Chlamydiosis of Scallops


Category 4 (Negligible Regulatory Significance in Canada)

Common, generally accepted names of the organism or disease agent


Scientific name or taxonomic affiliation

Chlamydial inclusions within the digestive tubule epithelia.

Geographic distribution

East coast U.S. shellfish hatcheries.

Host species

Argopecten irradians larvae and post metamorphic juveniles. The etiological agent is also suspected of infecting the digestive diverticula of older juveniles and adult A. irradians.

Impact on the host

Disruption of digestive gland epithelia, causing dysfunction of normal metabolic processes at a critical period for larval development and metamorphosis. Not normally associated with pathology in cultivated adult or wild populations but, adults may be carriers and epizootics in the hatchery may have been induced by the use of infected broodstock. It is believed that hatchery manipulation of holding temperatures and stocking densities may exacerbate infections and host stress, causing mortality.

Diagnostic techniques

Histology: Basophilic amorphous spheres within the epithelial cells of the digestive ducts and tubules.

Methods of control

Chlamydial infections of bay scallop are ubiquitous and, therefore, impossible to eradicate. The only associated pathology reported to date is limited to high stocking densities in two U.S. east coast shellfish hatcheries. Possible control measures could involve reduction of stocking densities or disposal of stocks being held in the most severely affected tanks.


Leibovitz, L. 1989. Chlamydiosis: a newly reported serious disease of larval and postmetamorphic bay scallops, Argopecten irradians (Lamarck). Journal of Fish Diseases 12: 125-136.

Morrison, C. and G. Shum. 1982. Chlamydia­like organisms in the digestive diverticula of the bay scallop Argopecten irradians. Journal of Fish Diseases 5: 173­184.

Citation Information

Bower, S.M. (1997): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Chlamydiosis of Scallops.

Date last revised: July 1997
Comments to Susan Bower

Date modified: