Classification of Hymenolepis nana
- Phylum – Platyhelminthes
- Class – Cestoda
- Genus – Hymenolepis
- Species – H. nana
Hymenolepiasis is primarily caused by the cestode species, Hymenolepis nana (the dwarf tapeworm, adults measuring 15 to 40 mm in length). Embryonated eggs of Hymenolepis nana are the infective form. They are either directly ingested by the host through contaminated hands, food and water or inhalation or arthropods carrying these parasites (cysticercoid).
Habitat and geographical distribution of Hymenolepis nana
They reside in the small intestine of host. Hymenolepis infection or infection with the dwarf tapeworm is found worldwide. It is most often seen in warm climates and in children in countries in which sanitation and hygiene are inadequate.
Life cycle of Hymenolepis nana
Eggs of Hymenolepis nana are immediately infective when passed with the stool and cannot survive more than 10 days in the external environment. When eggs are ingested by an arthropod intermediate host (various species of beetles and fleas may serve as intermediate hosts), they develop into cysticercoids, which can infect humans or rodents upon ingestion and develop into adults in the small intestine. When eggs are ingested by human/rodents (in contaminated food or water or from hands contaminated with feces), the oncospheres contained in the eggs are released. The oncospheres (hexacanth larvae) penetrate the intestinal villus and develop into cysticercoid larvae. Upon rupture of the villus, the cysticercoids return to the intestinal lumen, evaginate their scolexes, attach to the intestinal mucosa and develop into adults that reside in the ileal portion of the small intestine producing gravid proglottids.
Eggs are passed in the stool when released from proglottids through its genital atrium or when proglottids disintegrate in the small intestine. An alternate mode of infection consists of internal autoinfection, where the eggs released, penetrates the villus continuing the infective cycle without passage through the external environment. The life span of adult worms is 4 to 6 weeks, but internal autoinfection allows the infection to persist for years.
Clinical manifestation of Hymenolepiasis
An infection with H. nana may cause no symptoms even with a heavy worm burden. Some patients complain of headache, dizziness, anorexia, abdominal pain, diarrhea, or possibly irritability. Some patients may have a low‑grade eosinophilia of 5% or more. Heavy human infection can be attributed to internal autoinfection in which the eggs hatch in the intestine and follow the normal life cycle to the adult worm. This autoinfection feature of the life cycle can lead to complications in the immunocompromised patient.
Laboratory diagnosis of Hymenolepiasis
Microscopy – wet mount preparation for eggs of H. nana in concentrated stool specimens.
Treatment, prevention and control
- Praziquantel and niclosamide are drug of choice.
- Good hygiene, public health and sanitation programs, and elimination of infected rats help to prevent the spread of hymenolepiasis.
- Preventing fecal contamination of food and water in institutions and crowded areas is of primary importance.
- General sanitation and rodent and insect control (especially control of fleas and grain insects) are also essential for prevention of H. nana infection.