Diphyllobothriasis is caused by infection with the Diphyllobothrium latum organism, which is present in raw or undercooked fish. Infection occurs when the primary host (humans) ingests the cysts in the flesh of the intermediate host (fish). Cestodes are worms without an intestinal tract. They survive by attaching their head to the intestinal mucosa of the host and absorbing nutrients there. The body of the cestode contains segments called proglottids (male and female gonads) that produce up to one million eggs per day, per worm. These eggs are passed in the feces of the host. (See “Life cycle of D. latum.”)

Intestinal obstruction may occur as a result of massive infection. Other complications include cholecystitis or cholangitis, which result from migration of proglottids, and megaloblastic anemia, which is indistinguishable from pernicious anemia.

Diphyllobothriasis is frequently asymptomatic. Rarely, eggs may be discovered in feces. There has been some documentation of a patient vomiting a ball of fish tapeworms. When signs and symptoms do occur, they may include abdominal pain, diarrhea, malaise, anorexia, and weight loss. Less common signs include fatigue, dizziness, numbness of the extremities, hunger, and pruritus ani. Vitamin B₁₂ deficiency results from absorption of the vitamin by the tapeworm, which leads to pernicious anemia. Signs and symptoms of pernicious anemia include a smooth, beefy red, painful tongue; enlarged liver and spleen; dyspnea; weak extremities and disturbed coordination; irritability; depression; delirium; and ataxia. Signs of intestinal obstruction include abdominal distention and pain.