Parasites 4: Schistosomiasis

Question 1

What are the different types of schistosoma species and where are they found?

Answer 1

The three main species infecting humans are

• Schistosoma haematobium (urogenital schistosomiasis),
• Schistosoma mansoni (intestinal schisto)
• Schistosoma japonicum (intestinal schisto)

Schistosoma mekongi and Schistosoma intercalatum have a more limited distribution

Humans are infected while wading/bathing in fresh water infested with schistosome larvae.

Children aged 5 to 15 years are particularly at risk: prevalence and parasite load are highest in this age group.

Lifespan of schistosoma up to 30 years.

A S. mansoni egg is 50 x 150 µM and can be found in faecal smears using the Kato-Katz technique.

Question 2

What is the life-cycle of schistosoma?

Answer 2

Schistosoma eggs are eliminated with feces or urine, depending on species . Under appropriate conditions the eggs hatch and release miracidia , which swim and penetrate specific snail intermediate hosts . The stages in the snail include two generations of sporocysts and the production of cercariae . Upon release from the snail, the infective cercariae swim, penetrate the skin of the human host , and shed their forked tails, becoming schistosomulae . The schistosomulae migrate via venous circulation to lungs, then to the heart, and then develop in the liver, exiting the liver via the portal vein system when mature, . Male and female adult worms copulate and reside in the mesenteric venules, the location of which varies by species (with some exceptions) . For instance, S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine , and S. mansoni occurs more often in the inferior mesenteric veins draining the large intestine . However, both species can occupy either location and are capable of moving between sites. S. intercalatum and S. guineensis also inhabit the inferior mesenteric plexus but lower in the bowel than S. mansoni. S. haematobium most often inhabitsin the vesicular and pelvic venous plexus of the bladder , but it can also be found in the rectal venules. The females (size ranges from 7–28 mm, depending on species) deposit eggs in the small venules of the portal and perivesical systems. The eggs are moved progressively toward the lumen of the intestine (S. mansoni,S. japonicum, S. mekongi, S. intercalatum/guineensis) and of the bladder and ureters (S. haematobium), and are eliminated with feces or urine, respectively .

• Requirements for transmission:
  
  • Contamination of waer with viable eggs from a reservoir host
  • Presence in the water of susceptible snail intermediate gosts
  • Suitable environmental conditions for development in the snail
  • Human exposure to water containing cercariae

Question 3

How does the look of the different schistosoma eggs differ?

Answer 3

Question 4

How do the male and female schistosoma japonicum compare?

Answer 4

Question 5

How do the male and female schistsoma mansoni compare?

Answer 5

Question 6

How do the male and female schistosoma haematobium compare?

Answer 6

Question 7

What is the pathophysiology and what are the clinical features of schistosomiasis?

Answer 7

• Why do the eggs in schistosomiasis cause granulomas?
  
  • Tissue reaction to retained eggs -> circumoval granuloma formed
  • Granuloma formed of
    
    • epithelioid and giant cells
    • lymphocytes and eosinophils in concentric fashio
  • Precipitation of Ag-Ab complex on the egg surface helps activate inflammation
  • Response to a single egg lasts a few weeks
  • Eggs become deposited in clutches
• If bladder inflammation due to s. haemotbium is very persistent and prolonged over many years, it is associated with development of squamous cell bladder carcinomas
• Hepato- and splenomegaly are characteristically present in schistosomiasis

Question 8

How do we diagnose schistosomiasis?

Answer 8

Question 9

How do we treat schistosomiasis?

Answer 9

Praziquantel PO 40mg/kg single dose -> may need repeat after 3mo to be safe (in order to clear worms that were only maturing during initial illness if treatment commenced in acute schisto.)

Question 10

How do we prevent the spread of schistosomiasis?

Answer 10

• Mass tretment of whole population tio interrup transmission
• Snail control (chemical, biological, environmental)
• Improve sanitation
• Health education

Question 11

What is katayama fever?

Answer 11

A reaction to egg protein that starts when the paired flukes start to produce eggs