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About Hymenolepis nana

The common name of Hymenolepis nana: The Dwarf tapeworm

History and Distribution

  • Hymenolepis means a thin membrane covering the egg (Greek hymen—membrane, lepis—rind or covering), and nana means small size (nanus—dwarf).
  • 1st discovered by Bilharz in 1857
  • H. nana is the smallest intestinal cestodes that infecting to humans beings.

Epidemiology

  • Cosmopolitan in distribution but is more common in a warm climate than in cold climates.
  • Infection is most common in institutional populations and school children.
  • Common parasite of mice.

Habitat: Small intestine (proximal ileum of man), rodents like mice and rats (found in the posterior part of the ileum).

Morphology

  1. Adult Worm
  2. Eggs

Adult Worm:

Treponema pallidum
  • Hymenolepis nana is the smallest intestinal cestode that infecting to humans being.
  • Around 5 to 45 mm in length and less than 1 mm thick.

 

  • Scolex: It is globular with 4 suckers, & a retractile rostellum with a single row of hooklets(20-30), rostellum remains invaginated in the apex of an organ.
  • Neck: long slender.
  • Strobila:
    • Generally, it’s consisting of 200 or more proglottids, which are much broader than long.
    • Segment-0.3×0.9 mm.
    • Genital pores are marginal on the same side.
    • The testis is round and 3 in number and the Uterus has lobulated walls.
    • Eggs are released in the intestine by the disintegration of the distal gravid segments.

Egg:

Hymenolepis nana
  • Roughly spherical or ovoid in shape.
  • 30–45 μm in size in diameter.
  • It contains two distinct membranes:
    • The outer membrane is thin colorless.
    • Inner embryophore enclosing the hexacanth oncosphere with three pairs of hooklets.
    • The space between the 2 membranes is filled with yolk granules and 4–8 thread-like polar filaments arising from 2 knobs on the embryophore.
    • The eggs float in a saturated solution of salt.
    • Non-bile stained.
    • Instantly infective & unable to survive for more than 10 days in the external environment.

Mode of infection:

  • Ingestion of food contaminated with eggs.
  • Autoinfection: External and internal
    • External autoinfection occurs when a person ingests their own eggs by the fecal-oral route
    • Internal autoinfection: occur when the eggs released in the intestine hatch there themselves.
  • Rarely by ingestion of food contaminated with fleas harboring the cysticercoid larvae.

Life Cycle of Hymenolepis nana

    • Definitive host: Man
    • No intermediate host
    • Infective stage: embryonated egg
  • Direct
  • Indirect
  • Autooinfection (Internal, external)

Steps

  • H. nana is uncommon in that it undergoes multiplication in the body of the definitive host.
  • When a large number of eggs are swallowed, or in internal autoinfection, they hatch in the small intestine.
  • When the hexacanth embryo penetrates the intestinal villus and then develops into the cysticercoid larva.
  • It is a solid pyriform structure.
  • A solid pyriform contain the vesicular anterior end which containing the invaginated scolex and a short conical posterior end.
  • After about 4 days, the mature larva emerging out of the villus and then evaginates its scolex and attaches to the mucosae.
  • Now it’s starts strobilization, to become the mature worm, which begins producing eggs in about 25 days.

 

    • A different strain of H. nana infects rats and mice.
    • The eggs passed in rodent feces are ingested by rat fleas (Xenopsylla cheopis and others), which acts as the intermediate host.
    • The eggs develop into cysticercoid larvae in the hemocoel of these insects.
    • Rodents get infected when they eat these insects.
    • The murine strain does not appear or arrive to infect man.
    • However, the human strain may infect rodents, which may, therefore, constitute

Clinical features:

Generally asymptomatic

  • Abdominal pain
  • Diarrhea
  • Nausea
  • Pruritus-sometimes

Laboratory diagnosis:

  • Direct microscopy: demonstration of characteristic eggs in feces.
  • Concentration methods: like salt flotation and formalin ether may be used.
  • Serological methods: ELISA test: 80% sensitivity.

Treatment:

  • Niclosamide: 60-80 mg/kg for 5-7 days.
  • Praziquantel: 25 mg/kg in a single dose at one time (acts both against the adult worms and the cysticercoids in the intestinal villi).

Prophylaxis:

  • Maintenance of good personal hygiene.
  • Maintenance of sanitary improvements.
  • Avoid contaminated food and water.
  • Rodent control.

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