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Parasitology > Schistosomes - Exam Questions > Flashcards

Schistosomes - Exam Questions Flashcards

1
Q

What are the main characteristics by which schistosomes differ from other trematodes?

A
  • Blood parasites –> feed on RBCs
  • Dioecious –> female + male in separate
    forms
  • Cercariae penetrate skin of definitive host–>
    no metacercaria
  • Nonoperculate –> embryonated egg that
    hatches immediately after contacting
    water
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2
Q

Why is it so difficult to avoid getting infected with schistosomes, compared to other species of trematodes?

A

The cercariae are free-swimming in water + penetrate skin when human is in infected water

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3
Q

Why has this infection been so difficult to control?

A
  • Increased population –> need for food and
    electricity increases –> more eggs
  • Increased irrigation –> more snail habitat +
    more water contact
  • Increased dams –> more snail habitat + more
    water contact
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4
Q

What is the prevalence of the disease schistosomiasis?

A

240 million

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5
Q

What are the 3 species infecting humans, and what parts of the world does each occur?

A
  • S. mansoni
  • S. japonicum
  • S. haematobium
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6
Q

Which species overlap in distribution?

A

S. mansoni & S. japonicum

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7
Q

Where does each S. mansoni live in the definitive host?

A

inferior mesenteric veins (drain to liver via hepatic portal vein)

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8
Q

Where does each S. japonicum live in the definitive host?

A

superior mesenteric veins

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9
Q

Where does each S. haematobium live in the definitive host?

A

urinary tract

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10
Q

How do female S. mansoni get their eggs from the bloodstream to the outside world?

A

in feces

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11
Q

What is the role of the host’s immune system in this process? How efficient is this process in S. mansoni?

A
  • Paired adult worms migrate to mesenteric
    venules of bowel/rectum + lay eggs that
    circulate to the liver + shed in stools
  • Process is inefficient –> 50% of eggs swept
    into liver in hepatic portal vein
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12
Q
  1. Approximately how many eggs/female/day are oviposited in S. mansoni?
A

200 eggs/day/female

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13
Q

What is the main morphological identifying feature of the egg of S. mansoni?

A

large lateral spine

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14
Q

What is the intermediate host for S. mansoni?

A

Biomphalaria sp. (snail) –> aquatic

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15
Q

What are the intramolluscan stages of S. mansoni?

A
  • Miracidia penetrate snail tissue
  • Sporocysts in snail
  • Cercariae released by snail into water
    - free-swimming
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16
Q

What are the 2 types of penetration gland found in schistosome cercariae, and what are the secretory products of each?

A
  • Daughter sporocyst:
    - 2 pr preacetabular glands (cells) –>
    contain enzymes
    - 3 pr postacetabular glands (cells) –>
    contain mucus + enzymes
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17
Q

How long do cercariae live after leaving the snail? How do they locate the definitive host?

A
  • after leaving snail –> 1-3 days to find a host
  • locate the definitive host (humans) –> they’re attracted to warmth + skin lipids
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18
Q

What is the sequence of events by which the cercariae enter the body, and how long does this process take?

A
  • Cercaria attaches, drops tail, secretes gland
    contents
  • Penetrates epidermis into dermis (30
    minutes)
  • Enters blood vessel in dermis (by 24 hours)
  • Carried by blood to liver, mate, and mature
19
Q

What must happen in the liver in order for schistosomes to mature properly?

A
  • Granulomas transport eggs to lumen
  • Granuloma –> eggs secrete soluble egg
    antigen
20
Q

What is the prepatent period (period before eggs appear the feces) for S. mansoni?

A

Prepatent period –> adults migrate to mesenteric veins, produce eggs by 5-8 weeks post-infection (PI)

21
Q

What is the main identifying feature of the egg of S. japonicum?

A

smaller eggs with small lateral spine

22
Q

What is the intermediate host of S. japonicum, and what characteristics of this host make it difficult to eradicate?

A
  • intermediate host –> Oncomelania hupensis
    (snail)
    - amphibious, not aquatic
    - molluscicide treatment is difficult
23
Q

Why are infections with S. japonicum more pathogenic than those with S. mansoni?

A

10x higher egg production –> 2,000 eggs/day/female

24
Q

What are the comparative roles of reservoir hosts among the 3 species of human-infecting schistosomes?

A
  • S. mansoni –> no reservoir hosts
  • S. japonicum –> many reservoir hosts
    (zoonosis)
  • S. haematobium –> no important reservoir
    hosts
25
What are the snails associated with 3 species of human-infecting schistosomes?
- S. mansoni --> Biomphalaria sp. (aquatic) - S. japonicum --> Oncomelania hupensis (amphibious) - S. haematobium --> Bulinus truncates (aquatic)
26
What is acute schistosomiasis called, and when does it occur during the course of infection?
- Acute schistosomiasis = Katayama fever - When it occurs during infection --> 4-6 weeks post infection - Immune response to eggs
27
What is the underlying, fundamental cause of disease in chronic schistosomiasis?
due to eggs, not adults
28
Why is schistosomiasis considered a hypersensitivity disease?
Soluble egg antigen --> granuloma surrounding eggs --> fibrosis
29
What are the effects of eggs of S. mansoni and S. japonicum on the liver? What health consequences arise as a result of these effects?
Enlarged liver + granulomas --> scar tissue around eggs obstructs hepatic blood flow ( about 25% of cardiac output)
30
What is the major cause of death in severe schistosomiasis?
Blockage of blood flow thru liver --> 8% of cases
31
What problems arise when eggs enter the general circulation, and what pathology occurs in the digestive tract?
- Eggs enter general circulation --> CNS lesions + blockage of lung capillaries - Pathology that occurs in digestive tract --> gastrointestinal hemorrhage + fibrosis
32
What is unusual about the location of the adult worms of S. haematobium in comparison with S. japonicum/S. mansoni?
Venous plexuses surrounding urinary bladder
33
What is the main identifying feature of the egg of S. haematobium?
terminal spine
34
By what route do the eggs of S. haematobium reach the outside world?
thru urine
35
What is the intermediate host of S. haematobium?
Bulinus truncates (aquatic)
36
What organs are most affected by S. haematobium? What are some of the pathological effects?
- Organs most affected --> ureters + kidneys + bladder - pathological effects --> granulomas around eggs in wall of bladder, ureters --> damage to epithelium
37
What are the main epidemiological factors that contribute to successful maintenance of the schistosome life cycle in nature?
- Increased population --> need for food + electricity increases --> more eggs - Increased irrigation --> more snail habitats + more water contact - Increased dams --> more snail habitats + more water contact
38
What are the main mechanisms of exposure for humans to schistosomes?
- Contamination of snail habitats - Use of night soil as a fertilizer - No sewage treatment - Animal feces - Human water contact - Occupational - Domestic - recreational
39
What are some of the main control strategies employed against schistosomiasis, and what are some of the shortcomings of each?
- Snail control - Expensive - Recolonization - Environmental effects - Difficult for O. hupensis - Chemotherapy --> praziquantel - Immediate reinfection - Drug resistance - Public health education - Easier said than done
40
Given that adult schistosomes live in the blood, where they are exposed to the full range of host immune mechanisms, how do they avoid elimination by the host immune system?
Avoid elimination by host immune system --> IgE/complement-mediated eosinophil killing
41
What are the protective and pathological effects of the immune response during schistosomiasis?
hypersensitivity --> blocking inflammatory response with steroids causes expensive death of hepatocytes from egg secretions
42
Does a person ever become fully immune to schistosomes?
no
43
In what way is the schistosome dependent on the host’s immune response?
- Reliance on CD4 immunity for worm development - Granulomas escort eggs across intestine + bladder wall
44
What is schistosome dermatitis, where does it occur, what is the responsible parasite, and what medical/economic effect does it have?
- Schistosome dermatitis --> cercariae penetrate + die in skin causing inflammation - Where --> worldwide in freshwater + saltwater - Parasite responsible --> avian schistosomes

Parasitology (9 decks)

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