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