Hymenolepis nana: Life Cycle, Pathogenesis and Lab Diagnosis

  • Hymenolepis nana is one of the small intestinal cestodes infecting man.
  • Hymenolepis refers to the thin membrane covering the egg (Greek hymen—membrane, lepis- covering) and nana to its small size (nanus—dwarf).
  • Its common name is dwarf tapeworm due to its small size (1-4 cm long).
  • It causes hymenolepiasis.
  • H. Bilharz first discovered nana in 1857.
  • It is the most common tapeworm infection in humans.
  • It is cosmopolitan in its geographical distribution.

Habitat

  •   The abode of the adult worm is the small intestine (distal portion of the ileum) of man.
  •   It is also found in rodents, especially in mice and rats.

Note: It is unique that it is the only cestode that completes its life cycle in one host–humans.

Morphology

  • Length: 1 to 4 cm  and diameter: 1 mm ( maximum)
  • The worms may be present in large numbers (from 1000 to a maximum of 8000).
  • The lifespan of an adult worm is short (about 2 weeks).

Scolex (head): 

  • The minute scolex is 0.32 mm in diameter.
  • It’s in a rhomboidal shape.
  • Number of suckers: 4, which is 80 μm in the cross-section.
  • Presence of short rostellum, which is armed with a single row of hooklets numbering 20-30.
  • It is capable of invagination into the apex of the organ.
  • The rostellar hooklets are shaped like tuning forks.
  • The “neck” is long.

Proglottides (segments):

  • Number of segments: 200.
  • Mature sediment has length: 0.3 mm, breadth: 0.9 mm.
  • It begins with the short, narrow, and immature proglottids.
  • It then gradually becomes broader at the distal end.
  • Genital pores are marginal and are situated on the same side.
  • The uterus is a transverse sac with lobulated walls, while there are three testes.

Eggs: (please refer to the lab diagnosis section below).

Life Cycle of Hymenolepis nana

The life cycle of Hymenolepsis nana can be completed via a direct or indirect cycle. In the direct cycle, an intermediate host is not required, and the entire development from the larval to the adult stage occurs in humans. Arthropod (rat fleas and beetles) acts as an intermediate host in the indirect cycle.

Lifecycle of Hymenolepis nana
  1. When the eggs of Hymenolepis nana is passed in the stool, they are immediately infective. In the external environment, eggs cannot survive more than 10 days.
  2. Eggs develop into cysticercoids when ingested by the arthropod intermediate hosts such as beetles and fleas.
  3. Humans are infected when they ingest cysticercoid-infected arthropods.
  4. Humans can also get infected when they ingest food or water contaminated with embryonated eggs.
  5. Then after the ingestion, the oncospheres present in the eggs are released.
  6. Then in the small intestine, they can gradually develop into adults.
  7. Adults live in the small intestine in the ileal region and produce the gravid proglottids.
  8. Eggs get released from the genital atrium of the gravid proglottids or when the disintegration of the proglottids occurs in the small intestine.
  9. If eggs remain in the intestine, autoinfection may result. Intestinal villus is penetrated by the oncospheres (hexacanth larvae) and later develops into the cysticercoid larvae.

Pathogenesis

Mode of infection

  •  Ingestion of eggs can occur by:
    • Fecal-oral route
    • Contaminated food and water
    • Food contaminated with fleas which harbors the cysticercoid larvae (rarely)

Hymenolepiasis

  • Hymenolepiasis is caused by the Hymenolepis nana which occurs mostly in children.
  • Incubation period: 15-39 days
  • Most of the infections caused by Hymenolepis nana are asymptomatic. In the case of heavy infections, H. nana causes mechanical irritation of the intestine and the following symptoms may be seen because of the release of toxic metabolites;
    • Irritability
    • Diarrhea
    • Abdominal pain
    • Sleep disorders
    • Anal pruritus 
    • Nasal pruritus. 

Other rare symptoms are anorexia, nausea, and vomiting.

 Laboratory diagnosis

 Sample: Stool specimen

Collection of stool 

Collect all fecal specimens before the administration of antibiotics or antidiarrheal agents. 

  • The specimen of choice is a diarrheal stool and the optimum volume is a teaspoon amount.
  • Collect the fecal specimen in a clean, wide-mouthed container or newspaper, and transfer it to a container with a tight-fitting lid. Avoid contamination with urine or water from the toilet.
  •  Three specimens collected every other day or every third day should be adequate for parasitic examination. 
    • A single stool specimen may not exclude bacterial or parasitic pathogens as a cause of diarrhea.
    • Three consecutive negative specimens are often needed to rule out the carrier state for some organisms.
  • Avoid using mineral oil, bismuth, and barium before fecal collection since these substances may interfere with detecting or identifying intestinal parasites. 

Reject any specimen that appears to be dry on the surface or edges.

Transport

  • Transport the specimen to the laboratory as soon as possible, or keep it refrigerated until transport is possible. Dried specimens (diarrheic, semiformed, or formed) are not acceptable for fecal examination.
  • Smaller amounts can be examined, but the specimen will likely dry out before examination (unacceptable for testing).

Direct Microscopy

Eggs:

  • They are spherical or oval, hyaline, and measure 30-47 μm in diameter. 
  • There are two distinct membranes:
    • The outer membrane is thin and colorless.
    • The inner embryophore encloses an onchosphere with three pairs of lancet-shaped hooklets.
  • The space between the two membranes is filled with yolk granules and polar filaments (4 to 8) emanating from little knobs at either end of the embryophore.
  • Eggs of H. nana float in a saturated solution of common salt.

b.      Eosinophilia

c.       ELISA Test

Treatment

  • Drug of choice: Praziquantel
  • Niclosomide in a 60-80 mg/kg dose for 5-7 days (maximum dose 2gm/day) is used effectively.
  • Praziquantel in a single dose of 25 mg/kg is highly effective.
  • Mebendazole cures 50% of cases only.

Prevention and Control

  • Improve sanitary conditions by focusing on personal hygiene.
  • Prevention of food from contamination.
  • Controlling rodents in the house and surrounding environment.
  • Due to its direct lifecycle and zoonotic infection status, it’s difficult to control.

Reference

  • Chatterjee, K. D. (2009). Parasitology (Thirteenth). CBS Publishers & Distributors Pvt Ltd.
  • https://www.cdc.gov/dpdx/hymenolepiasis/index.html

Sushmita Baniya

Hello, I am Sushmita Baniya from Nepal. I am a postgraduate student of M.Sc Medical Microbiology. I am interested in Genetics and Molecular Biology.

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