8. Helminth TREMATODES

• Medically important trematode superfamilies

  • Schistosomatidae
  • Fasciolidae
  • Opisthorchidae
  • Troglotrematidae
• Medically important trematode genera

  • Schistosoma
  • Fasciola
  • Fasciolopsis
  • Clonorchis
  • Paragonimus
• Medically important trematode species

  • S. haematobium
  • S. mansoni
  • S. japonicum
  • F. hepatica
  • F. buski
  • C. sinensis
  • P. westermani
• Platyhelminthes


  • Tape-like
  • Flattened
  • Segmented
• Nemathelminthes


  • Leaf-like
  • Flattened
  • Unsegmented
• Cestoda


  • Elongated
  • Cylindrical
  • Unsegmented
• Trematoda


  • Leaf-like
  • Flattened
  • Unsegmented
• Scolex


  Suckers (often with hooks)
• Alimentary canal


  Absent in Platyhelminthes, Incomplete (no anus) in Trematoda, Complete (with anus) in Nematoda
• Body cavity


  Absent in Platyhelminthes and Trematoda, Present in Nematoda
• Trematodes are hermaphroditic except Schistosoma species
• Male reproductive system


  Two testes, vasa efferentia leading to a single vas deferens, terminal part of vas deferens modified into a muscular cirrus
• Female reproductive system


  Single ovary, ovary discharges ova into ootype, sperm enters Laurer's canal which leads to ootype, fertilised ova pass from ootype into uterus
• General life cycle of trematodes

  Egg (in water) -> Miracidium (in snail) -> Sporocyst (in snail) -> Redia (in snail) -> Cercaria (free-living) -> Metacercaria (no tail) -> Adult (in human)
• Helminth groups of medical importance

  • Nematoda (Roundworms)
  • Cestoda (Tapeworms)
  • Trematoda (Flukes)
• Fasciolopsis buski is the largest intestinal fluke found in humans
• Fasciolopsis buski


  Length: 20-75 mm, Width: 8-20 mm, Thickness: 0.5-3 mm, Oral sucker smaller than ventral sucker, Single branched ovary, Pair of highly branched testes
• Fasciolopsis buski egg


  Oval shape, Yellow colour, Thin shell, Indistinct operculum at one end, Rounded at both ends, Length ~150 μm, Width ~90 μm
• Life cycle of Fasciolopsis buski

  Unembryonated eggs in faeces -> Embryonated eggs in water -> Miracidia penetrate snail -> Sporocyst -> Redia -> Cercaria (free-swimming) -> Metacercaria (encyst on aquatic plants) -> Excyst in human duodenum -> Adult in small intestine
• Fasciolopsis buski pathogenesis


  Adult flukes attach to intestinal mucosa causing inflammation, ulceration and abscesses, Most infections are light and asymptomatic, Symptoms of heavy infections include diarrhoea, abdominal pain, nausea, fever, ascites, anasarca
• Praziquantel is effective treatment for Fasciolopsis buski infection
• Diagnosis is by direct wet mount microscopy observation of unembryonated eggs
• Fasciolopsis buski life cycle

  1. Metacercariae excyst in the duodenum

  2. Attach to the intestinal wall

  3. Develop into adult flukes in ~3 months
• Life span of adult Fasciolopsis buski


  About one year
• Definitive hosts for Fasciolopsis buski

  • Humans
  • Pigs
• Intermediate hosts for Fasciolopsis buski

  • Snails
  • Aquatic plants
• Fasciolopsis buski adult flukes

  • Attach to the intestinal mucosa
  • Cause inflammation, ulceration and abscesses
• Most Fasciolopsis buski infections

  Light and asymptomatic
• Symptoms of heavy Fasciolopsis buski infections

  • Diarrhoea
  • Abdominal pain
  • Nausea
  • Fever
  • Ascites
  • Anasarca
  • Intestinal obstruction
• Praziquantel


  Effective treatment for Fasciolopsis buski
• Avoid water chestnuts from contaminated waters to prevent Fasciolopsis buski
• Treat sewage to control Fasciolopsis buski
• Fasciolopsis buski eggs


  • Indistinguishable from Fasciola hepatica eggs
  • Both are operculated at the blunt end
• Important to observe for clinical symptoms in endemic areas for Fasciolopsis buski diagnosis
• Fasciola hepatica is a liver trematode
• Fasciola hepatica


  Also known as the Sheep Liver Fluke
• Fasciola hepatica adult


  • Large fluke found in human bile ducts
  • Has a cephalic cone with an oral and ventral sucker of equal size
  • Highly branched testes, intestine and vitellaria
  • Single small branched ovary
• Fasciola hepatica eggs


  • Oval/ellipsoidal shape
  • Thin shell
  • Indistinct operculum
  • Length ~170 μm, Width ~100 μm
• Fasciola hepatica life cycle

  1. Unembryonated eggs in feces

  2. Embryonated eggs in water release miracidia

  3. Miracidia infect snail intermediate host

  4. In snail, develop into sporocysts, rediae and cercariae

  5. Cercariae encyst as metacercariae on aquatic vegetation

  6. Humans/mammals ingest metacercariae

  7. Metacercariae excyst in duodenum, migrate to bile ducts and develop into adults
• Acute fascioliasis


  • Common in sheep, rare in humans
  • Caused by ingestion of large numbers of metacercariae
  • Migrating larvae invade liver and cause traumatic hepatitis, may rupture liver capsule causing peritonitis