Visual Atlas / Parasitology

Hymenolepis nana and H. diminuta Eggs

Bench-card visuals for dwarf and rat tapeworm egg identification: H. nana (thin-shelled, 6 hooklets, polar filaments in the space between oncosphere and eggshell - diagnostic) versus H. diminuta (same size range but no polar filaments, larger eggs).

Next step

Reinforce this visual pattern with mixed clinical microbiology questions.

Practice related review questions

Bench card visual

Hymenolepis eggs: polar filaments (H. nana) vs. no polar filaments (H. diminuta)

Read first

H. nana: 30-47 um, round to oval, thin shell, inner oncosphere with 3 hooklet pairs, polar filaments in the space between the oncosphere and outer shell - pathognomonic. H. diminuta: 70-85 um, larger, round, thick shell, inner onc...

H. nanaRound 30-47 um, polar filaments - diagnostic

30-47 um. Round to broadly oval. Thin outer shell. Inner oncosphere with 3 hooklet pairs (6 hooklets). Polar filaments in the space between the oncosphere and the outer shell - 4-8 thread-like filaments running from each pole of the oncosphere - this is the pathognomonic feature that distinguishes H. nana from all other cestode eggs. Most common tapeworm in the US. Direct fecal-oral transmission - no intermediate host required. Common in children and institutions.

H. diminutaLarger 70-85 um, no polar filaments, yellowish

70-85 um. Larger than H. nana. Round to oval, yellow-brown outer shell. Inner oncosphere with 3 hooklet pairs. NO polar filaments between oncosphere and outer shell - clear space is empty. Transmitted via accidental ingestion of insect intermediate hosts (flour beetles, grain beetles) - rare human infection. Rats and mice are the natural definitive hosts. H. diminuta infection in humans is uncommon and typically mild.

H. nanaRound 30-47 um, polar filaments - diagnostic

30-47 um. Round to broadly oval. Thin outer shell. Inner oncosphere with 3 hooklet pairs (6 hooklets). Polar filaments in the space between the oncosphere and the ou...

H. diminutaLarger 70-85 um, no polar filaments, yellowish

70-85 um. Larger than H. nana. Round to oval, yellow-brown outer shell. Inner oncosphere with 3 hooklet pairs. NO polar filaments between oncosphere and outer shell...

What to look for

Hymenolepis egg differential

Visual clue	Meaning	Student shorthand
Small round egg 30-47 um, thin shell, polar filaments in clear space	Hymenolepis nana	Polar filaments (thread-like strands between oncosphere and outer shell) are pathognomonic for H. nana
Larger round egg 70-85 um, yellow-brown shell, no polar filaments	Hymenolepis diminuta	Clear space between oncosphere and outer shell is empty (no filaments); size distinguishes from H. nana
3 hooklet pairs (6 hooklets) inside oncosphere	Hymenolepis or other cestode egg feature	Hooklets in inner oncosphere are shared with Taenia eggs; size and filament presence distinguish
Taenia egg (30-45 um) vs. H. nana (30-47 um)	Overlap in size - use shell and filaments	Taenia egg has thick radially striated embryophore; H. nana has thin shell + polar filaments; both about the same size
Direct fecal-oral transmission, no intermediate host	H. nana biology	H. nana is unique among tapeworms - eggs are infectious when swallowed directly; autoinfection can occur; no insect needed

Common trap

H. nana polar filaments may be subtle - use good optics and concentration

The polar filaments of H. nana can be difficult to see on direct wet prep, especially in fixed specimens. Formalin-ethyl acetate concentration and iodine staining improve visualization. The filaments are thread-like structures running from the poles of the inner oncosphere to the outer shell - do not mistake the clear space for simply having "no filaments."

H. nana has no intermediate host - children can have large worm burdens from autoinfection; treat all household contacts in institutions.
H. diminuta requires an insect intermediate host (grain beetle) - human infections are rare and typically self-limited.
Niclosamide or praziquantel are both effective for H. nana; repeat stool exam 3-4 weeks post-treatment to confirm clearance.

Interpretation table

Hymenolepis species identification guide

Pattern	What to call it	How to think about it
Small round 30-47 um, thin shell, polar filaments	Hymenolepis nana - positive	Report; praziquantel; treat household contacts and institutional contacts; repeat stool exam post-treatment
Larger round 70-85 um, yellow-brown shell, no filaments	Hymenolepis diminuta - positive	Report; praziquantel; inquire about grain beetle exposure; typically mild infection
Round cestode egg, filaments or no filaments uncertain	Hymenolepis spp. - needs clarification	Measure carefully; use concentration and iodine stain to look for filaments; consult reference material; report with description

Key takeaways

H. nana: small 30-47 um, thin shell, polar filaments between oncosphere and outer shell - pathognomonic.
H. diminuta: larger 70-85 um, yellow-brown shell, NO polar filaments - clear space between oncosphere and shell is empty.
H. nana is the most common US tapeworm - direct fecal-oral transmission, no insect needed; H. diminuta requires a beetle intermediate host.

Remember

Polar filaments = H. nana (pathognomonic). No filaments + larger + yellow-brown = H. diminuta. Size + filaments = the two distinguishing features.