Hymenolpsis nana

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Hymenolepis nana (Dwarf Tapeworm)

Nomenclature note: Recent molecular data have reclassified this organism to Rodentolepis nana (previously Hymenolepis nana) — Harrison's Principles of Internal Medicine 22E uses this new name, though H. nana remains widely used in clinical literature.

Classification

Class: Cestoda (tapeworm)
Common name: Dwarf tapeworm
Unique distinction: The only human cestode that does not require an intermediate host to complete its life cycle

Morphology

Feature	Detail
Adult length	2–4 cm (some sources up to 40 mm) — "dwarf" compared to Taenia (several metres)
Scolex	4 muscular suckers + crown of hooklets (rostellum)
Eggs	Oval, 30–47 µm, thin smooth outer shell; inner membrane with 4–8 polar filaments extending from two poles; contain a 6-hooked oncosphere (hexacanth embryo)

Life Cycle

Life cycle of H. nana (dwarf tapeworm) — Medical Microbiology 9e

Three routes of infection:

Direct (fecal-oral, no intermediate host) — Embryonated eggs ingested → oncosphere released → penetrates intestinal villi → develops into cysticercoid larva → re-enters intestinal lumen → scolex attaches to small intestinal mucosa → matures into adult in 10–12 days → gravid proglottids disintegrate in situ, releasing eggs into feces
Via arthropod intermediate host — Accidental ingestion of infected beetles (contaminating grain/flour) containing cysticercoids
Autoinfection (internal) — Eggs hatch within the intestinal lumen without leaving the host → new cysticercoids form → leads to hyperinfection with very heavy worm burdens
- Adult worm lifespan: ~4–10 weeks; infection perpetuated by repeated autoinfection cycles

Reservoir hosts: Mice (also beetles as intermediate hosts)

Epidemiology

Most prevalent tapeworm infection worldwide: estimated 50–75 million cases globally
Distribution: worldwide, including temperate and tropical regions
Most common tapeworm infection in North America
Highest risk: children, institutionalized patients, immunocompromised hosts
Transmission: primarily person-to-person fecal-oral route; also via contaminated grain/flour containing infected beetles
Capable of person-to-person spread (unlike most cestodes)

Clinical Features

Worm burden	Symptoms
Light infection	Asymptomatic (most common)
Heavy infection / hyperinfection	Diarrhea, abdominal pain, anorexia, headache, weight loss, malabsorption

Heavy infection is particularly seen in children harboring hundreds or thousands of worms, and in immunocompromised patients where autoinfection is less controlled.

Diagnosis

Stool microscopy: Detection of characteristic eggs in wet mount of concentrated stool — oval, 30–47 µm, with polar filaments and a 6-hooked oncosphere inside
More than one specimen may be required
Culture, serology, antigen detection, and PCR are not routinely used
Adult worms occasionally found during colonoscopy

Treatment

Drug	Dose	Notes
Praziquantel (drug of choice)	25 mg/kg once	Active against both adult worms and cysticercoids in the intestinal wall
Nitazoxanide (alternative)	500 mg twice daily × 3 days	Alternative when praziquantel unavailable
Niclosamide	Standard dose	Alternative (older agent)

Prevention & Control

Hand hygiene (especially in households, schools, day-care centers)
Improved sanitation
Mass chemotherapy in epidemic settings
Proper food storage to prevent beetle contamination of grain/flour

Sources: Medical Microbiology 9e | Harrison's Principles of Internal Medicine 22E | Goldman-Cecil Medicine | Tietz Textbook of Laboratory Medicine 7e

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