Definition

Fish tapeworm infection is an intestinal infection with a parasite found in fish.

Alternative Names

Diphyllobothriasis

Causes

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 nutrients from the food that the infected person eats. This may lead to vitamin B12 deficiency and anemia.

Symptoms

Most people who are infected have no symptoms. If symptoms do occur, they may include:

• Abdominal discomfort or pain
• Diarrhea
• Weakness
• Weight loss

Exams and Tests

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

Treatment

You will receive medicines to fight the parasites. You take these medicines by mouth, usually in a single dose.

The medicine of choice for tapeworm infections is praziquantel. If needed, your health care provider will prescribe vitamin B12 injections or supplements to treat vitamin B12 deficiency and anemia.

Outlook (Prognosis)

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

Possible Complications

Untreated, fish tapeworm infection may cause the following:

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

When to Contact a Medical Professional

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

Prevention

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.

References

Fairley JK, O'Connell EM. Tapeworms (cestodes). In: Blaser MJ, Cohen JI, Holland SM, et al, eds. Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases. 10th ed. Philadelphia, PA: Elsevier; 2026:chap 295.

Fischer PR, White AC. Adult tapeworm infections. In: Kliegman RM, St. Geme JW, Blum NJ, et al, eds. Nelson Textbook of Pediatrics. 22nd ed. Philadelphia, PA: Elsevier; 2025:chap 348.

White AC, Brunetti E. Cestodes. In: Goldman L, Cooney KA, eds. Goldman-Cecil Medicine. 27th ed. Philadelphia, PA: Elsevier; 2024:chap 325.