Trematode Metacercariae of Mussel


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Metacercarial infection of mussels.

Scientific name or taxonomic affiliation

Various species in the Digenea families Gymnophallidae, Echinostomatidae and Renicolidae. In addition to benign infections of encysted metacercariae, other Digenea such as Proctoeces maculatus (family Fellostomidae) and a few species in the family Bucephalidae are pathogenic for mussels.

Geographic distribution

Global, although each species probably has a confined distribution.

Host species

Metacercaria occur in most mussel species as well as other marine bivalves worldwide including oysters, clams and cockles.

Impact on the host

Most species are relatively nonpathogenic to mussels. However, infection can cause compression of adjacent tissues resulting in loss on normal organ architecture, reduced byssal production, impaired shell cleaning in young mussels, and induction of pearl formation which may seriously affect marketability of the mussels from some areas. Exposure of mussels to neurotoxins produced by the dinoflagellate Alexandrium fundyense, a species present in harmful algal blooms, caused a significant increase in the occurrence of Gymnophallidae metacercariae in the host tissue. This increased occurrence was considered opportunistic, taking advantage of the toxin-induced weakened immune status of the host (Galimany et al. 2008).

Diagnostic techniques

Gross Observations: Often not evident without adequate microscopic magnification. However, in some cases, dark pigmentation may form around the cyst causing it to appear as a speck of pepper in the tissue or the pearls induced by the metacercaria may become visible.

Squash Preparations: Thin slices (about 3 mm thick) of the mussel body pressed between two plexiglass plates and examined under a dissecting microscope (50 x) show metacercariae.

Histology: Cross sections of metacercariae can be observed within the tissues.

Methods of control

No known methods of prevention or control.


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Galimany, E., I. Sunila, H. Hégaret, M. Ramón and G.H. Wikfors. 2008. Experimental exposure of the blue mussel (Mytilus edulis, L.) to the toxic dinoflagellate Alexandrium fundyense: Histopathology, immune responses, and recovery. Harmful Algae 7: 702–711.

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Svärdh, L. and J. Thulin. 1985. The parasite fauna of natural and farmed Mytilus edulis from the west coast of Sweden, with special reference to Renicola roscovita. Meddelande Från Havsfiskelaboratoriet Lysekil 312: 1-16.

Citation Information

Bower, S.M. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Trematode Metacercariae of Mussels

Date last revised: June 2009
Comments to Susan Bower

Date modified: