Hymenolepis nana (Dwarf Tapeworm)

Taxonomic note: Recent molecular data have led to reclassification of this organism to the new name Rodentolepis nana (previously Hymenolepis nana), though the old name remains widely used in clinical practice. — Harrison's Principles of Internal Medicine 22e

Classification

• Type: Cestode (tapeworm), class Eucestoda
• Common name: Dwarf tapeworm
• Most common: The most common human cestode infection worldwide

Morphology

Life Cycle

Fig. 76.12 Life cycle of H. nana — Medical Microbiology 9e

1. No obligate intermediate host — the only human cestode with this property
2. Embryonated eggs are ingested → oncosphere released → penetrates intestinal villi → develops into cysticercoid larva in the epithelium
3. Cysticercoid re-enters lumen → scolex attaches to small intestinal mucosa → matures into adult tapeworm in 10–12 days
4. Adult produces egg-laden gravid proglottids → eggs passed in feces → immediately infective
5. Autoinfection: Eggs can hatch within the intestinal lumen and re-develop into cysticercoids without leaving the host → perpetuates and amplifies infection
6. Optional intermediate hosts: mice, beetles (grain/flour beetles); humans can ingest these

Epidemiology

• Worldwide distribution (temperate and tropical regions); endemic in developing countries
• Estimated 50–75 million people infected globally
• Most common tapeworm in North America
• Primarily fecal-oral transmission — person-to-person spread is possible (unique among tapeworms)
• Children are especially at risk; day-care centers and institutions are settings for outbreaks
• Also a common parasite of mice and rodents

Egg Morphology (Diagnostic Stage)

Fig. 76.13 H. nana eggs (30–45 μm diameter, thin shell, six-hooked embryo with polar filaments) — Medical Microbiology 9e

Clinical Features

• Heavy infections (hundreds to thousands of worms) occur especially in children via autoinfection/hyperinfection
• Adult worm lifespan: ~4–10 weeks, but perpetuated by autoinfection cycles

Diagnosis

• Stool microscopy — identification of characteristic eggs in wet mount preparations of concentrated stool
  • Eggs: 30–45 μm, thin-shelled, contain oncosphere with 6 hooklets and polar filaments (distinguishes from H. diminuta)
• More than one stool specimen may be required
• Culture, serology, antigen detection, and nucleic acid tests are not used in routine diagnosis

Treatment

Prevention & Control

• Improved sanitation and personal hygiene (hand washing)
• Control of rodent reservoirs
• Mass chemotherapy has been used to control institutional/community outbreaks
• Treat cases promptly to interrupt autoinfection cycles

Comparison with H. diminuta