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Microbiology · Cestodes (tapeworms)

Dibothriocephalus latus !

  • Parasite
  • For completeness

The longest human tapeworm; it comes from raw or undercooked fish. It can steal vitamin B12 and cause megaloblastic anaemia.

Updated 2026-11-01

Also called: Diphyllobothrium latum (former name), fish tapeworm, broad tapeworm

At a glance

Shape
Elongated scolex with two grooves (bothria), no suckers or hooks
Family
Diphyllobothriidae
Type
Cestode (diphyllobothriid), fish tapeworm
Size
Adult can exceed 10 m

Morphology

  • Proglottids broader than long, with a rosette-shaped uterus and one ventral genital pore.
  • Operculated egg, 55–75 × 40–50 µm, with a small knob at the opposite end.
  • Eggs are passed unembryonated and are usually abundant.

Transmission

  • Eating raw, marinated or undercooked fish with plerocercoid larvae.
  • Raw fish dishes (ceviche, sushi) made with fish that was not frozen.

Reservoir: Humans; Bears and other fish-eating mammals; Freshwater and some marine fish

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Life cycle

  1. Operculated eggs leave in stool and mature in water.
  2. The hatched coracidium is eaten by a small crustacean (copepod), where it forms the procercoid.
  3. A small fish eats the crustacean; the larva moves into its flesh as an infective plerocercoid.
  4. Larger predatory fish accumulate plerocercoids by eating the small ones.
  5. A person eats raw or undercooked fish; the adult grows in the bowel and lays eggs in 5–6 weeks.

Virulence factors

  • The worm absorbs vitamin B12 in the small bowel and competes with the host.
  • Prolonged deficiency can cause megaloblastic anaemia and neurological symptoms.
  • Most infections are mild; rarely obstruction or cholangitis.

Diseases it causes

Clinical picture

  • Asymptomatic or with mild GI complaints.
  • Passing chains of proglottids.
  • Pallor, glossitis and paraesthesia if B12 is deficient.

Diagnosis

  • Abundant operculated eggs on stool microscopy, visible without concentration.
  • Broad proglottids in stool.
  • Blood count and vitamin B12 levels.
  • PCR to identify the species, mainly in research.

Treatment

Cestocides: praziquantel (drug of choice) or niclosamide; vitamin B12 replacement if deficient.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Cook fish, or freeze it properly before eating it raw.
  • Sanitation so that human faeces do not reach rivers and lakes.

Test yourself

Think of the answer, then tap the question to see it.

Which vitamin deficiency can the fish tapeworm cause?

Vitamin B12 deficiency, with megaloblastic anaemia.

What does the Dibothriocephalus latus egg look like?

Operculated, 55–75 µm, with a small knob at the end opposite the operculum.

How is diphyllobothriasis acquired?

By eating raw, marinated or undercooked fish containing plerocercoid larvae.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: CDC - DPDx - Diphyllobothriasis (CDC, 2019); Fish tapeworm infection: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2025).

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Representative model, for educational purposes only.