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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
Q

What are the snails associated with 3 species of human-infecting schistosomes?

A
  • S. mansoni –> Biomphalaria sp. (aquatic)
  • S. japonicum –> Oncomelania hupensis
    (amphibious)
  • S. haematobium –> Bulinus truncates
    (aquatic)
26
Q

What is acute schistosomiasis called, and when does it occur during the course of infection?

A
  • Acute schistosomiasis = Katayama fever
  • When it occurs during infection –> 4-6 weeks
    post infection
    - Immune response to eggs
27
Q

What is the underlying, fundamental cause of disease in chronic schistosomiasis?

A

due to eggs, not adults

28
Q

Why is schistosomiasis considered a hypersensitivity disease?

A

Soluble egg antigen –> granuloma surrounding eggs –> fibrosis

29
Q

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?

A

Enlarged liver + granulomas –> scar tissue around eggs obstructs hepatic blood flow ( about 25% of cardiac output)

30
Q

What is the major cause of death in severe schistosomiasis?

A

Blockage of blood flow thru liver –> 8% of cases

31
Q

What problems arise when eggs enter the general circulation, and what pathology occurs in the digestive tract?

A
  • Eggs enter general circulation –> CNS lesions
    + blockage of lung capillaries
  • Pathology that occurs in digestive tract –>
    gastrointestinal hemorrhage + fibrosis
32
Q

What is unusual about the location of the adult worms of S. haematobium in comparison with S. japonicum/S. mansoni?

A

Venous plexuses surrounding urinary bladder

33
Q

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

A

terminal spine

34
Q

By what route do the eggs of S. haematobium reach the outside world?

A

thru urine

35
Q

What is the intermediate host of S. haematobium?

A

Bulinus truncates (aquatic)

36
Q

What organs are most affected by S. haematobium? What are some of the pathological effects?

A
  • Organs most affected –> ureters + kidneys +
    bladder
  • pathological effects –> granulomas around
    eggs in wall of bladder, ureters –> damage
    to epithelium
37
Q

What are the main epidemiological factors that contribute to successful maintenance of the schistosome life cycle in nature?

A
  • 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
Q

What are the main mechanisms of exposure for humans to schistosomes?

A
  • Contamination of snail habitats
    - Use of night soil as a fertilizer
    - No sewage treatment
    - Animal feces
  • Human water contact
    - Occupational
    - Domestic
    - recreational
39
Q

What are some of the main control strategies employed against schistosomiasis, and what are some of the shortcomings of each?

A
  • Snail control
    - Expensive
    - Recolonization
    - Environmental effects
    - Difficult for O. hupensis
  • Chemotherapy –> praziquantel
    - Immediate reinfection
    - Drug resistance
  • Public health education
    - Easier said than done
40
Q

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?

A

Avoid elimination by host immune system –> IgE/complement-mediated eosinophil killing

41
Q

What are the protective and pathological effects of the immune response during schistosomiasis?

A

hypersensitivity –> blocking inflammatory response with steroids causes expensive death of hepatocytes from egg secretions

42
Q

Does a person ever become fully immune to schistosomes?

A

no

43
Q

In what way is the schistosome dependent on the host’s immune response?

A
  • Reliance on CD4 immunity for worm development
  • Granulomas escort eggs across intestine + bladder wall
44
Q

What is schistosome dermatitis, where does it occur, what is the responsible parasite, and what medical/economic effect does it have?

A
  • 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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