The worms of the genus Diphyllobothrium spp. are, in their adult form, long, segmented and flat worms. This parasite is found in larval fish as well as in birds and mammals, including human adults.

Wildlife at risk

To develop and complete their life cycle, worms of the genus Diphyllobothrium spp. need to transit through several organisms called hosts.

Small crustaceans and fish

In its larval stage, Diphyllobothrium spp. may infect small plankton crustaceans and fish. Larvae have been identified in many species, such as:

  • Arctic char;
  • Brook char;
  • Walleye;
  • Northern pike;
  • Yellow perch;
  • Rainbow smelt;
  • Landlocked Atlantic salmon;
  • Rainbow trout.

Piscivorous birds and mammals

In adult stages, worms can infest fish-eating birds as well as mammals and humans. There are four species of Diphyllobothrium spp., and each species targets specific animals. For example, some species are found in birds while others are found in bears.

Signs of the parasite’s presence

In fish

Fish do not have any apparent symptoms and the infection is not severe. However, some factors can make it more damaging, such as:

  • the worm species. For example, larvae of D. dendriticum appear to be more harmful than those of D. ditremum;
  • the amount of larvae;
  • the location of larvae. For example, a larva in the heart muscle may have more impact than a larva in a skeletal muscle.

Inside the fish, the parasite can be seen in its free form or as a cyst in the viscera and muscles. In its free form, the parasite is white, elongated and 1 cm to 5 cm long. In the form of a cyst, it presents as a small spherical whitish mass, measuring about 2 mm in diameter. It can be invisible to the naked eye.

In addition to Diphyllobothrium spp., fish can be infected by larvae of other parasites (such as Triaenophorus crassus and Ligula intestinalis).

In piscivorous birds and mammals

The infection does not cause symptoms and is not severe, but digestive disorders (e.g., diarrhea) or non-specific disorders (e.g., weight loss) may develop. Worm segments are sometimes visible in feces. The definitive host usually only carries one parasite. The adult worm reaches up to 12 metres long by 1 to 2 cm wide and is located in the small intestine.

It should be noted that other intestinal worms may be a source of infection in these animals (e.g., Taenia hydatigena).

Life cycle and persistence in the environment

The life cycle of Diphyllobothrium spp. is maintained by contaminating bodies of water with fecal matter from infected mammals and birds, which contains the parasite’s eggs. Once in the water, the eggs continue to develop and become embryos. They are then ingested by a first intermediate host, a small planktonic crustacean, inside which they will transform into larvae. Fish will become infected after eating an infected crustacean. In fish, larvae move outside the intestine and move into muscles or organs. At this stage, larvae are infectious to birds and mammals, but can also transit through a second fish that has swallowed infected fish. The cycle ends when a fish containing infectious larvae is eaten by a bird or mammal. The larvae then reach the small intestine where they reach maturity. Five to six weeks after infection, the adult worm begins to evacuate up to a million eggs daily into the final host’s feces.

Protection and prevention

Risk for wildlife health

Animals often appear asymptomatic, but the actual impact of the parasite on wildlife populations remains difficult to assess.

Risk for domestic animal health

Dogs can be affected by Diphyllobothrium spp. species in the same way as other mammals. In order not to perpetuate the parasite cycle, domestic animals should not be fed raw or undercooked fish. If infected, dogs can be treated with a deworming agent.

Risk for human health

Eating raw fish (meat, eggs, liver or other viscera) or improperly prepared fish (insufficient cooking or salting) puts humans at risk of infection. The infection is most often invisible, but can cause digestive problems such as diarrhea, abdominal pain, nausea and vomiting. Rarely, in the case of a massive infection, mechanical obstruction of the intestine is possible. In general, deworming is effective in killing the parasite in a single dose.

To avoid contamination, learn about the precautions to take when preparing and cooking wild fish.

Surveillance and control

The presence in Québec of Diphyllobothrium spp. worms is known. It is not required to report suspect fish. To interrupt the life cycle of the worm, unused fish parts should not be discarded in the water. Instead, put them in the garbage or burn them.

Last update: January 8, 2024


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