trematodes

• schistosomes are parasitic flukes
• schistosomes means 


  split body
• intermediate host of snail
• schistosomiasis is classified as

  neglected tropical disease
• 3 major species, 2 minor
• secondary host

  • increased reproductive potential
  • range of parasite in space and time
  • intermediate host can channel parasite to definitive host
• transmission

  • pollution of water
  • snail host
  • human contact with water infected with cercariae
• life cycle (starting with release from snail)

  • cercariae
  • schistosomula
  • adult worms
  • eggs
  • miracidium
• life cycle in human host

  • break barrier as cercariae
  • migrate in circulation
  • mature from schistosomula to adult
  • pair up as adults
  • find home
  • reproduce (eggs)
• cercariae are photo-tropic, survive for 12-48 hours, forked tail to swim, water to swim and locate human
• cercariae shed glycocalyx to avoid the complement, shed tails to become 


  schistosomula
• schistosomula mature by migrating to lungs from skin, then to the liver
• adults are dioecious (males and females)
• females live in males
• live in venous system
• S. Haematobium live in veins surrounding the bladder
• schistosome species live in mesenteric plexus surrounding small and large intestines
• infection to beginning of egg laying is 25-30 days
• life cycle in snail

  • hatch to become motile (miracidia)
  • infect snail
  • transform so non-motile (sporocysts)
  • release cercariae
  • infect human host
• miracidia are 


  ciliated larval stage, free living and motile

  infective for snails for 6-8 hours after hatching

  find snails using external stimuli (light, snail derived chemicals)
• sporocysts

  • primary produce generations of secondary sporocysts
  • then produce cercariae
• clinical features

  • cercarial dermatitis
  • parasite maturation
  • established infection
  • late stage infection
• cercarial dermatitis

  • on secondary infection
  • release of histamines
  • resolves in 5 days
• parasite maturation

  • 2-8 weeks post infection
  • dry cough
  • mild/moderate hepatosplenomegaly
  • fever
  • weight loss
• established infection

  • painful urination
  • blood in urine
  • bladder inflammation
• pathogenesis: human schistosomiasis is immunopathological disease

  1. egg deposition
  2. inflammation
  3. granuloma formation
  4. obstruction of urinary tract or portal circulation
  5. fibrosis
• granuloma is concentric layer of cells forming distinct lesion
• granuloma characteristics

  • immune cells accumulating around egg
  • lead to disease
• fibrosis

  • permanent fibrous lesions, deposit fibrous tissue around egg
  • blockage of portal vein branches
• trapped eggs cause disease
• portal hypertension, build up of pressure due to blockages
• diagnosis

  • eggs in stool/urine
  • worm antigens in urine
  • ultrasound
  • bladder or rectal biopsy
  • serological testing (antibodies against adult worms)
• praziquantel

  • against trematodes and cestodes
  • single oral dose
  • half life is 0.8-1.5 hours
  • only adult worm cleared out
• can be re-infected after chemotherapy
• praziquantel is much less effective against

  immature worms
• no drugs to prevent infection of schistosomes