Parasitic Worms: Tapeworms, Flukes and Their Life Cycles
Tapeworms (Cestodes)
Cestode – Infective stage, Diagnostic stage, Key ID
Diphyllobothrium latum (Fish tapeworm)
Diphyllobothrium latum – fish tapeworm: LC, unembryonated eggs pass in feces; eggs embryonate in water, coracidia hatch and are eaten by crustaceans; in the body of the crustacean, a plerocercoid larva forms in fish; humans eat raw or undercooked fish; adults live in the small intestine.
Key ID: Operculated eggs; scolex has two groove-like bothria.
Taenia saginata (Beef tapeworm) and Taenia solium (Pork tapeworm)
Eggs pass in feces; cattle eat eggs; oncosphere enters cattle tissues; cysticercus develops in muscle; humans eat undercooked beef; adults live in the small intestine.
Key ID: Scolex has four suckers and no hooks (beef). Scolex has four suckers and an armed rostellum with hooks (pork).
Taenia pisiformis (Dog/rabbit tapeworm)
Eggs pass in dog feces; rabbit eats eggs; larvae migrate to the liver; cysticercus develops in tissue; dog eats infected rabbit tissue; adults live in the dog small intestine.
Key ID: Armed rostellum, four suckers.
Echinococcus granulosus (Hydatid tapeworm)
Adults live in the dog intestine; eggs pass in dog feces; sheep/humans swallow eggs; oncospheres migrate to organs; a hydatid cyst forms; dog eats infected organs; protoscolices develop into adult worms.
Key ID: Fluid-filled, single-chamber hydatid cyst.
Echinococcus multilocularis (Fox tapeworm)
Adults live in the fox/canid intestine; eggs pass in feces; rodents/humans swallow eggs; larvae develop in tissues, usually the liver; fox/canid eats infected rodent; adults develop in the intestine.
Key ID: Multilocular, invasive, tumor-like liver lesion.
Dipylidium caninum (Flea tapeworm)
Egg packets pass in proglottids; flea larva eats eggs; cysticercoid develops in the flea; dog/cat (or rarely human) swallows infected flea; adults live in the small intestine.
Key ID: Two lateral genital pores on each proglottid; eggs occur in packets.
Vampirolepis nana (Hymenolepis nana) (Dwarf tapeworm)
Life cycle (direct): eggs pass in feces; humans/rodents swallow eggs; oncospheres hatch and enter the intestinal villus; cysticercoid develops; returns to the intestinal lumen; adults develop in the small intestine; eggs pass in feces.
Indirect cycle: Insects can also act as intermediate hosts.
Key ID: Embryonated egg; eggs have polar filaments.
Hymenolepis diminuta (Rat tapeworm)
Life cycle: Eggs pass in rodent feces; beetles/insects eat eggs; cysticercoid develops in the insect; rodents or humans swallow the infected insect; adults live in the small intestine.
Key ID: Eggs are larger than H. nana eggs and lack polar filaments; an insect host is required.
Blood Flukes (Schistosomes)
Schistosoma mansoni (Blood fluke)
Eggs pass in feces; miracidia enter snails; sporocysts develop; cercariae leave snails; cercariae penetrate human skin; larvae travel through the blood to the liver; adults live in veins of the large intestine.
Schistosoma japonicum (Blood fluke)
Eggs pass in feces; miracidia enter snails; sporocysts develop; cercariae leave snails; cercariae penetrate human skin; larvae travel through the blood to the liver; adults live in veins of the small intestine.
Schistosoma haematobium (Blood fluke)
Eggs pass in urine; miracidia enter snails; sporocysts develop; cercariae leave snails; cercariae penetrate human skin; larvae travel through the blood to the liver; adults live in veins around the bladder.
Liver and Lung Flukes
Fasciola hepatica (Liver fluke)
Eggs pass in feces; eggs hatch in water; miracidia enter snails; sporocysts and rediae develop; cercariae leave snails; metacercariae encyst on aquatic plants; humans or animals eat plants; immature flukes migrate to the liver; adults live in bile ducts.
Fasciolopsis buski (Giant intestinal fluke)
Eggs pass in feces; eggs hatch in water; miracidia enter snails; sporocysts and rediae develop; cercariae leave snails; metacercariae encyst on aquatic plants; humans or pigs eat plants; adults live in the small intestine.
Paragonimus westermani / Paragonimus kellicotti (Lung fluke)
Eggs pass in sputum or feces; eggs hatch in water; miracidia enter snails; sporocysts and rediae develop; cercariae leave snails; cercariae enter crabs or crayfish; metacercariae develop in crustaceans; humans eat undercooked crustaceans; immature flukes migrate to the lungs; adults live in lung cysts.
Clonorchis sinensis (Liver fluke)
Eggs pass in feces; snails ingest eggs; sporocysts and rediae develop; cercariae leave snails; cercariae enter freshwater fish; metacercariae encyst in fish muscle; humans eat raw or undercooked fish; adults live in bile ducts.
Egg Identification Notes
Eggs: Vampirolepis nana (Hymenolepis nana) – clear, blob with two distinct circles; (pork) Taeniid egg – pink; Diphyllobothrium latum – bunch of blobs.
Schistosoma japonicum – blob.