Fish tapeworm infection is an intestinal infection with the tapeworm 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 to 6 weeks. The adult worm, which is segmented, attaches to the wall of the intestine. The tapeworm 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.
The tapeworm absorbs the nutrition from food that the infected person eats. This 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 praziquantel. Niclosamide can also be used. If needed, your health care provider will prescribe vitamin B12 injections or supplements to treat vitamin B12 deficiency and anemia.
Fish tapeworms can be removed with a single treatment dose. There are no lasting effects.
Untreated, fish tapeworm infection may cause the following:
Megaloblastic anemia (anemia caused by vitamin B12 deficiency)
Intestinal blockage (rare)
When to Contact a Medical Professional
Call your provider if:
You have noticed a worm or segments of a worm in your stool
Any family members have symptoms of anemia
Measures you can take to prevent tapeworm infection include:
Do not eat raw or undercooked fish.
Cook fish at 145°F (63°C) for at least 4 minutes. Use a food thermometer to measure the thickest part of the fish.
Freeze fish at -4°F (-20°C) or below for 7 days, or at -35°F (-31°C) or below for 15 hours.
King CH, Fairley JK. Tapeworms (cestodes). In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 8th ed. Philadelphia, PA: Elsevier Saunders; 2015:chap 291.
White Jr AC, Brunetti E. Cestodes. In: Goldman L, Schafer AI, eds. Goldman's Cecil Medicine. 25th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 354.
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, Boston, MA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.