Vl 13 Trematodes

Question 1

Describe the life cycle of Paragonimus westermani

Answer 1

1) infection by eating undercooked crustacean with parasite metacercariae
2) metacercariae excyst in the duodenum (zwölffingerdarm)
3) penetrate through intestinal wall, travel into the lungs, encapsulate and develop into adults - lay eggs (can also invade other tissues, but lifecycle can not be completed since eggs cant leave)
4) eggs excreted unembryonated in the sputum, or alternately swallowed and passed with stool
5) eggs embryonate in the external environment
6) miracidia hatch and invade the first intermediate host
(snail)
7) Miracidia go through several developmental stages inside the snail
8) cercariae emerge from snail⇒ invade second intermediate host (crustacean) encyst and become metacercariae⇒ infective stage for mammalian host

Question 2

How do Schistosoma eggs reach the intestine or the bladder?

Answer 2

• eggs are laid in blood vessels close to bladder or liver
• can not move actively⇒ adhere to endothelia, excrete proteins to induce TH2 response
• immune cells (macrophages and eosinophils) and antibodies gather around egg and excrete cell lysing enzymes
• blood epithel tissue is lyzed and eggs escorted to intestine or bladder

Question 3

Describe the aquatic part of the life cycle of Schistosoma spp.

Answer 3

eggs excreted via urine or feces⇒ hatch releasing miracidia⇒ swim and infect snail (intermediate host) ⇒ 2 different sporocyst stages in snail⇒ develop into cercariae⇒ released from snail, free swimming, can penetrate human skin

find host via lipids and fatty acids⇒ body temperature irrelevant

Question 4

Describe the temporal development of immune responses to a Schistosoma spp. infection.

Answer 4

• cercaria trigger TH1, but calmed down by parasite proteases (cleave antibodies and inhibit macrophages)
• adults don’t induce immune response ⇒ can dress up in host cell proteins (camouflage)
• adults can induce cancer in liver/bladder
• eggs proteins induce strong TH2 response to reach bladder/intestine, also to prevent other schistosomas from infecting the already infected host

Question 5

The pathogenesis of an infection by Schistosoma spp. is closely linked to the eggs. Explain!

Answer 5

1) egg proteins induce TH2 response, activating macrophages and eosinophiles to help them lyse blood vessel walls/tissues to get escorted to bladder and intestine ⇒ constantly irritating the organs/tissues
2) egg antigens lead to shift from TH1 to TH2 immune response, where IL-4 and IL-13 activate arginase in macrophages to deposit collagen and form granulomas ⇒ liver becomes enlarged