Intracellular Bacterial Disease of Scallops


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Intracellular bacterial disease.

Scientific name or taxonomic affiliation

Unidentified intracellular prokaryote, possibly a Mycoplasma or mycobacterium.

Geographic distribution

British Columbia, Canada.

Host species

Patinopecten yessoensis.

Impact on the host

Laboratory studies indicate that the aetiological agent infects the haemocytes and pathology is inversely dose dependant. Scallops (7-12 cm in shell height) inoculated intramuscularly with high concentrations of lesion homogenates from diseased scallops usually die with overwhelming infections 2-4 weeks after injection. These acute infections are often complicated by secondary septicemia. Injections of low concentrations resulted in a 2 month prepatent period and presence of pinkish-orange pustules up to 10 mm in diameter in the adductor muscle. Lesions observed histologically resembled those observed in scallops from six grow-out localities that experienced poor growth and high mortalities in 1989. This disease appears to be associated with stress induced by inappropriate scallop culture practices.

Diagnostic techniques

Gross Observations: Nonspecific indication of the disease is the presence of pinkish-orange pustules up to 10 mm in diameter in the soft tissues and possibly lack of growth (appearing like erosion) and concholin deposition in patches along the edge of the shell. The shell damage was initially attributed to the scallops "biting" each other or the culture apparatus but is probably caused by lesions that occur on the edge of the mantle.

Figure 1. Patinopecten yessoensis (upper valve removed) with gross signs typical of intracellular bacterial disease obtained from a commercial grow-out site. Note the "biting" damage to the edge of the shell (S) and the large pustule (P) in the centre of the adductor muscle.

Histology: Lesions occur in the connective tissues of all organs and vary in structure from irregular patches of infiltration, often with a core of necrosis, to encapsulated patches of haemocytes that usually contain necrotic cells. Characteristically there are few if any extracellular bacteria in the lesions. Oil immersion (1000 x) may reveal intracellular prokaryotes in a few of the haemocytes within the lesions.

Figure 2. Histological section at low magnification (10 x objective) of a large pustule in the adductor muscle of Patinopecten yessoensis with intracellular bacterial disease. The nuclei of numerous infiltrating haemocytes appear as purple spots in this figure. More normal looking muscle tissue occurs at the bottom of the figure. Haematoxylin and eosin stain.

Figure 3. A partially encapsulated lesion (between arrows) adjacent to a massive diffuse pustule in the adductor muscle of Patinopecten yessoensis. As for Fig. 2, the magnification is low (10 x objective). Haematoxylin and eosin stain.

Figure 4. Histological section at low magnification (10 x objective) of a large pustule (between arrows) in the digestive gland adjacent to the stomach wall of Patinopecten yessoensis. Haematoxylin and eosin stain.

Figure 5. Partially encapsulated pustule (arrows) in the gonad adjacent to the intestine of a male Patinopecten yessoensis. Note the various stages of haemocyte necrosis within the centre of the pustule. Haematoxylin and eosin stain.

Figure 6. Encapsulated small pustule (between arrows) in the connective tissue among the tubules of the digestive gland of Patinopecten yessoensis. Haematoxylin and eosin stain.

Figure 7. Histological section at higher magnification (100 x oil immersion objective) of cells within a pustule of Patinopecten yessoensis. Note haemocytes at various stages of necrosis and a haemocyte (M) that appears to be engulfing necrotic tissue. A few basophilic particles within the cytoplasm of a haemocyte are reminiscent of bacteria (B). Apart from these few "bacteria" pustules appear aseptic. Haematoxylin and eosin stain.

Electron Microscopy: Depending on the stage of infection, Mycoplasma-like prokaryotes can be observed within a few haemocytes associated with the lesions.

Figure 8. Electron micrograph showing several prokaryotes (arrows) within and beside a necrotic haemocyte from a lesion in the tissue of Patinopecten yessoensis. Uranyl acetate and led citrate stain.

Figure 9. Electron micrograph, from a lesion in the tissue of Patinopecten yessoensis, showing a healthy haemocyte (N indicates its nucleus) which appears to have engulfed a necrotic haemocyte that is infected with prokaryotes (arrows). Uranyl acetate and led citrate stain.

Methods of control

No known methods of prevention or control.


Bower, S.M. and G.R. Meyer. 1991. Disease of Japanese scallops (Patinopecten yessoensis) caused by an intracellular bacterium. Abstract. Journal of Shellfish Research 10(2): 513.

Bower, S.M., J. Blackbourn, G.R. Meyer and D.J.H. Nishimura. 1992. Diseases of cultured Japanese scallops (Patinopecten yessoensis) in British Columbia, Canada. Aquaculture 107: 201-210.

Bower, S.M. and G.R. Meyer. 1994. Causes of mortalities among cultured Japanese scallops, Patinopecten yessoensis, in British Columbia, Canada. In: Bourne, N.F., B.L. Bunting and L.D. Townsend (eds), Proceedings of the 9th International Pectinid Workshop, Nanaimo, B.C., Canada, April 22-27, 1993. Canadian Technical Report of Fisheries and Aquatic Science 1994: 85-94.

Contact Susan Bower, Pacific Biological Station, DFO, Nanaimo, BC, Canada V9R 5K6 for further information.

Citation Information

Bower, S.M. (1998): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Intracellular Bacterial Disease of Scallops.

Date last revised: December 2002
Comments to Susan Bower

Date modified: