Parasites III

Question 1

What is the difference between a definitive host and an intermediate host?

Answer 1

definitive host- sexual reproduction of the parasite happens here
intermediate host- larval development, but NOT sexual reproduction, occurs here

Question 2

What determines the parasite burden in helminth infection, in MOST cases?

Answer 2

the degree of exposure to the infectious form of the parasite..

Note: eggs/larvae may be produced independent of exposure, but usually they don’t mature into an adult parasite in the human- so a person with multiple adult worms usually has had multiple exposures

Question 3

What are the three most important general ways to prevent and eliminate helminth infections?

Answer 3

1. provide for sanitary disposal of human waste
2. insure a clean water supply
3. cook food properly
   FECAL MATTER is a problem

Question 4

What are the three kinds of helminths?

Answer 4

trematodes, aka flatworms/flukes
cestodes, aka tapeworms
nematodes, aka roundworms

Question 5

What are the four types of trematodes? What is their intermediate host? Generally, how do each of these types enter the body?

Answer 5

blood, liver, intestinal, and lung.
liver, lung, and intestinal acquired from undercooked food.
blood trematodes (schsitosoma) invade the body directly from skin contact with contaminated fresh water.
host: snail

Question 6

Clonorchis sinensis: helminth type, organ infected, and source of infection

Answer 6

trematode
liver
fish

Question 7

Fasciola hepatica: helminth type, organ infected, source of infection

Answer 7

termatode
intestinal
uncooked aquatic vegetation

Question 8

paragonimus westermani: helminth type, organ infected, source of infection

Answer 8

trematode
lung
crab

Question 9

What is one important functional and morphological feature of trematodes?

Answer 9

spikes that help them inch through the body

Question 10

Eosinophilia

What might cause this? What are some associated risks?

Answer 10

hallmark of helminth infection
reflects a hypersensitization to antigens released by larvae and eggs
can result in by-stander damage to cardiac tissue, esp. endocardium

Question 11

What are the three main species of schistosoma that cause human disease?

Answer 11

s. mansoni, S. hematobium, S. japonicum

Question 12

Where do we see schistosoma eggs excreted from the body, and why (multiple answers possible)?

Answer 12

schistosoma mansoi and schistosoma japonicum eggs excreted in the feces because these organisms like to live in the mesenteric venules of the portal venous system, near the large intestine
schistosoma hematobium excreated in the urine because this ogranism likes to live in the pelvic venous plexi.

Question 13

What is a disease that has been correlated with S. hematobium infection?

Answer 13

schistosoma hematobium infection has been correlated with ahigh incidence of bladder cancer, perhaps due to the inflammatory response.

Question 14

Why is schistosomiasis infection rare in the US?

Answer 14

it requires a snail vector, which can’t survive in the “cold” US water temperatures

Question 15

Describe the schistosomiasis lifecycle? (S. mansoni in particular)

Answer 15

1. Carcariae form of the parasite penetrates the skin directly
2. it migrates to the liver
3. Adults mate in the liver
4. Adults migrate to the mesenterics
5. Eggs are passed to the small intestine, large intestine, and then are excreted.
6. Eggs hatch in the water
7. hatched eggs enter the snail
8. infectious carcariae exit the snail

Question 16

How is schistosomiasis diagnosed?

Answer 16

presence of eggs in excrement, or in host tissues

Question 17

What is the interaction between schistosomiasis eggs and the immune system?

Answer 17

Eggs that get trapped in host tissue die. But, even dead eggs elicit a granulomatous immune response caused by hypersensitivity to egg/larval antigens. Leads to a fibrotic, eosiniphilic granuloma (100X the size of the egg itself!)
As egg burden increases, we do see a graudal desensitization to parasite antigens due to parasiite-induced modulation of cytokines that modulate the immune response.
parasite survival enhanced by reducing damage to the host: there is a dynamic balance between the parasite and the host.

Question 18

What is the infectious form of schistosomiasis?

Answer 18

cercaria/ cercarial larvae

Question 19

What is unique about schistosomiasis mansoni reproduction?

Answer 19

there are male and female worm forms.

all other schistosomiasis worms (S. japonicum and S. mansoni) are hermaphroditic

Question 20

Describe the three phases of schistosomiasis infection.

Answer 20

1. acute phase: rash, fever, abdominal pain
2. intermediate: immune complexes form- serum-sickness like disease with splenomegaly, diarrhea, and lympadenopathy
3. chronic: liver and spleen involvement, abdominal swelling due to ascites fluid accumulation (ascities fluid = in the peritoneal cavity). Damage to body mediated by immune response to egg/larval antigens. granuloma formation leads to inc. portal blood pressure leads to incr. portal varicies leads to bleeding and death (major cause of schistosomiasis death).

Question 21

What is concomitant immunity?

Answer 21

ongoing infection generates an immune response to adult, egg, and larval antigens. This immunity doesn’t help clear the ongoing infection but will destry a newledy arrived schistosomulum (infectious form- that’s what we call the ceracriae once its in the skin)

Question 22

For what three reasons do we see a decrease in the prevalence and intensity of schistosomiasis infection in people in the 20s (as opposed to kids)?

Answer 22

1. Spontaneous death of adult worms- they only live 3-5 yrs.
2. Slowly acquired immunity to super-infection mediated by increased numbers of eosinophils that can attack incoming infectious schistosomulums (schistosomula?)
3. behavior change- less water play

Question 23

What form schistosomiasis is susceptible to immune-mediated destruction?

Answer 23

schistosomulum

Question 24

How does the immune system attack the schistosomulum? What factors (cells and antibodies) are important?

Answer 24

1. Antibody dependent cell-mediated cytotoxicity is important: eosinophils + IgE abs is the most toxic. eosinophils + IgG or sensitized macrophages +IgE works too- but NOT IgM. IgM is produced against egg and larval antigens and are called blocking antibodies because they PREVENT the development of protective immunity (cross reactivity with schistosomulum antigens

Question 25

What toxins/systems are used to kill schistosomulums?

Answer 25

Complement, ROSs from eosinophils and macrophages, and toxic eosinophil granules: major basic protein, peroxidase, cationic peptides.

Question 26

Why isn’t the adult schistosomiasis worm susceptible to immune attack? Does it have any importance in immunity?

Answer 26

thick cuticle. but, abs against adult worm can react with schistosomulum surface

Question 27

Why must the immune system attack the schistosomulum quickly upon entry to the body?

Answer 27

otherwise, it can cause disease. also, it acquires host antigens on its surface within 5-7 days that allow it to mask its presence and pass as self.

Question 28

Why/how are IgMs called “blocking antibodies” in schistosomiasis infection?

Answer 28

IgM prevent binding of IgG and IgE to epitopes on schistosomulum surface. IgM is pentavalent- Fc portion inaccessible to immune effector cell receptors. IgM can’t participate in antibody dependent cell mediated cytotoxicity.

Question 29

How is concomitant immunity acquired as children age, given that IgM blocking abs exist? How might this be harnessed in the future?

Answer 29

At first, IgM predominates. As kids get older, IgE and IgG ab levels rise in comparison with IgM.
vaccine potential- you need to introduce antibodies that promote IgG and IgE immune responses rather than IgM responses.

Question 30

How do we treat schistosomiasis infection? Dosing info?

Answer 30

praziquantel- can be effective with a single dose. but, re-infection in endemic areas common, so multiple doses may be necessary. MOA unclear- potential interaction with parasitic use of Ca?

Question 31

What is swimmer’s itch? Where does it occur (geographically)? Reservoir?

Answer 31

occurs around the Great Lakes.
caused by ceracriae of schistosomes that infect water fowl and water mammals. in humans it is a cutaneous inflammation caused by death of cercariae in the skin; it is self-limiting.