Visual Atlas / Parasitology

Clonorchis sinensis / Opisthorchis spp.: Operculated Biliary Fluke Egg

Bench-card visuals for recognizing Clonorchis sinensis and Opisthorchis spp. eggs: the smallest common operculated helminth egg (~28-35 um), flask or vase shape, prominent shoulder rim at the opercular end, tiny knob at the abopercular end, and bile-staining.

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Bench card visual

Clonorchis / Opisthorchis egg: flask-shaped, opercular shoulder, abopercular knob

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Clonorchis sinensis (Chinese liver fluke) and Opisthorchis spp. eggs are morphologically identical. ~28-35 um - the smallest common operculated egg (smaller than Diphyllobothrium ~65 um). Flask/vase shape. Prominent shoulder rim (...

Clonorchis/Opisthorchis egg~30 um flask-shaped, shoulder rim, abopercular knob

28-35 um. Flask or vase shape (wider at opercular end, tapered toward the abopercular end). Prominent shoulder rim thickening at the opercular end - the operculum fits into this raised rim. Tiny knob or protuberance at the narrow (abopercular) end. Bile-stained golden-brown. Fully embryonated miracidium visible inside some preparations. Morphologically identical for C. sinensis, O. viverrini, and O. felineus - species differentiation requires geographic/exposure history or PCR.

Size and context comparatorSmallest operculated egg: ~30 um vs. Diphyllobothrium ~65 um

Clonorchis egg at ~30 um is the smallest of the four common operculated eggs. Diphyllobothrium latum (~65 um) is more than twice the size and has a more prominent abopercular knob. Heterophyes heterophyes and Metagonimus yokogawai eggs (~30 um) mimic Clonorchis in size but lack the shoulder rim and have a less prominent knob - differentiation requires careful morphology at oil immersion. Clinical context: East/Southeast Asian fish consumption; chronic infection is a risk factor for cholangiocarcinoma (especially O. viverrini).

Clonorchis/Opisthorchis egg~30 um flask-shaped, shoulder rim, abopercular knob

28-35 um. Flask or vase shape (wider at opercular end, tapered toward the abopercular end). Prominent shoulder rim thickening at the opercular end - the operculum fi...

Size and context comparatorSmallest operculated egg: ~30 um vs. Diphyllobothrium ~65 um

Clonorchis egg at ~30 um is the smallest of the four common operculated eggs. Diphyllobothrium latum (~65 um) is more than twice the size and has a more prominent ab...

What to look for

Clonorchis/Opisthorchis egg recognition guide

Visual clue	Meaning	Student shorthand
Flask/vase shape, ~28-35 um, prominent opercular shoulder rim	Clonorchis sinensis or Opisthorchis spp.	Shoulder rim distinguishes from Heterophyes/Metagonimus which lack this thickening
Tiny knob at narrow (abopercular) end	Clonorchis/Opisthorchis - supportive feature	Knob is small but visible at oil immersion; Diphyllobothrium knob is larger and more prominent
Bile-stained golden-brown, fully embryonated	Clonorchis/Opisthorchis - eggs shed embryonated	Miracidium visible inside well-preserved eggs; contrast with Diphyllobothrium (undeveloped embryo)
~28-35 um - smallest common operculated egg	Size anchors the identification	Measure with calibrated micrometer; size overlaps with Heterophyes at lower end; shoulder rim is the differentiator
East/Southeast Asia exposure + raw freshwater fish history	Epidemiology supports Clonorchis/Opisthorchis	Endemic in China, Korea, Vietnam, Thailand; raw or undercooked cyprinid fish; cholangiocarcinoma risk with long-term infection
Morphologically identical to O. viverrini, O. felineus	Species cannot be distinguished by morphology	Geographic exposure + PCR or serology required for species-level differentiation; C. sinensis (East Asia) vs. O. viverrini (Southeast Asia)

Common trap

Heterophyes and Metagonimus eggs mimic Clonorchis - opercular shoulder is the key differentiator

Heterophyes heterophyes and Metagonimus yokogawai eggs overlap with Clonorchis in size (~28-30 um). The key differentiator is the opercular shoulder: Clonorchis/Opisthorchis have a prominent thickened shoulder rim that is absent or much less defined in Heterophyes/Metagonimus. Careful oil-immersion examination is required when the eggs are on the small end of the Clonorchis range.

Chronic Clonorchis/Opisthorchis infection (especially O. viverrini in Southeast Asia) is a Group 1 IARC carcinogen and a major risk factor for cholangiocarcinoma.
Praziquantel is highly effective and is the treatment of choice for all liver fluke infections.
Stool O&P is the primary diagnostic method; serologic tests (ELISA) are available for population surveys but have cross-reactivity with other trematodes.

Interpretation table

Clonorchis/Opisthorchis identification and reporting guide

Pattern	What to call it	How to think about it
Flask-shaped ~30 um egg, opercular shoulder rim, tiny abopercular knob	Clonorchis sinensis / Opisthorchis spp. - positive	Report as liver fluke egg consistent with Clonorchis/Opisthorchis; praziquantel treatment; liver imaging for cholangiopathy
Similar-sized egg ~30 um, no prominent shoulder rim	Heterophyes/Metagonimus - consider alternative	Shoulder rim absence or minimal = Heterophyes or Metagonimus; report with morphologic notes; geographic context
Equivocal morphology at size limit	Confirm with repeat prep, PCR, or serology	If species differentiation is clinically important, request PCR or reference lab confirmation; clinical context guides

Key takeaways

Smallest common operculated egg ~28-35 um: flask shape + opercular shoulder rim + tiny abopercular knob = Clonorchis/Opisthorchis.
Shoulder rim at opercular end is the key differentiator from Heterophyes/Metagonimus (which lack this rim at the same size).
Chronic infection (especially O. viverrini) = major cholangiocarcinoma risk; always note exposure history when reporting.

Remember

Clonorchis = smallest operculated egg (~30 um). Flask shape. Shoulder rim at opercular end. Tiny abopercular knob. Bile-stained. Cholangiocarcinoma risk.