Fish tapeworm: symptoms, detection and therapy

Fish tapeworm: symptoms, detection and therapy

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The fish tapeworm is a parasite. It is about 20 meters long with 3000 to 4000 proglottids, which are segments that each form a fully-fledged sexually reproductive system. The skolex, the front end, serves to attach itself to the intestine of the host. The fish tapeworm has two elongated suction cups. Its eggs are colored yellow and about 45 x 70 µm in size.


The fish tapeworm occurs mainly in subtropical, temperate and subarctic zones, and there in the area of ​​lakes. It is particularly common in North America, Siberia, Scandinavia, the Baltic States, Japan and Chile. Inuit in Alaska and northern Canada very often contract this tapeworm. Raw fish is the main food for those affected. In Japan, and now also in the USA and Europe, the consumption of sushi is the main reason to ingest such a tapeworm.

The infestation with fish tapeworms can be triggered in Europe and North America by one of the following types, the symptoms are largely identical:

  • Diphyllobothrium latum in Europe, especially on the Iberian Peninsula
  • Diphyllobothrium dendriticum
  • Diphyllobothrium ditremum

The life cycle

Humans become infected with the parasite by eating raw or underheated fish. The fish tapeworm is not specialized in humans as hosts: it can infect all mammals that eat fish. Amongst the domestic animals, these are mainly dogs and cats, sled dogs in the subarctic, which mainly feed on fish waste, are particularly affected.

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The proglottids contain eggs, and these worm eggs excrete the fish eaters with their faeces. If the excrement gets into the water cycle, it contaminates the water, and the eggs grow into the coracidia, eyelashed larvae.

These use small crabs of the genera Cyclops and Diaptomus as the first intermediate hosts. Fish eat these crabs, then the fish serve as second intermediate hosts. They are often carp-like and other plankton-eaters, sometimes also predatory fish such as pike.

The larvae mature in the fish

In the muscles of the fish, the coracidia grow into pleroceroids, into infectious larvae. If people eat these fish now, the infectious larvae enter the human small intestine, where they can live for several years. The feces contain the first eggs 3 weeks after the infection. The worm excretes these eggs over the last segment.
The worm grows by 9 to 15 cm a day.

There are usually no symptoms

Those affected often do not notice anything about the worm infestation, because usually there are no symptoms. Only with a large amount of tapeworm segments in the body can diarrhea occur, or the worm balls block the bile duct or the intestinal lumen. If the worms get stuck in the upper small intestine, a vitamin B 12 deficiency can result.

The fish tapeworm absorbs more vitamin B 12, and the human body then lacks this when it forms the red blood cells. The result is a special anemia, pernicious anemia.


The eggs usually appear in bulk in the stool (up to a million a day) and are therefore easy to identify.


Worm infestation is easy to treat. 90% of patients fully recover if they take praziquantel worm once. Since usually only one tapeworm of these species affects a person, the detection of the Skolex in the faeces is evidence that the parasite has died.

Danger in Germany?

Local fish rarely carry the fish tapeworm in them. Imported fish from Northwest Russia, the Baltic States and Scandinavia are at higher risk of becoming infected. Tourists are particularly at risk if they eat the popular raw fish dishes in Scandinavia or the Baltic States, Japan or South America.

In North America and Switzerland, fish tapeworms were infected by sushi fashion.

How do you protect yourself?

Raw fish is only a problem if it comes from fresh water. Saltwater fish never carry the parasite. Boiled, smoked, fried and deep-frozen fish can be eaten without hesitation because the heat and cold kill the pathogen.

Danger from sushi?

Veterinary examinations showed that in 70% of all sushi products there were no borderline germ levels. Before the preparation, this amount was as high as 83%, which meant that 13% of the germs got into the sushi during the preparation. The added washabi horseradish also has a disinfectant effect.

In Japan, however, the increased consumption of sushi has meanwhile led to several hundred registered cases of worm-induced diseases.

German legislation now stipulates that raw fish must first be cooled to at least - 20 degrees Celsius for at least 24 hours before it is consumed. That is enough to eliminate the tapeworm. In addition, sushi is often made from frozen fish anyway, almost exclusively in most sushi bars in Germany.

The freshwater carpaccio, which has been popular with gourmets for several years, poses a major problem, in which salmon, trout or pike are cut into thin slices. As these are gourmet restaurants, guests are of course not served any deep-frozen fish. (Dr. Utz Anhalt)

Author and source information

This text corresponds to the specifications of the medical literature, medical guidelines and current studies and has been checked by medical doctors.

Dr. phil. Utz Anhalt, Barbara Schindewolf-Lensch


  • Richard D. Pearson: Diphyllobothriasis (fish tapeworm infection), MSD Manual, (accessed 06.09.2019), MSD
  • G. Piekarski: Cestodes, tapeworms Diphyllobothrium latum, medical parasitology. Springer Verlag, (accessed on September 6, 2019), Springer

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