Viral Gametocytic Hypertrophy of Oysters


Category 2 (In Canada and of Regional Concern)

Common, generally accepted names of the organism or disease agent

Viral gametocytic hypertrophy, VGH, Ovacystis disease.

Scientific name or taxonomic affiliation

Pathology has been associated with a papilloma-like virus (also referred to as papova-like and polyoma-like viruses) with morphological characteristics similar to those of the Papillomaviridae and Polyomarviridae families.

Geographic distribution

InCrassostrea virginica from Atlantic Canada, the Eastern U.S. and the northeastern Gulf of Mexico coast of Florida. Similar lesions reported from other species of oysters from the west coast of the U.S., Korea and Japan.

Host species

Crassostrea virginica, and unconfirmed reports in Crassostrea gigas, Saccostrea (=Crassostrea) commercialis, Crassostrea rhizophorae and Ostrea conchaphila (=Ostrea lurida). Other bivalves including Pinctada maxima and Mya arenaria were reported to be infected by papova-like viruses.

Impact on the host

Causes massive hypertrophy of individual gametes and gametogenic epithelium by replicating in the host cell nucleus. Host response to infection is negligible to a light to moderate haemocyte infiltration into localised areas adjacent to infected gametes. Level of infection generally low (usually fewer than five infected cells per 6 µm tissue section) but prevalence of infection up to 80% occur sporadically (McGladdery 1999). In Atlantic Canada, this infection is also found in immature oysters where germinal cells are infected (McGladdery et al. 1993). Despite the massive hypertrophy and tissue displacement surrounding infected cells, no adverse effect on oyster health or fecundity, and no indication of associated mortality was detected (McGladdery 1999). Experimental investigation is required to determine if the virus could adversely affect hatchery broodstock productivity (McGladdery et al. 1993).

Diagnostic techniques

Histology: Examine transverse sections through the gonad and observe abnormally enlarged basophilic (with haematoxylin and eosin stain) or Feulgen positive hypertrophic ova (up to 500 µm in diameter), sperm and germinal cells. Hypertrophied ova had scant cytoplasm and granular nuclear-inclusions that stained Feulgen-positive indicating the presence of DNA.

Electron Microscopy: Presence of icosahedral, non-enveloped viral particles measuring 43-55 nm in diameter in the nucleus of hypertrophied gonadal cells.

Methods of control

No known methods of prevention or control.


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Citation Information

Bower, S.M. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Viral Gametocytic Hypertrophy of Oysters.

Date last revised: February 2010
Comments to Susan Bower

Date modified: