Trematodes

• Class: Trematoda or Digenea
• hermaphroditic (self-fertilizing)
• dioecious (parasites that reproduce via separate sexes)
• Common to all trematodes is their complex life cycles, which almost always include mollusks (snails) as an intermediate host.
• The eggs, which are the primary morphologic form recovered in human specimens, vary in appearance.
• Some contain a lidlike structure that under the appropriate conditions flips open to release its contents for further development, called an operculum, such as in Fasciolopsis and Fasciola.
• Other members of the trematodes may be distinguished by the presence and location of spines, as seen in the Schistosoma spp.
• The rarely seen adult flukes are thin and nonsegmented, resembling leaves in shape and thickness.
• those that reside in the intestine, bile duct, or lung (organ-dwelling)
• those that reside in the blood vessels around the intestine and bladder (blood-dwelling)
• The organ-dwelling flukes include all trematodes except those belonging to the genus Schistosoma.
• Human infection of such organ-dwelling flukes occurs following the ingestion of water plants (e.g., water chestnuts), fish, crab, or crayfish contaminated with the encysted form of the parasite known as metacercaria.
• On entrance into the intestinal tract, the encysted metacercaria excysts and migrates to the intestine, bile duct, or lung.
• all organ-dwelling flukes are hermaphroditic
• On contact with fresh water, the miracidium (contents of the egg) emerges from each egg.
• Numerous rediae (a larval stage that forms in the sporocyst) result and ultimately produce many cercariae (final-stage larvae).
• The cercariae emerge from the snail and encyst on water plants or enter a fish, crab, or crayfish, which serves as the second intermediate host.
• schistosomule (the morphologic form that emerges from cercariae following human penetration)
• The first intermediate host for all the trematodes is snail.
• The specimen of choice for the recovery of trematode organisms is species-dependent.
• Intestinal Species

  • Fasciolopsis buski
  • Heterophyes heterophyes
  • Metagonimus yokogawai
• Liver Species

  • Fasciola hepatica
  • Clonorchis sinensis
• Blood Species

  • Schistosoma mansoni
  • Schistosoma japonicum
  • Schistosoma haematobium
• Serologic tests, such as the ELISA, are also available for the diagnosis of the blood flukes (Schistosoma spp.)
• Like the cestodes, the trematodes belong to the phylum Platyhelminthes.