Diphyllobothriasis represents an intestinal parasitic infection caused by the cestode Diphyllobothrium (also known as “fish tapeworm” or “broad tapeworm”). Diphyllobothriasis is acquired by ingestion of late larvae in undercooked or raw fish. The clinical presentation of diphyllobothriasis varies from asymptomatic, nonspecific symptoms to intestinal obstruction, and/or vitamin B12 deficiency. Identification of eggs or proglottids in the stool can provide the diagnosis. Management includes anthelmintic therapy and, if needed, vitamin B12 supplementation.

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General Characteristics and Epidemiology

General features of Diphyllobothrium

Diphyllobothriasis is caused by a parasitic infection from the cestode (tapeworm) Diphyllobothrium

  • The largest human parasite (length up to 10 meters)
  • Adult morphology:
    • Scolex
      • No hooks 
      • Paired slit-like attachment grooves (bothria)
    • Neck
    • Strobila with up to 3,000–4,000 proglottids
      • Segmented
      • Contains sets of reproductive organs
  • Eggs:
    • Ellipsoidal or oval
    • Operculum (lid-like structure) at 1 end
    • Mature in water within 3 weeks
  • Feeds through absorption

Clinically relevant species

  • D. latum (most common)
  • D. nihonkaiense
  • D. dendriticum
  • D. pacificus


  • Approximately 20 million people are infected worldwide.
  • Traditionally found in areas where raw fish consumption is common:
    • Northern Europe
    • North America
    • Japan
    • South America (rare)
  • No reported age or sexual predilection
  • No racial predilection



Definitive hosts:

  • Humans
  • Mammals
  • Birds

Intermediate hosts:

  • Freshwater fish (most common) 
  • Marine fish
  • Crustaceans


Diphyllobothriasis is transmitted through the consumption of raw or undercooked fish.

Life cycle

  • Immature eggs are passed in the feces of a definitive host.
  • Eggs mature → oncospheres → develop into coracidia → ingested by crustaceans (1st intermediate host)
  • Develop into procercoid larvae
  • Crustacean ingested by fish (2nd intermediate host) → procercoid larvae develop into plerocercoid larvae (infectious stage) 
  • 2nd intermediate host can be eaten by larger predators → plerocercoid larvae migrate to musculature
  • Humans (definitive host) consume infected raw or undercooked fish → develop into adult worms in the small intestine → produce eggs
  • Immature eggs are passed in the feces → cycle continues
Life cycle of diphyllobothriasis

The life cycle of the Diphyllobothrium species (causal agent of diphyllobothriasis)

Image: “Diphyllobothrium” by CDC. License: Public Domain

Clinical Presentation

General signs and symptoms

  • Most patients are asymptomatic.
  • Passage of proglottids in the stool can occur.
  • Nonspecific symptoms:
    • Fatigue
    • Abdominal pain
    • Diarrhea
    • Dizziness
    • Weight loss


The following may be caused by heavy infections or aberrant migration of the tapeworms:

  • Vitamin B12 deficiency → D. latum has a high affinity for vitamin B12
    • Anemia 
    • Paresthesias
    • Weakness
    • Hyporeflexia
    • Ataxia
    • Encephalopathy
  • Bowel obstruction → occurs if worms become entangled
  • Biliary disease → migration of proglottids 
    • Cholecystitis 
    • Cholangitis

Diagnosis and Management


  • Microscopic examination of the stool can reveal operculated eggs or proglottids.
  • Laboratory testing is largely nonspecific, but may reveal: 
    • Eosinophilia
    • Megaloblastic anemia
    • ↓ Vitamin B12 


  • Anthelmintic therapy:
    • Praziquantel (preferred)
    • Niclosamide
  • Correction of vitamin B12 deficiency


  • Cook fish properly.
  • If eating sashimi or sushi, freeze fish to kill the tapeworm larvae.
  • Water sanitation measures

Comparison of Tapeworm Species

Table: Characteristics and diseases of different tapeworm species
OrganismDiphyllobothrium latumTaenia saginataEchinococcus granulosus
  • Approximately 10 meters long
  • No hooks
  • Bothria present
  • > 3,000 proglottids
  • < 5 meters long
  • No hooks
  • No neck
  • Approximately 1,000 proglottids
  • 2–7 mm long
  • Hooks present
  • 3–6 proglottids
TransmissionEating raw, infected fishEating raw, infected beefFecal to oral (ingestion of contaminated food or water)
DiseaseDiphyllobothriasisTaeniasisCystic echinococcosis
  • Abdominal discomfort
  • Weight loss
  • Vitamin B12 deficiency
  • Bowel obstruction
  • Usually asymptomatic
  • Mild GI symptoms
Depends on location and size of hydatid cysts
DiagnosisEggs or proglottids in stoolEggs or proglottids in stool
  • Imaging
  • Serology
  • Praziquantel
  • Niclosamide
  • Praziquantel
  • Niclosamide
  • Albendazole
  • Percutaneous drainage
  • Surgical excision
  • Freeze fish.
  • Thoroughly cook fish.
  • Water sanitation measures
Cook beef thoroughly.
  • Personal hygiene
  • Avoid contact with stray dogs.
  • Avoid potentially contaminated food.
  • Improve water sanitation.

Differential Diagnosis

  • Ascariasis: an infection caused by the parasitic roundworm Ascaris lumbricoides. Transmission occurs from the ingestion of water or food contaminated with Ascaris eggs. Patients may be asymptomatic or experience cough and hemoptysis. A large worm burden can cause intestinal obstruction and impair growth in children. Examination of the stool may show the presence of worms or eggs.  Management is with anthelmintic therapy.
  • Pernicious anemia: causes vitamin B12 deficiency and megaloblastic anemia due to a deficiency of intrinsic factor, which is required for vitamin B12 absorption. Patients may have fatigue, cognitive decline, neuropathy, ataxia, and glossitis. Low vitamin B12 levels, anti-intrinsic factor antibodies, and the Schilling test can be used for diagnosis. Management includes vitamin B12 replacement.
  • Small intestinal bacterial overgrowth: Aerobic and anaerobic microbes normally present in the colon grow excessively in the small intestine. Approximately 90% of cases are due to motility disorders and chronic pancreatitis. Presentation includes bloating, flatulence, watery diarrhea, and abdominal discomfort; vitamin B12 deficiency can occur. The diagnosis can be made with breath testing. The mainstay of treatment is antibiotics and the correction of nutritional deficiencies.
  • Inflammatory bowel disease (IBD): characterized by chronic inflammation of the GI tract due to a cell-mediated immune response to GI mucosa. Crohn’s disease and ulcerative colitis are inflammatory bowel diseases. Symptoms include diarrhea, abdominal pain, weight loss, and extraintestinal manifestations. Diagnosis includes imaging, endoscopy, and biopsy. Management involves steroids, aminosalicylates, immunomodulators, and biologic agents.


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