Hymenolepis nana: Morphology, Life cycle, Lab diagnosis-NotesMed

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:

• 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:

• 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.