Trematodes

• What is snail fever and bilharzia?

  schistosomes`
• what is the schistosomiasis distribution?

  251.4 million people required preventative treatment for schistosomiasis 2021.

  only 75.3 million reported to have been treated
• what is a NTD?

  Neglected tropical disease
• what is human schistosomiasis caused by?

  3 major species:

  • schistosoma mansoni (africa)
  • S. haematobium (Africa and middle East)
  • S. japonicum (S.E Asia and china)

  2 minor species:

  • S. intercalatum (central and W.Africa)
  • S. mekongi (Cambodia and Laos)
• what is the advantages of secondary host?

  Increased reproductive potential - asexual reproduction can take place in the alternative host.

  Increases the range of the parasite in space and time - infecting more than one host species allows parasite to survive periods when one host is scarce
• How can intermediate host channel the parasite towards the definitive host?

  Intermediate host releases parasites into host environment or is part of the definitive host's food chain
• What are the 3 major factors responsible for maintaining schistosome transmission?

  pollution of water with excreta containing eggs

  presence of suitable snail host

  human contact with water infected with cercariae
• what is the lifecycle within the human host?

  1. break the barrier
  2. migrate in circulation
  3. mature
  4. pair up
  5. find a home
  6. reproduce (lay eggs0
• Describe breaking the barrier:

  S. mansoni cercariae are photo-trophic

  shed by intermediate snail host

  free living, survive 12-48h

  very motile, use forked tail to swim

  Use water turbulence and skin-derived fatty acids to locate human host

  Cercariae attach to skin and use proteases to break through the epidermis
• How long are cercariae?

  500 micrometers
• How long doesit take to make it through the first layer of skin?

  10 minutes, the shed their glycocalyx to become schistosomula
• describe finding a home:

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

  mature into adult worms in the liver
• what are the developmental stages during migration?

  Skin schistosomula

  lung schistosomula

  Liver schistosomula
• what does dioecious mean?

  Having separate male and female reproductive organs on different individuals of the same species.
• what does sexually dimorphic mean?

  Sexual dimorphism refers to the phenotypic differences between males and females of the same species beyond differences in their sexual organ
• Where does the female schistosome live?

  within the males gynaecophoric canal
• What is the life span of schistosome?

  up to 5 years
• Where do schistosome live?

  in venous system

  S. haematobium - veins (vesical plexus) surrounding the bladder

  other schistosome species surrounded small and large intestine
• Do schistosomes have multiple partners?

  No, they mate for life
• how many eggs are produced per day?

  300-3000
• how long does it take for the adult worm to start laying eggs after initial infection of the host?

  25-30 days
• How to identify egg species?

  spine
• where do 50% of the the eggs remain trapped?


  in the host tissue
• What is the life cycle within the snail host?

  1. hatch to become motile
  2. infect snail
  3. transform so non-motile
  4. make copies
  5. release cercariae
  6. infect human host
• what is a miracidia?

  This is a ciliated larval stage which is free living and motile
• How are miracidia produced?

  eggs which reach suitable freshwater conditions hatch to release miracidia
• how long do snails remain infective for?

  6-8 hours after hatching
• How do miracidia locate suitable snail host?

  using external stimuli such as light and snail derived chemicals
• how are sporocysts produced?

  inside snail tissue miracidium transforms into a non-motile primary sporocyst
• what do primary sporocysts produce?

  secondary sporocysts
• what do secondary sporocysts produce?

  cercariae
• what are the 4 clincal phase of schistosomiasis?

  1. cericarial dermatitis
  2. parasite maturation
  3. established infection
  4. late stage infection
• what causes cercarial dermatitis?

  cercariae burrowing through the skin
• what is cercarial dermatitis also known as?

  swimmers itch
• when do the symptoms of cercarial dermatitis occur?

  on secondary exposure to infection

  present before 15 minutes after exposure

  symptoms continue to develop for 2-3 days

  resolves within 5 days
• what is parasitic maturation also known as?

  acute schistosomiasis or katayama syndrome
• when does schistosomiasis begin?

  2-8 weeks after primary infection but resolves after several days or weeks
• what are common symptoms of schistosomiasis?

  dry cough

  mild to moderate hepatosplenomegaly

  pyrexia (fever)

  weight loss

  giant urticaria (hives)
• what is giant urticaria?

  transient, slight elevated patches of skin redder or paler than surrounding skin
• what symptoms are often associated with giant urticaria?

  intense itching caused by release of vasoactive mediators (mainly histamine) from mast cells