Intestinal flukes (trematodes) are flat, hermaphroditic worms that range in size from a few millimeters to several centimeters. Approximately 70 trematode species have been reported to colonize the human intestinal tract. The geographic distribution is worldwide; the highest prevalences are in East and Southeast Asia. Infection in the United States can be acquired via importation as well as locally; the most common species include Nanophyetus, Alaria, and Heterophyes . Sources for transmission of human infection include fish, crustaceans, and aquatic plants.
Four of the most frequently described intestinal flukes will be reviewed here: Fasciolopsis buski, Heterophyes heterophyes, Metagonimus yokagawi, and Echinostoma species.
Issues related to liver flukes are discussed separately. (See "Liver flukes: Clonorchiasis and opisthorchiasis" and "Liver flukes: Fascioliasis".)
Fasciolopsis buski is the largest intestinal fluke of humans; pigs are also mammalian hosts. The infection is common in Southeast Asia and the Far East, especially in areas where humans raise pigs and consume freshwater plants.
The lifecycle begins with release of immature eggs in mammalian stool (figure 1). These eggs become embryonated in water and release miracidia, which enter snail intermediate hosts. In the snail, the parasites undergo several developmental stages (sporocysts, rediae, and cercariae). The cercariae are released from the snail and encyst as metacercariae on aquatic plants, where they can survive for prolonged periods (often up to a year). Mammalian hosts become infected by ingesting metacercariae on the aquatic plants. After ingestion, the metacercariae excyst in the duodenum and attach to the intestinal wall, where they develop into adult flukes in approximately three months. Adult worms are 2 to 7.5 cm long and 1 to 2 cm wide; they have a lifespan of about one year and produce approximately 25,000 eggs daily .