Perkinsus karlssoni of Scallops


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Scallop Perkinsus disease.

Scientific name or taxonomic affiliation

Perkinsus karlssoni. However, the identity of this parasite is in question. It was proposed that stages from several organisms were combined to create the description and life cycle of P. karlssoni. Thus, Goggin et al. (1996) indicated that this parasite should not be considered as a species of Perkinsus. Although no longer considered to be a valid species, the identity of the parasite encountered in bay scallops by Karlsson (1991), McGladdery et al. (1991, 1993) and Whyte et al. (1993, 1994) remains unknown (McGladdery et al. 2006).

Geographic distribution

Atlantic coast of Canada and eastern United States.

Host species

Argopecten irradians.

Impact on the host

Tissue lesions and mortalities in broodstock being conditioned for spawning in warm (20 °C) sea water. Parasites spread throughout the body but are principally located between basal membrane and epithelial ducts, intestine and stomach, as well as in the connective tissue of the digestive gland, gonad, and mantle. Proliferation results in disruption of the mantle and digestive epithelia, as well as haemocyte infiltration. This host response ranged from focal encapsulation by haemocytes to formation of abscesses filled with ceroid-containing cells. Timing of proliferation appears to coincide with scallop gamete maturation (when water temperatures exceed 20 °C) and the effect of the parasite is difficult to distinguish from normal post-spawning mortality, characteristic of bay scallops. Transmission of P. karlssoni from adult to larvae can occur during spawning which may explain the persistence of this disease for at least four generations of A. irradians held in quarantine. The detection of this parasite in open-water cultured A. irradians without associated mortalities suggests that this parasite may not have a significant adverse effect on scallop survival during grow-out (Ball and McGladdery 2001).

Diagnostic techniques

Histology: Focal swirl-like haemocyte encapsulation with the formation of abscesses containing necrotic haemocytes, parasites (trophozoites and schizonts), and ceroid bodies (brown cells). Parasite visibility is frequently masked by the heavy, ceroid cell aggregations. These lesions occur in a range of host tissues including mantle, digestive gland, intestinal wall, gill and gonad.

Culture: Examine tissues which have been placed in Fluid Thioglycollate Medium for approximately 120 h for Lugol positive prezoosporangia (diameter of 80-138 µm).

Methods of control

No known methods of prevention or control. Scallops from areas with records of the disease should not be imported into areas with no record of P. karlssoni.


Ball, M.C. and S.E. McGladdery. 2001. Scallop parasites, pests and diseases: implications for aquaculture development in Canada. Bulletin of the Aquaculture Association of Canada 101-3: 13-18.

Bower, S., E. Burreson and K. Reece. 2003. Annex 10: Review of molecular techniques used to differentiate the various species/isolates of Perkinsus. Report of the Working Group on Pathology and Diseases of Marine Organisms, Aberdeen, UK, 11-15 March 2003. Mariculture Committee, ICES CM 2003/F:03, Ref. ACME, pg. 54-60 ( for electronic version see: (pg 60 of 101)).

Goggin, C.L., S.E. McGladdery, S.K. Whyte and R.J. Cawthorn. 1996. An assessment of lesions in bay scallops Argopecten irradians attributed to Perkinsus karlssoni (Protozoa, Apicomplexa). Diseases of Aquatic Organisms 24: 77-80.

Karlsson, J.D. 1991. Parasites of the bay scallops, Argopecten irradians (Lamarck, 1819). In: S.E. Shumway and P.A. Sandifer (eds.). An International Compendium of Scallop Biology and Culture. World Aquaculture Society. Baton Rouge, p. 180-190.

McGladdery, S.E., B.C. Bradford and D.J. Scarratt. 1993. Investigations into the transmission of parasites of the bay scallop, Argopecten irradians (Lamarck, 1819), during quarantine introduction to Canadian waters. Journal of Shellfish Research 12: 49-58.

McGladdery, S.E., R.J. Cawthorn and B.C. Bradford. 1991. Perkinsus karlssoni n. sp. (Apicomplexa) in bay scallops Argopecten irradians. Diseases of Aquatic Organisms 10: 127­137.

McGladdery, S.E., S.M. Bower and R.G. Getchell. 2006. Diseases and parasites of scallops. In: Shumway, S.E., G.J. Parsons (eds.) Scallops: Biology, Ecology and Aquaculture, Second ed. Elsevier B. V., Amsterdam. pp. 595-650.

Whyte, S.K., R.J. Cawthorn, R.J. MacMillan and B. Despres. 1993. Isolation and purification of developmental stages of Perkinsus karlssoni (Apicomplexa: Perkinsea), a parasite affecting bay scallops Argopecten irradians. Diseases of Aquatic Organisms 15: 199-205.

Whyte, S.K., R.J. Cawthorn and S.E. McGladdery. 1994. Co-infection of bay scallops Argopecten irradians with Perkinsus karlssoni (Apicomplexa, Perkinsea) and an unidentified coccidian parasite. Diseases of Aquatic Organisms 18: 53-62.

Citation Information

Bower, S.M. (2010): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Perkinsus karlssoni of Scallops.

Date last revised: November 2010
Comments to Susan Bower

Date modified: