Rickettsia-like and Chlamydia-like Organisms of Scallops


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent

Rickettsia-like and Chlamydia-like organisms.

Scientific name or taxonomic affiliation

Intracellular microcolonies of organisms belonging to the Rickettsiales.

Geographic distribution


Host species

Placopecten magellanicus, Patinopecten yessoensis, Pecten maximus, Chlamys varia, Chlamys opercularis, Argopecten irradians and Argopecten purpuratus as well as a wide variety of other marine bivalves including oysters, mussels, clams and cockles.

Impact on the host

Microcolonies in the epithelial cells of the gills and digestive gland. Infections usually of light intensity and not associated with disease. Although, rickettsiosis has been associated with mass mortalities in P. magellanicus (Gulka et al. 1983)and P. maximus (Le Gall et al. 1988), the mortalities were eventually attributed to other adverse environmental factors (Gulka and Cheng 1984, Le Gall et al. 1991). Chlamydiosis was also associated with serious disease among hatchery-reared larval and postmetamorphic A. irradians. Other bivalve species could not be infected by the rickettsia-like organism from P. magellanicus through the water column or by direct injection (Gulka and Cheng 1984).

Diagnostic techniques

Histology: Basophilic microcolonies in cytoplasm of epithelial cells of the digestive gland, kidney or gills. Infected cells may or may not be hypertrophied.

Electron Microscopy: A dimorphic species (one form about 0.8 × 1.63-2.45 µm in the epithelial and subepithelial cells of the gills, labial palps and oesophagus and a second form 1.4-4.1 × 2.7-8.2 µm in the epithelail cells of the gills and labial palps) has been reported in P. yessoensis from Uchiura Bay, Hokkaido, Japan (Friedman 1994).

Biochemical characterisation: Rickettsiales-like organisms purified from P. maximus by differential and isopycnic centrifugation on sucrose and renografin density gradients had 15 major constituent proteins with molecular weights ranging between 148 and 16 kD and 10 enzymes including acid phosphatase and catalase, which could be related to pathogenicity (Le Gall and Mialhe 1992).

Immunological Assay: Monoclconal antibodies have been produced against the gill Rickettsiales-like organisms of P. maximus by Le Gall et al. (1992) (IFREMER, Station de La Tremblade, Mus-du-Loup, B.P. 133, 17390 La Tremblade, France).

DNA Probes: Oligonucleotide primers for nucleic acid hybridisation and polymerase chain reaction (PCR), that amplified rickettsia-like organisms but not scallop DNA, were identified from a 1500 bp sequence of a cloned genomic DNA fragment extracted from purified rickettsia-like organisms from the gills of infected P. maximus (Kellner-Cousin et al. 1993).

Methods of control

No known methods of prevention or control. Most species are probably not pathogenic.


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Citation Information

Bower, S.M. (2004): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Rickettsia-like and Chlamydia-like Organisms of Scallops.

Date last revised: December 2004
Comments to Susan Bower

Date modified: