Anguillicola (syn. Anguillicoloides) crassus


Anguillicola crassus is a round worm that affects Atlantic eels, i.e. the European eel and the American eel.

Wildlife at risk

The Anguillicola crassus worm needs to transit through several organisms called hosts to develop itself and complete its life cycle.

Small crustaceans and fish

At the larval stage, the parasite is typically found in small planktonic crustaceans (copepods). The parasite may also be present in fish such as:


The adult parasite lives and reproduces in the swim bladder (an internal organ that allows fish to float and swim) of eels. Eels can be infected with Anguillicola crassus at an early stage of life.

The Anguillicola crassus worm was first observed in Asia, in the Japanese eel. In general, the presence of the parasite does not have a negative effect on this species. However, the parasite is known to have adverse effects in Atlantic eels.

Signs of the parasite’s presence

The presence of the parasite in an eel generally does not cause external signs. It is impossible to determine if a fish is infested by observing it.

Anguillicola crassus is known to damage the eel’s swim bladder, causing inflammation, lesions, bleeding, fibrosis and wall thickening.

To diagnose the presence of Anguillicola crassus in an eel, one must examine the eel’s swim bladder. Therefore, this can only be done in a dead eel. A method to detect the presence of Anguillicola crassus in a body of water from the examination of intermediate hosts is being developed. 

Life cycle and persistence in the environment

The Anguillicola crassus parasite develops in an intermediate host (small crustaceans) before completing its life cycle in its final host (eel). It can also go from an intermediate host to a fish, which is optional in the parasite’s life cycle.

In the adult stage, the parasite becomes established in the eel’s swim bladder when an infected host ingests it. Adult worms, both female and male, live and breed there. The eggs and larvae produced are then expelled to water. Larvae settle on the substrate and may be ingested by an intermediate host or fish. Larvae reach the wall of the eel’s swim bladder when the eel consumes an infected prey. The worms moult and become sexually mature.

Although it has several stages, the Anguillicola crassus life cycle is short and completed in 2 to 4 months.

The parasite has a high tolerance to environmental and physicochemical conditions of water, such as salinity and temperature. It can live in waters between 4°C and 20°C and develop rapidly with rising temperatures.

Protection and prevention

Risk for wildlife health

The Anguillicola crassus parasite can affect the ability of eels to swim and reduce their ability to migrate to their breeding area. Eels also become more susceptible to disease. Their growth rate slows, and if the infestation is severe enough, they can die. The effect of the parasite on the ability of American eels in the St. Lawrence watershed to reach their breeding area and on their survival remains to be determined. However, this is an additional threat to this migratory species exposed to other threats, such as habitat fragmentation, mortality from turbination, commercial fishing, bioaccumulation of toxic substances, and climate change. Anguillicola crassus is often identified as one of the contributing factors to its decline.

Risk for human health

Anguillicola crassus is not known to be transmissible to humans, since it is specific to eels. However, for aesthetic reasons, removal of the swim bladders may be recommended prior to the treatment and sale of fished eels.

Surveillance and control

Anguillicola crassus is found in American eels living in coastal and inland waters of the eastern United States, New Brunswick, Nova Scotia and the St. Lawrence River watershed. In Québec, it was first detected in 2015. Reporting of suspect fish is not mandatory.

No fully effective treatment or control methods have been found for this parasite. In farming, a medical treatment of swimming fish in a solution containing a nematicide, or feeding it, has been tested, with varying degrees of success.

Last update: January 8, 2024


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