Medical Helminthology Lab 3: Cestode Notes

Cestode: Hymenolepis spp. and Diphyllobothrium latum

Hymenolepis nana (Dwarf Tapeworm)

  • Most common tapeworm in humans.
  • Also known as the dwarf tapeworm due to its small size.
  • Located in the small intestine (ileum).
  • Size: 1–4 cm long (rarely 9 cm) and 1 mm wide (approximately 200 segments).
  • Composed of a head (scolex), neck, and segmented body.

Hymenolepis nana Egg

  • Shape: Typically oval to subspherical.
  • Size: Approximately 40–60 µm in length and 30–50 µm in width.
  • Found: Passed in the feces of infected individuals.
  • Inside the egg: Contains an oncosphere (hexacanth embryo), which is the infective larval stage.
  • Oncosphere: Equipped with six characteristic hooklets, used for tissue penetration during the infection process.

Hymenolepis nana Larvae (Cysticercoids)

  • Infection: Results from peroral ingestion of eggs.
  • Process: Oncospheres hatch in the small intestine, penetrate into the villi, and develop into larvae (cysticercoids).
  • Development: Larvae return to the intestinal lumen and develop into adult tapeworms within two to three weeks.

Hymenolepis diminuta (Rat Tapeworm)

  • Commonly found in rats and mice; infrequently in humans.
  • Complex life cycle.
    • Intermediate host: Cysticercoid or larval stage in arthropods (fleas).
    • Definitive host (final host): Adult stage found in rats and mice, and accidentally in humans, especially children.

Diphyllobothrium latum (Fish Tapeworm or Broad Tapeworm)

  • Longest human-infecting cestode, reaching lengths up to 10 meters.
  • Acquisition: Consuming raw or undercooked freshwater fish containing the infective plerocercoid larva.
  • Location: Inhabits the small intestine of humans and other fish-eating mammals.
  • Disease: Causative agent of Diphyllobothriasis.

Diphyllobothrium latum Morphological Features

Eggs

  • Shape: Oval, operculated (lid-like cap).
  • Size: 50–70 µm by 35–50 µm.
  • Color: Yellowish-brown.
  • State: Unembryonated when passed in feces.

Adult Proglottids

  • Shape: Broader than long (a key identifying feature).
  • Uterus: Each contains a rosette-shaped uterus.
  • Genital Pore: Midline genital pore (not lateral like in Taenia).

Scolex (Head)

  • Shape: Almond-shaped.
  • Attachment: Two longitudinal sucking grooves (bothria) for attachment (not suckers or hooks).

Diphyllobothrium latum Hosts and Stages

  • Definitive hosts: humans (and other fish-eating mammals).
  • Intermediate hosts: planktonic crustacean and one or more freshwater fish.
  • Infective stage: plerocercoid larvae.
  • Diagnostic stage: eggs or segments of the tapeworm in a stool sample with a microscope.

Laboratory Materials Needed

  • Fecal sample (real or preserved).
  • Microscope slides and cover slips.
  • Normal saline or iodine solution.
  • Compound light microscope.
  • Gloves and lab coats.

Lab Procedures

1. Microscopic Examination of Eggs

  • Prepare a wet mount of the stool sample using saline.
  • Observe under 10x and 40x objectives.
  • Look for oval, operculated eggs with a knob at the opposite end.

2. Stained Egg Observation (Optional)

  • Use iodine to enhance visualization of internal egg structures (oncosphere).

3. Observation of Proglottids (if available)

  • Mount preserved proglottid segment on a slide.
  • Observe under low magnification.
  • Identify rosette-shaped uterus and central genital pore.