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Intestinal Trematodes
More than 70 species of zoonotic, hermaphroditic intestinal flukes can parasitize the human intestine. Most human infections are asymptomatic, but heavy infections are more likely to result in clinical disease.
Adult flukes are flat and leaf-shaped, ranging in length from a few millimeters to several centimeters. They attach to the intestinal mucosa by means of oral and ventral suckers and release eggs having a characteristic operculum at one end of the shell. When deposited into freshwater, the operculum opens to release a miracidium, the free-swimming, ciliated larval stage of the parasitic fluke that finds and penetrates an appropriate host, typically a snail in which further replication occurs. The infected snail eventually releases thousands of cercariae into water that encyst (as metacercariae) on aquatic plants or on or in a second intermediate host (i.e., fish, mollusks, crustaceans, and amphibians). When ingested by humans and other definitive hosts, the metacercariae develop into adult flukes.
Some of the more common intestinal trematodes are Fasciolopsis buski, Heterophyes heterophyes, Metagonimus yokogawai , and the echinostomes ( Table 283.1 ). However, Metorchis conjunctus, Nanophyetus salmincola, Gastrodiscoides hominis, Centrocestus formosanus, Haplorchis species, and others also infect humans. , Intestinal trematodes can be treated easily with oral praziquantel.
TABLE 283.1
Parasitic Intestinal Trematodes of Humans
| Common Name of Disease | Species | Intermediate Host | Location of Adult Trematode | Distribution | |
|---|---|---|---|---|---|
| Primary | Secondary | ||||
| Fasciolopsiasis | Fasciolopsis buski | Snails | Aquatic plants | Small intestine | Southeast Asia, primarily southern China |
| Heterophyiasis | Heterophyes heterophyes | Snails | Fish | Small intestine | Middle East |
| Metagonimiasis | Metagonimus yokogawai | Snails | Fish | Small intestine | Far East |
| Echinostomiasis | Euparyphium ilocanum | Snails | Fish, amphibians, crustaceans, mollusks | Small intestine | Far East |
Fasciolopsiasis
Description
Fasciolopsiasis is a small bowel infection caused by the largest of the intestinal trematodes, F. buski, which can measure up to 7.5 × 2.5 cm. Eggs passed in feces hatch, and miracidia infect planorbid snails. Cercariae that emerge from the snails encyst on freshwater vegetation (i.e., bamboo shoots, water chestnuts, water caltrop, water lilies, hyacinths, and morning glory) and remain infectious for up to 1 year. After ingestion, the metacercariae develop into mature flukes in 3 months.
Epidemiology
F. buski is found throughout the Far East and Southeast Asia, including southern China, Taiwan, Bangladesh, India, Indonesia, Thailand, Malaysia, Sumatra, Cambodia, Philippines, Vietnam, Laos, and Myanmar. , Pigs are important reservoirs of the infection, although dogs and rabbits also can be infected. , Children, who tend to eat plants with encysted cercariae, have the highest prevalence rates.
Clinical Manifestations
Most infections are asymptomatic. , Ulcers or abscesses develop at the site of attachment to the intestinal mucosa in the duodenum and jejunum, and those infected may complain of epigastric pain indistinguishable from peptic ulcer disease. Children may come to medical attention because of systemic allergic manifestations attributed to the absorption of parasite metabolites. In cases of heavy infections, an edematous condition resulting from protein-losing enteropathy has been reported. Large numbers of flukes in the intestine may cause ileus or intermittent obstruction.
Laboratory Findings and Diagnostic Tests
Eosinophilia is common, and anemia can be seen in severe cases. The diagnosis is made by identification of large (135 × 80 μm), operculated eggs in stool. Eggs can be confused with those of Fasciola hepatica, Fasciola gigantica, or the echinostomes, which are similar morphologically. Stool concentration techniques, which allow examination of 1 g or more of stool, enhance detection of light infections.
Treatment
The drug of choice for children and adults is praziquantel, administered as 75 mg/kg/day divided into 3 doses. Confusion of fasciolopsiasis eggs with eggs from fascioliasis (i.e., liver fluke infection) results in ineffective treatment for the latter, which is resistant to praziquantel.
Prevention
In endemic areas, freshwater plants should be cooked before being eaten. Better control of the infection can be achieved by avoiding the use of human or pig feces as fertilizer and proper disposal of human and pig excreta.
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