Diphyllobothrium latum: Operculated Fish Tapeworm Egg
Bench-card visuals for the fish tapeworm egg: Diphyllobothrium latum produces the largest egg of the common intestinal cestodes - broadly oval, operculated, yellow-brown, with a small knob at the abopercular end and an unembryonated interior.
D. latum egg: operculated, smooth-rimmed, yellow-brown, 58-76 um
D. latum: 58-76 x 40-50 um - broadly oval, yellow-brown, thin shell. Operculum at one pole (flat lid, no shoulder). Small knob (abopercular button) at the opposite pole. Unembryonated when passed. Found in stool. Acquired from raw...
58-76 x 40-50 um. Broadly oval. Yellow-brown thin shell. Operculum at one pole - a flat lid with a smooth rim (no shoulder or prominent ridge at the opercular junction - key difference from Paragonimus). Small abopercular knob (button-like) at the opposite pole. Unembryonated when passed. Found in stool. The largest common intestinal cestode egg. Acquired by ingestion of raw or undercooked freshwater fish. May cause megaloblastic anemia via vitamin B₁₂ competition.
Paragonimus westermani (80-120 um): larger, prominent shouldered opercular rim (the operculum sits in a groove with a widened shell rim - absent in D. latum). Fasciola hepatica (130-150 um): largest operculated egg, no opercular shoulder, no abopercular knob, very large. Clonorchis sinensis (28-34 um): much smaller, vase-shaped with prominent shoulder and knob. D. latum occupies the middle size range with a smooth rim and small knob - no other common trematode/cestode occupies this morphologic space.
58-76 x 40-50 um. Broadly oval. Yellow-brown thin shell. Operculum at one pole - a flat lid with a smooth rim (no shoulder or prominent ridge at the opercular juncti...
Paragonimus westermani (80-120 um): larger, prominent shouldered opercular rim (the operculum sits in a groove with a widened shell rim - absent in D. latum). Fascio...
D. latum vs. operculated trematode eggs
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Broadly oval, 58-76 um, smooth opercular rim, small abopercular knob | Diphyllobothrium latum (cestode) | The smooth rim (no shoulder) + small knob + intermediate size = D. latum; not a trematode |
| Shouldered opercular rim (groove at opercular junction) | Paragonimus westermani (trematode) | Paragonimus is larger (80-120 um) and has a prominent shouldered rim; D. latum rim is smooth |
| Very large operculated egg (130-150 um), no shoulder, no knob | Fasciola hepatica (trematode) | Fasciola is the largest common operculated egg; lacks both opercular shoulder and abopercular knob |
| Very small flask-shaped egg (28-34 um), prominent shoulders | Clonorchis sinensis (trematode) | Much smaller than D. latum; prominent opercular shoulders; abopercular knob present |
| Vitamin B₁₂ deficiency + operculated egg in stool | D. latum - fish tapeworm | D. latum competes for vitamin B₁₂ in the terminal ileum - megaloblastic anemia is a complication in heavy infection |
Diphyllobothrium latum identification guide
| Pattern | What to call it | How to think about it |
|---|---|---|
| Broadly oval 58-76 um, smooth opercular rim, abopercular knob | Diphyllobothrium latum - positive | Report; praziquantel; measure vitamin B₁₂; dietary history (raw freshwater fish); avoid raw fish going forward |
| Similar egg without knob or with shoulder | Reconsider - see comparator table | With shoulder = Paragonimus; much larger = Fasciola; much smaller = Clonorchis; measure and re-examine |
| Broad proglottid (wider than long) in stool | D. latum proglottid | Typical proglottid is 2-4 mm wide x 10-15 mm long; much wider than long - distinctive cestode segment morphology |
- D. latum: broadly oval 58-76 um, smooth opercular rim (no shoulder), small abopercular knob, yellow-brown - the largest common intestinal cestode egg.
- Smooth rim (no shoulder) distinguishes D. latum from Paragonimus; smaller size distinguishes it from Fasciola.
- Vitamin B₁₂ deficiency + megaloblastic anemia in a fish-eating patient = D. latum until proven otherwise.
D. latum: smooth rim + small knob + yellow-brown + 58-76 um. No shoulder (≠ Paragonimus). Freshwater fish source. May deplete vitamin B₁₂.