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Diphyllobothriasis is an infection from a fish tapeworm.

The fish tapeworm (Diphyllobothrium latum), is the largest parasite that infects humans. Humans become infected when they eat raw or undercooked freshwater fish that contain tapeworm cysts.

The infection is seen in many areas where humans eat uncooked or undercooked fish from rivers or lakes. Diphyllobothriasis is seen in Eastern Europe, North and South America, African countries in which freshwater fish are eaten, and in some Asian countries.

After a person has eaten infected fish, the larva begin to grow in the intestine. They are fully grown in 3 - 6 weeks. The adult worm, which is segmented, may reach a length of 30 feet. Eggs are formed in each segment of the worm and are passed in the stool. Occasionally, parts of the worm may also be passed in the stool.

Fish tapeworm infection may lead to vitamin B12 deficiency and megaloblastic anemia.

Most people who are infected have no symptoms. Symptoms may include:

• Abdominal discomfort or pain
• Nausea, vomiting, or diarrhea
• Loss of appetite and weight loss

People who are infected sometimes pass segments of worm in their stools. These segments can be seen in the stool.

Tests may include:

• Complete blood count, including differential
• Stool examination for eggs and parasites

You will receive medicines to fight the parasites. Medicines are taken by mouth, usually in a single dose.

The drug of choice for tapeworm infections is niclosamide. Praziquantel and albendazole can also be used. You may need vitamin B12 injections or supplements to treat megaloblastic anemia that can occur with this infection.

Fish tapeworms can be removed with a single treatment dose. There are no lasting effects.

• Megaloblastic anemia
• Intestinal blockage

Call your health care provider if:

• You have noticed a worm or segments of a worm in your stool
• Any family members have symptoms of pernicious anemia

Avoiding raw freshwater fish and cooking fish enough (to more than 140 degrees F for 5 minutes) will prevent infection with the fish tapeworm. Freezing fish to -4 degrees F for 24 hours also kills fish tapeworm eggs.

Fish tapeworm infection

King CH. Cestode infections. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 375.

Updated by: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and Jatin M. Vyas, MD, PhD, Assistant Professor in Medicine, Harvard Medical School, Assistant in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.