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 freshwater fish from rivers or lakes, including:

• Africa
• Eastern Europe
• North and South America
• Scandinavia
• Some Asian countries

After a person has eaten infected fish, the worm larva begins 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 (9 meters). 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
• Diarrhea
• Weakness
• 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
• Blood tests to determine the cause of anemia, if present
• Vitamin B12 level
• Stool exam for worm eggs and parasites

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 treated with a single treatment dose. There are no lasting effects.

Untreated, fish tapeworm infection may cause the following:

• Anemia caused by vitamin B12 deficiency (megaloblastic anemia)
• Intestinal blockage (rare)

Contact your provider if:

• You have noticed a worm or segments of a worm in your stool
• You have symptoms of anemia
• Any family members are diagnosed with a fish tapeworm infection

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 -31°F (-35°C) or below for 15 hours.

Diphyllobothriasis

Alroy KA, Gilman RH. Tapeworm infections. In: Ryan ET, Hill DR, Solomon T, Aronson NE, Endy TP, eds. Hunter's Tropical Medicine and Infectious Disease. 10th ed. Philadelphia, PA: Elsevier; 2020:chap 130.

Fairley JK, King CH. Tapeworms (cestodes). In: Bennett JE, Dolin R, Blaser MJ, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 9th ed. Philadelphia, PA: Elsevier; 2020:chap 289.

Updated by: Jatin M. Vyas, MD, PhD, Associate Professor in Medicine, Harvard Medical School; Associate in Medicine, Division of Infectious Disease, Department of Medicine, Massachusetts General Hospital, Boston, MA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.