DefinitionFish tapeworm is an infection with a parasite found in fish.Alternative NamesDiphyllobothriasisCauses, incidence, and risk factorsThe 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 eatenEastern EuropeNorth and South AmericaScandinaviaSome Asian countriesAfter 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 megaloblastic anemia.SymptomsMost people who are infected have no symptoms. If symptoms do occur, they may include:Abdominal discomfort or painDiarrheaWeaknessWeight lossSigns and testsPeople 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 differentialStool examination for eggs and parasitesTreatmentYou 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 megaloblastic anemia that can occur with this infection.Expectations (prognosis)Fish tapeworms can be removed with a single treatment dose. There are no lasting effects.ComplicationsMegaloblastic anemiaIntestinal blockageCalling your health care providerCall your health care provider if:You have noticed a worm or segments of a worm in your stoolAny family members have symptoms of pernicious anemiaPreventionAvoiding 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.ReferencesWhite Jr AC, Brunetti E. Cestodes. In: Goldman L, Ausiello D, eds. 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.Reviewed ByReview Date: 08/24/2011David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine; and 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, Medical Director, A.D.A.M., Inc.