Section 1: Digenetic Trematodes (Life Cycles)

Simple life cycle of Fasciola hepatica (tissue fluke)

1. unembryonated eggs passed in feces

• life cycle continues in water

2. eggs become embryonated in water

3. miracidia hatch from eggs

• look for snail (intermediate host)

4. Miracidia penetrate snail body (intermediate host)

• development in snail: miracidia → sporocysts → rediae → cercariae

5. free swimming cercariae encyst on aquatic vegetation

6. cyst (metacercariae) on vegetation ingested by definitive host

• definitive host: humans or ruminants

7. cysts travel to duodenum

• excyst (immature fluke)

• penetrate intestinal wall, migrate through liver to biliary ducts (liver)

• mature to adult, reproduce (selfing/outcross)

8. adult flukes in biliary ducts

9. unembryonated eggs passed in feces

Simple life cycle of Clonorchis sinensis (tissue fluke)

1. embryonated eggs leave in feces

2. eggs ingested by the snail (intermediate host)

• miracidia hatch from eggs in snail gust and penetrate gut

• miracidium develop into sporocysts

• sporocysts give rise to rediae

• rediae release cercariae

3. free-swimming cercariae encyst in flesk/skin of freshwater fish

4. metacercariae in flesh/skin of fresh water fish

5. infected fish ingested by definitive host (humans)

5. excyst in doudenum

• migrate to and enter biliary duct

6. adult in biliary duct

• reproduce and lay eggs

7. embryonated eggs leave in feces

Simple life cycle of Schistosoma mansoni/haematobium

1. eggs passed from body

• S. haematobium: eggs shed in urine

• S. mansoni: eggs shed in feces

2. eggs hatch and release miracidia

3. miracidia penetrate snails and develop into mother sporocysts

• daughter sporocysts develop within mother sporocysts

4. daughter sporocysts develop in snail to cercariae

5. cercariae released from snail into water

• free-swimming cercariae released from snail into water

6. cercariae penetrate skin

• lose tails

7. become schistosomulae

8. schistosomulae enter circulation (penetrate human skin)

9. carried to liver where they mature

10. worms pair in liver

• migrate to mesenteric veins around the intestine (shed in feces)

• migrate to venous plexus of bladder (shed in urine)

11. eggs passed from body

• S. haematobium: eggs shed in urine

• S. mansoni: eggs shed in feces

Simple life cycle of Fasciolopsis buski

1. unembryonated eggs passed in feces

2. eggs become embryonated in water

• miracidia hatch

3. miracidia penetrate snail

4. develop in snail

• sporocysts → rediae → cercariae

5. free-swimming cercariae released from snails

6. metacercariae encyst on water plant

• ingested (human or pig)

7. excyst in duodenum

8. adults in small intestine

9. unembryonated eggs passed in feces