The fish tapeworm, scientifically named Diphyllobothrium latum, represents one of the giant tapeworm species. Humans become infected when they eat raw or undercooked fish that contain tape worm larvae (sparganum).

The infection is seen in many areas where humans consume 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. The adult worm, which is segmented, may attain a length of 30 feet. Eggs are formed in each segment (proglottid) of the worm and are passed in the stool. Occasionally, a string of proglottids may be passed in the stool.

Fish tapeworm infection may lead to Vitamin B12 deficiency and the subsequent development of megaloblastic anemia.

The vast majority of infected individuals have no symptoms. Symptoms may include:

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

Infected individuals sometimes pass visible segments of worm in their stools.

Tests may include:

• Complete blood count
• Stool smear

Medicines to fight the parasites are given in a single dose to treat the tapeworm infection. Vitamin B12 injections or supplements may be needed to treat megaloblastic anemia.

Fish tapeworms can be eradicated 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 the stool. Also call if any family members have symptoms suggestive of pernicious anemia.

Avoiding raw freshwater fish and cooking fish sufficiently will prevent infection with the fish tapeworm.

Sampaio JLM, Andrade VP, Lucas MC, et al. Diphyllobothriasis. Emerg Infect Dis. 2005;11:1598-1600.