Trematodes

• Phyla of Helminths

  • Platyhelminthes (flukes, tapeworms and other flatworms)
  • Nemathelminthes (round worms and their relatives)

  
• Classes of Helminths

  • Cestoda (Tape worms)
  • Trematoda (Flukes)
  • Nematoda (Round worms)

  
• Cestoda

  • Tape-like, segmented
  • Not separate sexes, i.e., hermaphrodite (monoecious)
  • Suckers present, often with hooks
  • Alimentary canal absent
• Trematoda

  • Leaf-like, unsegmented
  • Not separate sexes (monoecious), except Schistosomes
  • Suckers present, no hooks
  • Incomplete alimentary canal, no anus
• Nematoda

  • Elongated, cylindrical, unsegmented
  • Separate sexes (diecious)
  • No suckers, no hooks, well-developed buccal capsule in some
  • Complete alimentary canal, anus present
  • Body cavity present
• Sub-Classes of Trematoda

  • Monogenea
  • Aspidogastrea
  • Digenea
• Monogenea


  • Parasitic on cold-blooded aquatic vertebrates (fishes, amphibians and reptiles)
  • Mostly ectoparasitic
  • Direct life-history
  • None parasitic on domestic animals
• Aspidogastrea

  • Parasitic on fishes, turtles, mollusca
  • None parasitic on domestic animals

  
• Digenea

  • Parasitic in domesticated animals
  • Life-histories require one, two or more intermediate hosts
  • Veterinary importance
• Digenetic trematodes (Digenea)


  Includes all the trematode species parasitic in domesticated animals
• Body Shape of Digenetic Trematodes

  • Dorsoventrally flattened, some being long and narrow, some leaf-shaped
  • A few, the amphistomes, have thick fleshy bodies
  • The schistosomes are long and worm-like
  • Cuticle/tegument is smooth or spiny

  
• Organs of Attachment of Digenetic Trematodes

  • Two strong muscular cup-shaped depressions, called suckers - oral sucker and ventral sucker or acetabulum
  • Body cavity absent
• Alimentary System of Digenetic Trematodes

  • Incomplete alimentary canal, anus is absent
  • Oesophagus bifurcates in front of the ventral sucker into a pair of blind intestinal caeca which may be simple, branched or reunite to form a single caecum
  • Food - blood, mucous and tissues taken in to the caeca via the oral sucker
  • Absorption of amino acids and monosaccharides occurs through the tegument

  
• Other Systems of Digenetic Trematodes

  • Excretory and nervous systems are present
  • Excretory system consists of "flame cells" and collecting tubules which open posteriorly, into the excretory pore
  • Sexes are not separate, i.e., each individual worm is a hermaphrodite (monoecious) except the Schistosomes (unisexual)
  • Reproductive system is highly developed and complete in each individual
• Trematode Eggs

  • All operculated (except those of Schistosomes) and can develop only in water
  • Mature egg is yellow because of the tanned protein shell
  • Do not float in a saturated salt solution
• Life Cycle of Digenetic Trematodes

  1. Eggs pass out of the final host, usually in faeces or urine

  2. Larval stages develop in a molluscan intermediate host

  3. For a few species, a second intermediate host is involved, but the mollusc is essential for all members of the group

  4. One trematode egg may eventually develop into hundreds of adults (paedogenesis)

  5. Egg cannot withstand desiccation

  6. Hatching is controlled by factors like light, temperature, and salinity

  7. Miracidium hatches out from the egg and infects snail

  8. Miracidium develops into sporocyst, which contains germinal cells

  9. Germinal cells in sporocysts develop into rediae, which migrate to the hepato-pancreas of the snail

  10. From the germinal cells of rediae arise the final stages, the cercariae

  11. Cercariae swim, attach to vegetation, shed their tails and encyst as metacercariae

  12. Metacercariae have great potential for survival, extending to months

  13. Once ingested, the emergent juvenile fluke penetrates the intestine and migrates to the predilection site where it becomes an adult after several weeks