Schistosomiasis Flashcards
What three species of Schistosoma are of vast medical significance?
Schistosoma mansoni
Schistosoma japonicum
Schistosoma haematobium
Where in the body do adult Schistosoma worms typically reside?
Adult worms live within the veins that drain certain organs of their host’s abdomen. Each of the three main species have distinct preferences.
- S. haematobium lives mainly in the veins of the urinary bladder plexus
- S. mansoni prefers the portal veins draining the large intestine
- S. japonicum is concentrated in the veins of the small intestine
Within which veins does S. haematobium typically reside?
Schistosoma haematobium typically resides in the veins of the urinary bladder plexus.
Within which veins does Schistosoma mansoni reside?
S. mansoni resides within the portal veins that drain the large intestine.
What veins do Schistosoma japonicum worms typically reside?
S. japonicum worms are concentrated in the veins of the small intestine.
How many people worldwide suffer from severe Schistosomiasis?
20 million people
What does Schistosoma mean?
Split body- referring to the gynecophoral canal of the male
What types of eggs does Schistosoma mansoni produce?
Laterally spined eggs.
Where is Schistosoma japonicum distributed?
Japan, China, Taiwan, the Philipines and SE Asia.
What term describes the ventral, longitudinal groove on male Schistosoma?
Gynecophoral canal.
Where does the female Schistosoma reside when moving?
The gyncecophoral canal of the male.
What does Schistosoma do to improve motility upstream from smaller veins?
The female Schistosoma positions itself within the gynecophoral canal of the male.
Where does copulation between male and female Schistosoma take place?
Within the gynecophoral canal.
Where are the eggs of Schistosoma deposited?
Within the small venules of the host.
What obstacle must eggs traverse before they can be expelled by the host?
The wall of the venules (small vein) and gut or bladder.
What process aids egg expulsion from the host?
The endothelial lining of the venule moves over the Schistosome eggs to exclude them from the lumen.
What processes may the worm complete to aid transport of the eggs to the gut or bladder lumen?
It may exploit host immune responses.
What process occurs once a Schistosoma egg has been forced out of the venule?
The expelled egg stimulations the formation of a granuloma (mass of cells containing eosinophils, plasma cells and macrophages).
Once the Schistosome granuloma has formed where does it move to?
The granuloma, containing the Schistosome egg moves to the intestinal or bladder lumen.
Once the Schistosome granuloma reaches the bladder or intestinal lumen what occurs?
The granuloma disperses and the egg contained within is expelled in the urine or faeces.
What fraction of Schistosome eggs are retained and not expelled from the host via urine or faeces?
Around 2/3 of eggs are not excreted and accumulate within the bladder or gut wall.
What stimulus triggers the hatching of the Schistsome egg?
Contact with fresh water releases free-swimming miracidia.
Once the miracidia are released in water what process occurs?
Cilia activation continues which allows the miracidium to enter a high spin state.
Once the miracidia reaches its high spin state what occurs?
Osmotically controlled vents open which allows the miracidium to emerge.
How long can a miracidium swim for to find a host?
6 hours maximum
What tropism do miracidium possess?
Schistsome miracidia are positively phototropic
What are the important intermediate snail hosts for Schistosoma haematobium?
Bullinus and Physopsis
What are the important intermediate snail hosts for Schistosoma mansoni?
Biomphalaria spp
What are the important intermediate snail hosts for Schistosoma japonicum?
Oncomelania spp
When a miracidium penetrates a snail host what occurs?
The miracidium sheds its epithelium and enters a new stage of development, the mother sporocyst
How long does the mother sporocyst stage last?
2 weeks
After two weeks the mother sporocyst gives birth to daughter sporocysts that migrate to other organs within the snail.
How long does the mother sporocyst produce daughter sporocysts for?
6 weeks
What emerges from the daughter sporocysts?
Tail forked (furcocercous) CERCARIAE
How long does it take between initial penetration of the snail by miracidium and the emergence of cercariae from daughter sporocysts?
~ 4 weeks
How do cercariae penetrate human skin?
The cercariae lack an oral sucker
They have a head organ with penetration glands and a ventral sucker covered in spines.
How long do the cercariae survive in water?
The cercariae survive for 1-3 days and swim to the surface and sink the bottom in cycles.
Once the cercariae come into contact with a human host how do they find a suitable penetration site?
They creep until they find a suitable penetration site.
What attracts the cercariae to the human host?
Cercariae are attracted to skin secretions, particularly arginine
What skin secretion is particularly attractive to free-swimming cercariae?
Arginine
Stimulation of the cercariae allows it to produce arginine itself via the postacetabular glands, attracting other cercariae to the vicinity.
Via which gland do cercariae produce arginine?
Postacetabular gland
Why does the cercariae produce its own arginine when it is stimulated by host skin arginine?
Increased arginine attracts more cercariae to the penetration site.
How does the cercariae complete penetration?
The cercariae can penetrate the surface within seconds and through the epidermis in less than 30 minutes.
Vigorous wiggling and secretion of products from the head organ assist penetration.
What happens to the cercariae tail after penetration?
The tail is lost.
Once the cercariae lost their tails where do the schistosomules move to?
Within 24 hours the schistosomules enter the peripheral circulation and are swept to the heart.
Once the schistosomules leave the right heart where to they go?
They attempt to move through the pulmonary capillaries to gain access to the left heart and circulation.
What percentrage of schistosomules are lost trying to migrate through lung capillaries?
Up to 70%
What route must schistosomules take to reach the liver for further growth?
Schistosomules must enter the mesenteric arteries and traverse the intestinal capillary bed to reach the liver via the hepatoportal system.
Where do schistosomules continue their development after migrating through the heart, lungs and circulation?
The liver via the mesenteric arteries and intestinal capillaries.
How long do the schistosomules develop within the liver sinusoids?
3 weeks
Once developed within the liver sinusoids what do the young worms do?
The worms pair up and migrate to the gut or bladder wall (depending on species)
Where do the schistosomes produce eggs?
Within the gut or bladder wall (depending on species)
How long does the prepatent period last? (time between infection and detection of eggs in excreta)
5-8 weeks
How long can adult schistosomes live for?
20-30 years
What happens to unpaired female worms?
They do not become sexually mature and may starve.
When a female pairs with a male what process assists growth?
Growth stimulating factors are released that may assist the immature female to pump blood into her intestine.
What is required to stimulate Schistosome development?
Host immune cues
TNF stimulates egg production
IL7 and thyroxin are required for growth
CD4 lymphocytes produce important immune signals for the parasite
What is the main epidemiological factor in Schistosomiasis?
Human waste in water
What is responsible for all the pathogenesis in Schistosomiasis?
The Schistosome eggs, the worms do not contribute to pathogenesis.
How are the Schistosome eggs packaged to traverse the gut or bladder wall?
They are contained within a granuloma
