Life Cycles of Medically Important Trematodes (Flukes)

## 1. Schistosoma spp. (S. mansoni, S. haematobium, S. japonicum) Unique: No metacercaria stage; cercariae directly penetrate human skin. ### Life Cycle: - Eggs in urine (S. haematobium) or stool (S. mansoni/japonicum). - Eggs hatch in freshwater, releasing miracidia. - Miracidium infects snail (intermediate host). - Inside snail: sporocysts produce cercariae. - Cercariae emerge from snail and penetrate human skin. - Transform to schistosomula, migrate to blood, lungs, and then liver. - Mature into adult worms in venous plexuses: - S. haematobium: bladder veins. - S. mansoni: mesenteric veins (colon). - S. japonicum: mesenteric veins (small intestine). - Eggs released cause inflammation and pathology. ## 2. Fasciola hepatica (Liver fluke) Infection from water plants (e.g., watercress); involves a metacercarial stage. ### Life Cycle: - Eggs passed in stool hatch in water to release miracidium. - Miracidium infects Lymnaea snail. - Inside snail: sporocyst → redia → cercaria. - Cercariae exit snail and encyst on aquatic vegetation to become metacercaria. - Humans ingest metacercaria on water plants. - Excyst in duodenum, penetrate gut wall, migrate through peritoneum to the liver. - Mature in bile ducts and lay eggs. ## 3. Fasciolopsis buski (Intestinal fluke) Similar to Fasciola but resides in the intestine; also acquired from aquatic vegetation. ### Life Cycle: - Eggs in stool hatch in water to release miracidium. - Miracidium infects snail (e.g., Segmentina species). - Sporocyst → redia → cercaria. - Cercariae encyst on water plants as metacercaria. - Human eats contaminated plants; metacercariae excyst in duodenum. - Adult worms attach to intestinal mucosa and release eggs. ## 4. Clonorchis sinensis (Chinese liver fluke) Infection via undercooked freshwater fish. ### Life Cycle: - Eggs in stool are ingested by snail (eggs do not hatch in water). - In snail: miracidium → sporocyst → redia → cercaria. - Cercariae exit snail and penetrate freshwater fish to become metacercaria. - Humans ingest infected fish. - Excyst in duodenum and migrate up bile ducts. - Mature into adults and lay eggs passed in feces. ## 5. Paragonimus westermani (Lung fluke) Infection from undercooked crab or crayfish. ### Life Cycle: - Eggs in sputum or swallowed and passed in stool. - Eggs reach water and hatch into miracidium. - Miracidium infects snail → sporocyst → redia → cercaria. - Cercariae infect crab or crayfish to become metacercaria. - Human eats raw/undercooked crustacean; metacercaria excysts in gut. - Penetrates intestinal wall, migrates through diaphragm to the lungs. - Adults form cysts in lungs and lay eggs.