Trematoda (trematode, fluke)

Introduction

Includes both sexual forms (schistosomes) & hermaphroditic forms (others). All species that infect humans have an oral sucker through which the digestive tract opens & a ventral sucker used for attachment. Adults vary in length from 1 mm (Metagonimus) to 7 cm (Fasciola gigantica).

Epidemiology

• eggs are passed into the environment through the feces, urine or sputum depending upon species of trematode
• hermaphroditic forms produce unembryonated operculate eggs (except Clonorchis & Opisthorchis)
• schistosome eggs are not operculated & each contains a mature larvae when passed from the host
• trematode larvae (miracidia) are ciliated & capable of penetrating tissues of its 1st intermediate host, a snail
• each species of trematode uses a particular species of snail as its 1st intermediate host
• free-swimming larvae termed cercariae are produced within the snail
• schistosome cercariae are capable of penetrating human skin directly, causing schistosomiasis
• cercariae of hermaphroditic trematodes encyst on aquatic vegetation or invade tissues of 2nd intermediate hosts such as fish or crabs
• ingestion of encysted larvae (metacercariae) then results in human infection
• most common in South & East Asia
• species most commonly infecting humans:
  • Fasciolopsis buski
  • Heterophytes heterophytes
  • Metagonimus yokogawai
  • Echinostoma

Clinical manifestations

• symptoms vary depending upon the number of trematodes parasitizing the host
• adult flukes may cause inflammation, ulceration & mucus secretion at the site of intestinal wall attachment
• most infections are asymptomatic
• loose stools, weight loss, malaise abdominal pain may be noted

Laboratory

• diagnosis of trematode infection is made by recovery & identification of eggs* in feces, urine, sputum or occasionally tissues
• direct mounts & formalin-ethyl acetate concentration methods are most useful for recovery of eggs*.
• flukes* may also be recovered

* images

Management

• 3 doses of praziquantel over 1 day

More general terms

• platyhelminth (flatworm)

More specific terms

• Dicrocoeliidae
• Digenea
• Echinostomatidae
• Fasciolidae
• Heterophyidae
• Nanophyetus
• Opisthorchidae
• paramphistomatidae
• Schistosomatidae
• Troglotrematidae

Additional terms

• trematodiasis