Extracellular Giant "Rickettsiae" of Oysters


Category 1 (Not Reported in Canada)

Common, generally accepted names of the organism or disease agent

Extracellular giant "rickettsiae", Pleomorphic basophilic vesicles (PBV), Hyperplastic growths, Multiple tumor-like growths on gills.

Scientific name or taxonomic affiliation

Pleomorphic prokaryotic microorganism but not in the family Rickettsiales.

Geographic distribution

Atlantic coast of Spain.

Host species

Crassostrea gigas.

Impact on the host

Disappearance of apical microvilli and cilia with concomitant lysis of gill epithelial cells. Multiple tumor-like growths on the gill lamellae. Prevalence varied between 0 and 13.3%, infection had no association with season, and mortalities were not significant.

Diagnostic techniques

Gross Observations: Tumor-like growths that appear as papillary fleshy masses with a rough surface are up to 10 mm in width and 5 mm in height on the gill surface. They may also appear as whitish elevations of the surface, spreading from the proximal to distal margin and as sparce patches with whitish protuberances. Most affected oysters had one to several growths on different lamellae.

Histology: Numerous anucleate pleomorphic microorganisms (5-7 µm in length but 9 µm in some sections) among the cilia and in contact with the apical region of the ciliated gill epithelium. (Although not intracellular, the designation "rickettsia" was initially chosen for PBV due to the pleomorphnic nature of the organism which are very large, irregularly shaped cells and have prokaryotic ultrastructural characteristics.) Wide areas of the hyperplastic epithelium may be lined with PBV and adjacent connective tissue was heavily infiltrated by haemocytes. Some areas of hyperplastic epithelium that was lined with PBV or in close proximity to lined areas may be in the process of degeneration.

Electron Microscopy: Internal features are ribosome-like particles and DNA strands which are distinguishable in some light areas. Some cells show one or two vacuole-like dense inclusions formed by curved concentric bands about 4 nm thick separated by a 5 nm light surrounding space (myelin-like bodies).

Methods of control

No known methods of prevention or control.


Azevedo, C. and A. Villalba. 1991. Extracellular giant rickettsiae associated with bacteria in the gill of Crassostrea gigas (Mollusca, Bivalvia). Journal of Invertebrate Pathology 58: 75-81.

Elston, R.A. 1993. Infectious diseases of the Pacific oyster Crassostrea gigas. Annual Review of Fish Diseases 3: 259-276.

Villalba, A., C. Azevedo and C. Rodriguez. 1993. Occurrence of multiple hyperplastic growths on the gills of Pacific oyster, Crassostrea gigas and their relationship with associated pathologic conditions. Journal of Invertebrate Pathology 61: 296-302.

Citation Information

Bower, S.M. (1996): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Extracellular Giant "Rickettsiae" of Oysters.

Date last revised: September 1996
Comments to Susan Bower

Date modified: