Hymenolepis nana
Also known as: dwarf tapeworm
Hymenolepis nana, the dwarf tapeworm, is the smallest and most common tapeworm infecting humans. It spreads by fecal-oral ingestion of eggs, can complete its entire life cycle in one host, and is treated with praziquantel.
Hymenolepis nana is a cestode (tapeworm) and, at only a few centimeters long, the smallest tapeworm that routinely infects humans — hence the common name dwarf tapeworm. It is also the most prevalent human tapeworm worldwide, with infection concentrated in children, institutional settings, and regions with limited sanitation.
Its distinguishing biological feature is that it needs no obligatory intermediate host. Eggs ingested through the fecal-oral route hatch in the small intestine, where larvae develop inside intestinal villi and then mature into adult worms in the lumen. Because eggs are immediately infectious, autoinfection can occur — eggs released by adult worms hatch within the same host — allowing the worm burden to build without any new exposure. Insects such as grain beetles can serve as optional intermediate hosts.
Most infections are asymptomatic. Heavier worm burdens can cause abdominal pain, diarrhea, anorexia, and irritability, and eosinophilia may be present. Diagnosis rests on finding the characteristic eggs — oval, with polar filaments between two membranes — in a stool ova-and-parasites examination. Treatment is praziquantel, dosed to kill both adult worms and developing larvae, with hygiene measures to prevent reinfection.
On USMLE Step 1, Hymenolepis nana appears in parasitology as the tapeworm with a single-host life cycle and fecal-oral spread — a contrast with Taenia species (undercooked meat) and Diphyllobothrium (raw fish) — with praziquantel as the answer for treatment.
Key takeaways
- Hymenolepis nana is the dwarf tapeworm, the smallest and most common human tapeworm.
- It completes its entire life cycle in a single host, and immediately infectious eggs allow autoinfection.
- Transmission is fecal-oral; children and institutional settings carry the highest burden.
- Diagnosis is by eggs in stool examination; treatment is praziquantel.
- USMLE Step 1 tests it as the single-host, fecal-oral tapeworm, contrasted with meat- and fish-borne cestodes.