Intracellular Ciliates of Mussels


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Mussel protozoan X (MPX), Intracellular ciliate of the digestive tubule, Ciliophora-like organism (CILO), Digestive gland ciliates.

Scientific name or taxonomic affiliation

Rhynchodid-like Phyllopharyngea ciliates.

Geographic distribution

Appears to be ubiquitous. Reported from mussels in German East-Frisian coast (North Sea between Cuxhaven and Emden, pers. com. Dr. Burkard Watermann, e-mail:, northwest Spain (Robledo et al. 1994, Villalba et al. 1997), Gulf of Trieste in Slovenia at the north end of the Adriatic Sea (Gombac et al. 2008) and the east and west coasts of North America including Canada (Bower 1992, McGladdery et al. 1993, Moret et al. 1999).

Host species

Mytilus edulis, Mytilus trossulus, Mytilus galloprovincialis.

Impact on the host

Large numbers elicit no obvious host-response despite disruption of the digestive tubule epithelia and no mortalities have been associated with these infections. Prevalances up to 40% have been detected in Spain but on both coasts of North America and on the German coast, prevalence is usually less than 1%. In Slovenia, a prevalence of 21.4% was detected but the majority of infections (94%) were classified as mild (less than 20 ciliates per section) (Gombac et al. 2008).

Diagnostic techniques

Histology: Spindle-shaped ciliates (9-16 µm × 4.5-6.0 µm) occur inside the digestive tubule epithelia. Less commonly found partially imbedded or lying loose within the tubule lumens. The macronucleus is polymorphic (oval to globular) and densely basophilic. The micronucleus is spherical and stains less intensely. The cilia are retained by ciliates within the host cell (unlike the intracellular stages of certain sphenophryiid ciliates).

Figures 1a and 1b. Histological sections through the digestive gland of Mytilus trossulus from British Columbia, Canada with ciliates (arrows) within the cytoplasm of the epithelial cells.

Methods of control

Prevention and control impractical. These ciliates appear to be ubiquitous, although different species may occur in different areas.


Bower, S.M. 1992. Diseases and parasites in mussels. In: E. Gosling (ed.) The Mussel Mytilus: Ecology, Physiology, Genetics and culture. Elsevier Press, Amsterdam, p. 543-563.

Figueras, A. J., C. F. Jardon and J. R. Caldas. 1991. Diseases and parasites of mussels (Mytilus edulis Linneaus, 1758) from two sites on the east coast of the United States. Journal of Shellfish Research 10: 89-94.

Gombac, M., M. Pogacnik, I. Fonda and V. Jencic. 2008. Intracellular ciliates of cultured Mediterranean mussels (Mytilus galloprovincialis) in the Gulf of Trieste in Slovene Adriatic Sea. Bulletin of the European Association of Fish Pathologists 28: 217-221.

McGladdery, S. E. and M. F. Stephenson. 1991. Parasites and diseases of suspension- and bottom-grown shellfish from eastern Canada. Bulletin Aquaculture Association of Canada 91-3: 64-66.

McGladdery, S.E., R.E. Drinnan and M.F. Stephenson. 1993. A manual of parasites, pests and diseases of Canadian Atlantic bivalves. Canadian Technical Report of Fisheries and Aquatic Sciences 1931, p. 72-73.

Moret, K., K. Williams, C. Couturier and J. Parsons. 1999. Newfoundland cultured mussel (Mytilus edulis) industry 1997 health survey. Bulletin of the Aquaculture Association of Canada 99-3: 35-37.

Robledo, J.A.F., M.M. Santarém and A. Figueras. 1994. Parasite loads of rafted blue mussels (Mytilus galloprovincialis) in Spain with special reference to the copepod, Mytilicola intestinalis. Aquaculture 127: 287-302.

Villalba, A., S.G. Mourelle, M.J. Carballal and C. López. 1997. Symbionts and diseases of farmed mussels Mytilus galloprovincialis throughout the culture process in the Rías of Galicia (NW Spain). Diseases of Aquatic Organisms 31: 127-139.

Citation Information

Bower, S.M. (2013): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Intracellular Ciliates of Mussels.

Date last revised: May 2013
Comments to Susan Bower

Date modified: