Intestinal Trematodes

Published on 08/02/2015 by admin

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Last modified 08/02/2015

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Intestinal Trematodes

The intestinal trematodes (flukes) are members of the phylum Platehelminthes (flatworms), are dorsoventrically flattened, and require at least one intermediate host (a freshwater snail). Human infection occurs by ingestion of metacercariae encysted on freshwater vegetation or fish. Most trematodes are hermaphroditic (both ovaries and testes are contained within each adult worm). The parasites are typically identified from eggs shed in the feces.

The adult worms are located in the small intestine, where they lay eggs that may be embryonated or remain unembryonated until they are shed from the body via feces. The egg continues developing after reaching the water, and a ciliated, free-swimming miracidium larva is released. The miracidium enters a snail host and develops into a redia (cylindrical larvae), followed by development into tailed cercariae. The cercariae emerge from the snail and encyst as a metacercariae (encrusted larvae) on water plants or fish. A human host ingests raw or undercooked plants (Fasciolopsis buski) or fish (Heterophyes heterophyes, Metagonimus yokogawai) containing the metacercariae, which exycyst in the intestinal tract, attach, and mature into adults (Figure 56-1).

Fasciolopsis Buski

General Characteristics

The adults of F. buski have an elongated shape and range from 20 to 75 mm long to approximately 8 to 20 mm wide (Figure 56-2). They have an oral sucker at the anterior end and a ventral sucker located about midway to the posterior end. The eggs, which are indistinguishable from those of Fasciola hepatica (Figure 56-3), are oval and elongated, transparent, and yellow-brown with an operculum (lid) at one end, and they range in size from 130 to 140 µm long to 80 to 85 µm wide and may be unembryonated.