Cestode Parasitism of Oysters


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Cestode parasitism of oysters, Larval cestode infection.

Scientific name or taxonomic affiliation

Unidentified metacestode species of Tylocephalum sp. in the order Lecanicepalidea.

Geographic distribution

Tropical and subtropical waters with most reports from Crassostrea virginica in eastern parts of the Gulf of Mexico (Winstead et al. 2004), but also in Hawaii and Georgia, U.S.A. (Sinderman and Rosenfield 1967) and with reports from other locations (Maryland and North Carolina, USA) attributed to oysters transplanted from the Gulf of Mexico (Lauckner 1983). In Crassostrea gigas from southern Japan and Taiwan, in Crassostrea madrasensis from India, in oysters (Saccostrea glomerata and Striostrea mytiloides) from Australia and pearl oysters Pinctada spp. from the South Pacific (Lauckner 1983).

Host species

Metacestodes (a term used for any larval form between the egg and adult cestode and also referred to as larval cestodes or acaudate glando-procercoids) reported from Crassostrea virginica, Crassostrea gigas, Crassostrea madrasensis, Saccostrea (=Crassostrea) glomerata (=commercialis =cucullata), Striostrea mytiloides (=Saccostrea =Crassostrea echinata) and Pinctada sp. as well as from other pelecypods including scallops (e.g., Argopecten irradians), clams (e.g., Mercenaria mercenaria) and molluscivorous gastropods (Cake 1976).

Impact on the host

Oysters and other pelecypods appeared to serve as primary intermediate hosts while molluscivorous gastropods as well as other marine organisms including crustaceans and fish appeared to serve as secondary intermediate or paratenic hosts. All species are thought to have elasmobranch definitive (final) host (especially demersal elasmobranchs) where the cestodes became sexually mature. Pelecypods became infected by ingesting eggs containing oncospheres released from the intestinal tract of the definitive host. Reports of oysters being infected by free-swimming coracidia (ciliated larval stages of some cestodes but possibly not lecanicehhalidean cestodes) of Tylocephalum sp. have been questioned (Lauckner 1983).

In some locations, the prevalence of infection with metacestodes can reach 100% with several hundred metacestodes per oyster (Lauckner 1983, Winstead et al. 2004). Heavy infections can cause physiological stress and may affect growth and reproduction as well as reduce oyster marketability because of the poor condition (transparent and watery consistence) of the tissues (Sparks 1985). However, some oysters with heavy parasite burdens (up to 125 metacestodes per C. virginica) may not exhibit any detrimental affects (Lauckner 1983).

Diagnostic techniques

Squash Preparations: Squash the dissected organs, especially the stomach, intestine and digestive gland, of oysters between glass plates and examine with the aid of a stereo-zoom, dissection microscope. Usually the metacestode is located within a thick-walled fiberous capsule of host tissue (Lauckner 1983). Metacestodes can be removed from the tissues intact by permitting the visceral tissue to putrefy in a Petri dish containing filtered seawater. In order to examine the morphology of the metacestodes, they should be killed in an expanded or relaxed condition achieved by emersion in tepid tap water or hot (about 50°C) AFA (80 parts 70% ethanol, 10 parts formaldehyde and 10 parts glacial acetic acid). Preserve in 70% ethanol and 5% glycerine. Stain larvae with Ehrlich's acid haematoxylin and mount in Permount using standard helminthological techniques.

Histology: Metacestodes of Tylocephalum sp. usually occur in the vesicular connective tissue near the digestive diverticula and stomach and less frequently in the connective tissues of the gills and mantle. In C. virginica, metacestodes usually elicit a host response which includes a thick encapsulation consisting of an eosinophilic intercellular fibrocyte-like inner layer that has a positive periodic acid-Schiff (PAS) reaction (Rifkin and Cheng 1968) and an outer accumulation of haemocytes (Winstead et al. 2004). Crassostrea virginica can resorbe encapsulated metacestodes indicating that this oyster species may not be a totally compatible host for Tylocephalum sp. (Cheng 1967).

Methods of control

No known methods of prevention or control. However, elasmobranchs infected with the adult stage of these cestodes must occur in the vicinity of the oysters in order to initiate the infection.


Cake, E.W. 1976. A key to larval cestodes of shallow-water, benthic mollusks of the northern Gulf of Mexico. Proceedings of the Helminthological Society of Washington 43: 160-171.

Cheng, T.C. 1967. Marine molluscs as hosts for symbioses with a review of known parasites of commercially important species. In: Russell, F.S. (ed.) Advances in Marine Biology, Vol. 5. Academic Press, London/New York. pp. xiii + 424.

Couch, J.A. 1985. Prospective study of infectious and noninfectious diseases in oysters and fishes in three Gulf of Mexico estuaries. Diseases of Aquatic Organisms 1: 59-82.

Lauckner, G. 1983. Diseases of Mollusca: Bivalvia. In: Kinne, O. (ed.) Diseases of marine animals. Volume II: Introduction, Bivalvia to Scaphopoda, Vol. 2. Biologische Anstalt Helgoland, Hamburg. pp. 477-961 (section on Cestoda, pg. 762-784).

Rifkin, E. and T.C. Cheng. 1968. The origin, structure, and histochemical characterization of encapsulating cysts in the oyster Crassostrea virginica parasitized by the cestode Tylocephalum sp. Journal of Invertebrate Pathology 10: 54-64.

Sindermann, C.J. and A. Rosenfield. 1967. Principal diseases of commercially important marine bivalve Mollusca and Crustacea. Fishery Bulletin 66: 335-385.

Sparks, A.K. 1985. Synopsis of invertebrate pathology exclusive of insects Elsevier Science Publishers B.V., Amsterdam. 423 pp.

Stephen, D. 1978. First record of the larval cestode Tylocephalum from the Indian backwater oyster Crassostrea madrasensis. Journal of Invertebrate Pathology 32: 110-111.

Winstead, J.T., A.K. Volety and S.G. Tolley. 2004. Parasitic and symbiotic fauna in oysters (Crassostrea virginica) collected from the Caloosahatchee River and estuary in Florida. Journal of Shellfish Research 23: 831-840.

Citation Information

Bower, S.M. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Cestode Parasitism of Oysters.

Date last revised: July 2009
Comments to Susan Bower

Date modified: