Fish tapeworm is an infection with a parasite found in fish.
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 fish tapeworm cysts.
The infection is seen in many areas where humans eat uncooked or undercooked fish from rivers or lakes, including:
African countries in which freshwater fish are eaten
North and South America
Some Asian countries
After a person has eaten infected fish, the larva begin to grow in the intestine. Larvae 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. Sometimes, parts of the worm may also be passed in the stool.
Fish tapeworm infection may lead to vitamin B12 deficiency and anemia.
Most people who are infected have no symptoms. If symptoms do occur, they may include:
Abdominal discomfort or pain
Exams and Tests
People who are infected sometimes pass segments of worm in their stools. These segments can be seen in the stool.
You will receive medicines to fight the parasites. You take these medicines by mouth, usually in a single dose.
The drug of choice for tapeworm infections is praziquantal. Niclosamide can also be used. You may need vitamin B12 injections or supplements to treat the vitamin B12 deficiency and anemia that can occur with this infection.
Fish tapeworms can be removed with a single treatment dose. There are no lasting effects.
Intestinal blockage (rare)
When to Contact a Medical Professional
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 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.
White Jr AC, Brunetti E. Cestodes. In: Goldman L, Ausiello D, eds. Goldman's Cecil Medicine. 24th ed. Philadelphia, PA: Saunders Elsevier; 2011:chap 362.
King CH, Fairley JK. Cestodes (tapeworm). In: Mandell GL, Bennett JE, Dolin R, eds. Principles and Practice of Infectious Diseases. 7th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2009:chap 290.
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, Bethanne Black, and the A.D.A.M. Editorial team.