and ventral. Except for schistosomes, which have separate sexes, all human parasitic trematodes are hermaphroditic. Their life cycles involve a mammalian/human definitive host, in which sexual reproduction by adult worms takes place, and an intermediate host (snails), in which asexual multiplication occurs. Some species of trematodes have more than one intermediate host.

Humans are infected either by direct penetration of intact skin (schistosomiasis) or by ingestion of raw freshwater fish, crustaceans, or aquatic plants with metacercariae—the infective larval stage.

Significant trematode infections of humans may be divided according to the location of the adult worms: blood, liver (biliary tree), intestines, or lungs (Table 229-1). Adult worms do not multiply within the mammalian host but can live for up to 30 years. Infections are often chronic.

TABLE 229–1Major Human Trematode Infections

image Although it is relatively rare to encounter patients with trematode infections in the United ...

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