Trematodes - Overview

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Trematoda - Courtesy of C. Hermosilla, Royal Veterinary College

The trematodes are typically flat, leaf-shaped worms. Two groups are of veterinary interest – the monogeneans and the digeneans.

Monogenean Trematodes

  • These are ectoparasites of fish and other aquatic animals
  • They have a prominent attachment organ, the haptor
  • The life-cycle is direct

Digenean Trematodes

  • These are endoparasites with species parasitic to humans, domesticated and wild animals
  • They have complex indirect life-cycles using molluscan intermediate hosts

The most obvious external features of digenean trematodes are the ventral and oral suckers. The mouth leads from the latter to a muscular pharynx which pumps food into two blind-ending caecae. In some species, the caecae have multiple branches to increase surface area. Most species are hermaphrodite, but individuals cross-fertilise.

Structure of tegument

  • Flukes are covered by a metabolically highly active tegument
  • Tegumental cells in the sub-surface parenchyma donate cytoplasm to the surface syncitium

The tegument plays an important role in the evasion of host immunity. Different trematode species have evolved different strategies

  • Rapid turnover; sloughing of antibody and adhering host cells every 5-6 hours
  • Molecular mimicry; host does not recognise parasite as foreign (e.g. Schistosoma)
  • Release of immunomodulatory factors (e.g. lymphocyte responses in cattle chronically infected with Fasciola are TH-2 type = non-protective IgG1 response)
  • Release of enzymes that cleave immunoglobulins (e.g. Fasciola)
  • Antigenic variation (e.g. with Fasciola, three types of tegumental cell contribute cytoplasm during different phases of the parasitic life-cycle, each presenting different antigens to the host)

Life-cycle stages

Egg: the Fasciola egg is a large egg (twice the size of a standard strongyle egg). It is oval and brown with an operculum at one end. It is a relatively dense structure that will not float in saturated salt solution (used for routine egg-counts), so a flotation fluid with a higher specific gravity is required (e.g. ZnSO4). Better still, a sedimentation technique can be used. Initially, the egg is full of nutrient material. It need moisture and temperatures greater than 10°C for the miracidium to develop inside the egg.

Miracidium: the first larval stage develops inside the egg. It has two light sensitive spots and will only hatch out of the egg if the light intensity is correct and the egg is covered with a film of water. After hatching, it has to find the molluscan intermediate host. It is therefore covered with cilia for locomotion. When penetrating the snail, it releases proteases.

Sporocyst: this is a motile sac-like structure whose function is asexual multiplication, producing more sporocysts, and eventually rediae.

Redia: cylindrical with a pharynx. Further asexual multiplication takes place, producing cercariae, which are expelled through a birth pore.

Cercaria: the stage that leaves the snail. It is heart-shaped and has a tail for swimming.

Metacercaria: the cercaria swims onto vegetation, loses its tail, secretes a tough protective wall around itself to form the metacercaria which waits to be ingested by a suitable host.

Immature fluke: on ingestion, the immature fluke excysts from the metacercaria and migrates to its predilection site (this is the liver in the case of Fasciola).

Adult fluke: in the predilection site, the fluke matures and eventually starts to lay eggs.

Occlusion of pancreatic ducts by flukes sometimes happens Pancreas - Parasitic Pathology

May cause peritonitis

Peritoneal Cavity - Parasitic Pathology