Trematatodes

• Where are trematodes endemic

  In tropical countries
• What type of organism is a schistosome

  It is a trematode
• Schistosome life cycle


  1. Eggs are present in human urine or faeces
  2. Eggs hatch to produce miracidia which are motile
  3. They infect a snake
  4. They become non-motile
  5. Asexual replication produces Cercariae
  6. Cercariae penetrate the skin
  7. They migrate in the circulation and pair up
  8. They migrate to the portal blood and mature into adults
  9. They reproduce in the venules of the bowel and rectum
  10. They can then penetrate the venous plexus of the bladder
• What is the secondary or intermediate host for Schistosomes

  Water snails
• Describe the feature of Schistosome Mansoni

  
  

  They are phototropic and motile.
• How do cercaria enter the skin

  They use proteases to break the epidermis
• Describe the features of mircacidia

  They are produced from the hatching of Schistosome eggs.

  They are a ciliated larval stage.
• What are the 4 stages of human Schistosomiasis

  Cercarial dermatitis.

  Acute Schistosmiasis.

  Species specific symptoms.

  Symptoms that appear years after primary infection.
• What causes Cercarial dermatitis

  When the Cercariae burrow throguht the skin resulting in an allergic reactions.
• what causes Acute Schistosmiasis

  Mast cells release histamine in response to the antigens present on eggs.
• What are the symptoms of Acute Schistosmiasis

  Dry cough

  Hepatosplenomegaly

  Pyrexia which is fever

  Weight loss

  Hives
• Cercariae can make it through the first layer of skin within 10 minutes


  Shed their tails to become Schistosomula
• Migration of Schistosomula


  1. Migrate from the skin to the liver via the lungs in the vasculature and lymphatics

  2. Progress through 3 developmental stages during migration: skin schistosomula, lung schistosomula, liver schistosomula

  3. Mature into adult worms in the liver
• Only 30-50% of cercariae that penetrate host reach maturity
• Adult schistosomes


  • Dioecious and sexually dimorphic
  • Female lives within male's gynaecophoric canal
  • Lifespan up to 5 years depending upon species
  • Location within the human body: venous system, with S. haematobium in veins surrounding the bladder and other species in veins of the mesenteric plexus surrounding intestines
• Schistosomes mate for life
• Schistosome egg production


  • Paired female worms release eggs throughout adult life
  • Daily egg production: 300-3,000 eggs per day
  • Period of time between infection of human host to beginning of egg laying by adult worms ~ 25 to 30 days
  • Eggs of most species have a characteristic spine for species identification and diagnosis
  • ~50% of the eggs remain trapped in host tissue
• Free-living and within the snail host stages of schistosome life cycle

  1. Eggs hatch to release miracidia

  2. Miracidia infect snail

  3. Transform into non-motile sporocysts in snail

  4. Sporocysts produce generations of cercariae

  5. Cercariae released from snail can infect human host
• Miracidia


  • Ciliated larval stage which is free living and motile
  • Remain infective for snails for ~6-8 hours after hatching
  • Able to locate suitable snail hosts using external stimuli such as light and snail derived chemicals
• Clinical phases of untreated human schistosomiasis

  • Cercarial dermatitis (swimmers itch)
  • Parasite maturation (acute schistosomiasis or Katayama Syndrome)
  • Established infection
  • Late stage infection
• Cercarial dermatitis (swimmers itch)

  • Caused by cercariae burrowing through the skin and causing allergic reaction
  • Occurs on secondary exposure to infection
  • Presents within <15 minutes after exposure
  • Continues to develop for 2-3 days
  • Resolves within 5 days
• Acute schistosomiasis or Katayama Syndrome


  • Begins 2-8 weeks post-infection
  • Usually mild and transient but can be severe/life threatening
  • Common clinical features include dry cough, mild to moderate hepatosplenomegaly, pyrexia, weight loss, giant urticaria
• Symptoms of established schistosome infection

  • S. haematobium: Haematuria, dysuria, abdominal pain, bladder inflammation
  • Other species (S. mansoni etc.): Abdominal pain, diarrhoea, hepatomegaly with or without splenomegaly
• Late stage clinical syndromes of schistosomiasis

  • S. japonicum: Intestinal schistosomiasis
  • S. mansoni: Hepatosplenic schistosomiasis
  • S. japonicum: Pulmonary schistosomiasis
  • S. japonicum: CNS schistosomiasis
  • S. haematobium: Urinary tract schistosomiasis
• Pathogenesis of chronic schistosomiasis

  • Immunopathological disease caused by host immune response against trapped eggs
  • Over 50% of eggs remain trapped in host body
  • Egg deposition leads to inflammation, granuloma formation, obstruction of urinary tract or portal circulation, and fibrosis
• Granuloma formation


  • Concentric layers of cells forming a distinct lesion around trapped eggs
  • Response to antigens released by egg/miracidium, with peak response 4-8 days after egg deposition
  • Immune cells accumulate around egg, miracidia die, and granuloma protects host from toxins
  • But egg-induced granulomas result in pathological changes in liver leading to disease
• Fibrosis


  • Egg-induced granulomas transformed into permanent fibrous lesions by deposits of fibrous tissue around egg
  • Pipe-stem fibrosis causes portal vein branches to resemble sections of clay pipe stems
  • Blockage of portal vein branches and development of anastomoses
• Hepatosplenic disease


  • Clinical symptoms include hepatosplenomegaly, portal hypertension, cirrhosis of liver, and ascites
• Epidemiology of schistosomiasis


  • Infection rises rapidly through childhood, peaks in older children/young adults at low transmission rates, then declines into older age
• Diagnostic techniques for schistosomiasis

  • Presence of eggs in stool or urine samples
  • Presence of worm antigen CCA in urine
  • Ultrasound
  • Bladder or rectal biopsy
  • Serological testing for antibodies against adult worms
• Praziquantel


  • Effective chemotherapy against trematodes (and cestodes)
  • Single oral dose, well absorbed but short serum half-life
  • Few side effects and contra-indications
  • But increasing reports of drug resistant schistosomes
• Praziquantel mode of action

  • Increases cell membrane permeability
  • Causes imbalance in ion transport
  • Loss of intracellular calcium
  • Massive contraction and paralysis of musculature
  • Disintegration of the schistosome tegument
• Re-infection after praziquantel chemotherapy is very common due to presence of snail host, and praziquantel is less active against immature worms
• Schistosomiasis prevention methods


  • Avoid swimming in freshwater in endemic countries
  • Drink 'safe water' - filter, heat to 50°C for 5 minutes or allow to stand for 24 hours
  • Swimming in chlorinated water and sea water is safe
• No vaccines available and no drugs to prevent schistosomiasis infection