Trematodes

• Trematodes

  Unsegmented helminths, also known as flukes, that appear flat, broad, and leaf-shaped except for schistosomes
• All trematodes are hermaphrodites except schistosomes
• All trematode eggs are operculated except for schistosomes
• Trematodes


  • Possess suckers but no hooks
  • Are oviparous
  • Have an oral sucker, pharynx, ceca, ventral sucker, uterus, vitellaria, testes
• Trematode eggs

  • Immature: Fasciola hepatica, Fasciolopsis buski, Echinostoma ilocanum, Paragonimus westermani
  • Mature: Clonorchis sinensis, Opistorchis felineus, Heterophyes heterophyes, Metagonimus yokogawai, Dicrocoeulum dendriticum, Schistosoma species
• The infective stage for all Trematodes is the metacercaria except for schistosomes
• Trematode infections by location

  • Blood (blood flukes): Schistosoma haematobium, Schistosoma mansoni, Schistosoma japonicum
  • Lungs (lung flukes): Paragonimus westermani
  • Liver/bile passages (liver flukes): Clonorchis sinensis, Fasciola hepatica, Opistorchis spp.
  • Intestines (intestinal flukes): Fasciolopsis buski, Heterophyes heterophyes, Echinostoma ilocanum
• General life cycle of Trematodes

  1. Definitive hosts: mammals, humans

  2. 1st Intermediate hosts: freshwater mollusks, snails

  3. 2nd Intermediate hosts: fish, crab

  4. Eggs in the environment hatch in water to miracidia

  5. Miracidia develop within 1st IH to sporocysts, rediae, cercariae

  6. Cercariae escape 1st IH and encyst as metacercariae in 2nd IH

  7. Definitive host infected by ingestion of metacercariae (except schistosomes which penetrate skin)
• Sporocysts

  • Miracidium sheds cilia and becomes sac-like, where germ balls are formed (asexual replication)
• Mature cercariae


  • Schistosomes have a forked tail and infect DH by skin penetration, other trematodes have an unsplit tail and infect DH by ingestion of metacercariae
• Trematodes have 2 muscular shaped suckers (distomata), lack a body cavity, circulatory system, and respiratory system, and undergo self-fertilization (hermaphroditic) except schistosomes (dioecious)
• Male and female schistosomes live in copula
• Medically important Schistosoma species

  • Asian schistosomiasis: S. japonicum, S. mekongi
  • African schistosomiasis: S. mansoni, S. haematobium, S. intercalatum
• Schistosoma life cycle development

  1. Embryonated eggs hatch to miracidia, which penetrate snail intermediate host

  2. Miracidia lose cilia and develop into sporocysts

  3. Sporocysts reproduce asexually to give rise to free-living cercariae

  4. Cercariae penetrate skin, develop into schistosomula, migrate to hepatic portal vein (adults)
• Schistosomes ingest red blood cells, possess a protease (hemoglobinase), and utilize glucose at a rapid rate
• Schistosoma species and their hosts

  • S. japonicum: man, domestic animals, rodents
  • S. mansoni: man
  • S. haematobium: man
  • S. intercalatum: man
  • S. mekongi: man
• Schistosoma morphology

  • Cylindrical body, leaf-like and unsegmented, covered by thick, tuberculated, syncytial tegument
  • Large sucker, ventral sucker, gonophore
  • Incomplete digestive system, bifurcated, inverted Y-shape excretory system
  • Females are longer and thinner, occupy gynecophoric canals of males, deposit eggs in portal vein
  • Males are short and thick, testes in one row, hold females in gynecophoral canal
• Schistosoma cercariae


  • Have a forked tail, oral sucker, small ventral suckers, most abundant in early evening
• Pathogenesis of schistosomiasis

  Skin penetration (incubation period), acute infection (Katayama disease), ectopic granulomas (colon, liver, spleen, lungs), cancer
• Schistosoma haematobium

  • Endemic in Africa and West Asia, inhabit vesical and pelvic venous plexuses
  • Adults covered in finely tuberculated cuticle, 2 muscular suckers, small oral sucker, large ventral sucker, gravid female may contain 20-30 eggs
  • Eggs are ovoid, non-operculated, with terminal spine, excreted in urine
• Pathogenesis of S. haematobium

  Cercarial dermatitis, acute systemic schistosomiasis, oviposition leading to painless terminal hematuria, bladder hyperplasia and inflammation, squamous cell carcinoma
• Schistosoma mansoni

  • Widely distributed in Africa, South America, Caribbean, adults inhabit inferior mesenteric vein
  • Resemble S. haematobium, appear smaller, integument covered in coarse tubercles, gravid female contains 1-3 eggs
  • Eggs are ovoid, non-operculated, with lateral spine, excreted in feces
• Pathogenesis of S. mansoni

  Cercarial dermatitis, acute systemic schistosomiasis with Katayama fever and intestinal symptoms, oviposition causing granulomas, hyperplasia, fibrosis in intestine and liver
• Schistosoma japonicum

  • Found in Far East, adults inhabit superior mesenteric venules
  • Similar morphology to S. haematobium and S. mansoni, male has smooth cuticle, female uterus contains up to 100 eggs
• Schistosoma japonicum pathogenesis is similar to S. mansoni but more severe, including Katayama fever, colicky abdominal pain, dysentery, anemia, hepatomegaly with periportal fibrosis and portal hypertension
• Distinguishing features of Schistosoma species

  • S. japonicum: non-tubercular integument, ovary in middle of body, uterus contains 50+ eggs
  • S. mansoni: grossly tuberculated integument, ovary in anterior half, uterus contains 1-3 eggs
  • S. haematobium: finely tuberculated integument, ovary in posterior one-third, uterus contains 20-30 eggs
• Other Schistosoma species

  • S. mekongi: eggs oval with minute lateral spine/knob like S. japonicum but smaller
  • S. intercalatum: eggs elongate with terminal spine like S. haematobium but larger
  • S. malayensis: AFS (+) eggs found in feces, diamond-shaped
• Female


  Contains as many as 100 eggs at one time
• Female

  May pass out 3500 eggs daily
• Eggs

  • Appear ovoid and non-operculated
  • Have a small lateral knob
  • Contain ciliated miracidium
  • Are excreted in feces
• Pathogenesis

  Similar to that of S. mansoni but manifestations are more severe
• Katayama fever


  May develop in acute phases
• Intestinal manifestations

  Colicky abdominal pain and dysentery
• Patient

  • May develop anemia
  • Hepatomegaly with periportal fibrosis and portal hypertension may be present
• Schistosomes

  • S. japonicum
  • S. mansoni
  • S. haematobium
• Schistosomes

  • Integument: non-tubercular
  • Ovary: in the middle of the body
  • Uterus: contains 50 or more eggs
• S. mansoni

  • Integument: grossly tuberculated
  • Ovary: in the anterior half of the body
  • Uterus: contains 1–3 eggs
• S. haematobium

  • Integument: finely tuberculated
  • Ovary: in the posterior one-third of the body
  • Uterus: contains 20-30 eggs
• Other Schistosomes

  • S. mekongi
  • S. intercalatum
  • S. malayensis
• S. mekongi

  • Eggs are oval with minute lateral spine or knob resembling S. japonicum but smaller