Rickettsia-like and Chlamydia-like Organisms of Clams and Cockles


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 organisms belonging to the Rickettsiales.

Geographic distribution


Host species

Many species of clam including Cerastoderma edule, Donax trunculus, Mercenaria mercenaria, Mya arenaria, Mactromeris polynyma, Venerupis (=Tapes) philippinarum, Tapes (=Ruditapes) decussatus, Siliqua patula, Tapes pullastra Tellina tenuis, Tridacna crocea, Protothaca staminea, Clinocardium nuttali, Venerupis obscurata, and Hippopus hippopus as well as a wide variety of marine molluscs including oysters, mussels and scallops.

Impact on the host

Microcolonies occur in the epithelial cells of the gills and digestive gland. Infections are usually light and usually not associated with disease. However, extensive inflammation of gill tissue and mortalities in cultured H. hippopus were associated with heavy gill infections of Rickettsiales-like organisms in the Philippines and Micronesia.

Diagnostic techniques

Histology: Microcolonies in the cytoplasm of epithelial cells which may or may not be hypertrophied. Extensive inflammation was associated with gill infections of H. hippopus.

Figure 1. Histological section through the digestive gland of Tapes philippinarum with a colony of Rickettsia-like or Chlamydia-like organisms (arrow) in an epithelial cell. Note the low intensity of infection (only one colony in this field). Haematoxylin and eosin stain.

Figure 2. Higher magnification of Fig. 1 showing the Rickettsia-like or Chlamydia-like colony (C) in the cytoplasm adjacent to the nucleus (N) of the epithelial cell. Haematoxylin and eosin stain.

Figure 3. Two basophilic rod-shaped colonies (B) next to a Richettsia-like or Chlamydia-like colony (R) in the gills of Protothaca staminea. Note the size difference in individuals in each type of colony. Both types of colonies cause distortion of the host cell but there was no apparent host response. Haematoxylin and eosin stain.

Methods of control

No known methods of prevention or control. It was suggested that overcrowding and low exchange rates of water in land based culture tanks predisposed H. hippopus to increased intensity of infection, clinical disease, and mortalities.


Carballal, M.J., D. Iglesias, J. Santamarina, B. Ferro-Soto and A. Villalba. 2001. Parasites and pathologic conditions of the cockle Cerastoderma edule populations of the coast of Galicia (NW Spain). Journal of Invertebrate Pathology 78: 87-97.

Comps, M. and R. Raimbault. 1978. Infection rickettsienne de la glande digestive de Donax trunculus Linné. Sciences et Pêches, Bulletin de l'Institute des Pêches Maritimes 281: 11-12. (In French).

Comps, M. 1982. Étude morphologique d'une infection rickettsienne de la palourde Ruditapes philippinarum Adam and Reeves. Revue des Travaux de l'Institut des Pêches Maritimes 46: 141-145. (In French.).

Comps, M. 1983. Infections rickettsiennes chez les mollusques bivalves des côtes francaise. Rapports et Procès-verbaux des Réunions du Conseil International pour l'Exploration de la Mer 182: 134-136. (In French).

Elston, R.A. 1986. Occurrence of branchial rickettsiales-like infection in two bivalve molluscs, Tapes japonica and Patinopecten yessoensis, with comments on their significance. Journal of Fish Diseases 9: 69-71.

Harshbarger, J.C., S.C. Chang and S.V. Otto. 1977. Chlamydiae (with phages), mycoplasmas, and rickettsia in Chesapeake Bay bivalves. Science 196: 666-668.

MacCallum, G.S., J. Blackbourn, S.E. McGladdery, S.M. Bower and J.T. Davidson. 2001. Disease issues relevant to the culture of shellfish in Atlantic and Pacific Canada. Bulletin of the Aquaculture Association of Canada 101-3: 5-12.

Marshall, W.L., S.M. Bower and G.R. Meyer. 2003. A comparison of the parasite and symbiont fauna of cohabiting native (Protothaca staminea) and introduced (Vennerupis philippinarum and Nuttalia obscurata) clams in British Columbia. Journal of Shellfish Research 22: 185-192.

Meyers, T.R. 1979. Preliminary studies on a chlamydial agent in the digestive diverticular epithelium of hard clams Mercenaria mercenaria (L.) from Great South Bay, New York. Journal of Fish Diseases 2: 179-189.

Norton, J.H., M.A. Shepherd, M.R. Abdon-Naguit and S. Lindsay. 1993. Mortalities in the giant clam Hippopus hippopus associated with rickettsiales-like organisms. Journal of Invertebrate Pathology 62: 207-209.

Citation Information

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

Date last revised: May 2004
Susan Bower

Date modified: