Oyster Velar Virus Disease (OVVD)
Category 1 (Not Reported in Canada)
Common, generally accepted names of the organism or disease agent
Oyster velar virus disease, OVVD, Blister disease.
Scientific name or taxonomic affiliation
Icosahedral DNA virus thought to be an irido-like virus but affiliations with the morphologically similar viruses that cause gill disease in Crassostrea angulata and haemocyte infection virus disease (HIV) are unknown.
Washington State, USA (diagnosed in two shellfish hatcheries but believed to be widespread including virtually all temperate waters of the world where this oyster is found). Reports by the industry of the occurrence of this disease in British Columbia have not been confirmed.
Crassostrea gigas midstage umbo larvae more than 150 µm in shell height.
Impact on the host
Viral infection of the velar epithelium, can cause severe hatchery losses (nearly 100%) in affected tanks. Usually appears in March to May, but may also occur throughout the summer. The effect of subclinical exposure to this disease on subsequent remote settings of the larvae and grow-out of the seed is not known.
Wet Mounts: Larvae greater than 150 µm in shell length and more than 10 days of age (when cultured at 25 to 30 °C) demonstrate virus infected cells on the velum. Infected cells are hypertrophied and devoid of cilia causing the "blisters" which characterise the gross appearance of this disease. Inclusion bodies (1.2 to 2.4 µm in diameter) within infected cells stain green with acridine orange. The entire velum may also lose its cilia, however, this is not a feature specific to OVVD.
Histology: Ciliated velar epithelium with intracytoplasmic inclusion bodies (1.2-2.4 µm diameter) which are spherical, dense, Feulgen positive, and basophilic in early stage infections but become irregular and less basophilic as virions form. Inclusion bodies occasionally occur in velar-supporting esophageal and oral epithelia and rarely in mantle epithelia. Electron microscopy confirmation of viral presence is recommended where experience with such infections is limited.
Electron Microscopy: Velar epithelial cells with viroplasm (observed as cytoplasmic inclusion bodies in histology) that form the viral particles. The viroplasms may be adjacent to the cell nucleus, especially early in the virogenic process, and may cause an indentation in the host cell nucleus. Viral capsids (particles), with icosahedral symmetry (228 ± 7 nm diameter) and with a capsid consisting of two bilayered membranes, are formed on the periphery of the viroplasm and mature in the host cell cytoplasm. Complete viral particles have a dense inner core separated from the capsid by a moderately dense zone.
Methods of control
Larvae known to be infected with the virus should be destroyed and discarded in an approved manner. No known control methods other than quick recognition of the problem by hatchery personnel followed by immediate disinfection of tanks and equipment to prevent tank-to-tank spread. The source of the virus is not known but has been speculated to be from the broodstock. Thus, the tracking of broodstock groups or individuals may eventually assisting in understanding the source of infection for the future implementation of management techniques.
Elston, R.A. 1979. Virus-like particles associated with lesions in larval Pacific oysters (Crassostrea gigas). Journal of Invertebrate Pathology 33: 71-74.
Elston, R. 1980. Ultrastructural aspects of a serious disease of hatchery reared larval oysters, Crassostrea gigas Thünberg. Journal of Fish Diseases 3: 1-10.
Elston, R.A. 1993. Infectious diseases of the Pacific oyster, Crassostrea gigas. Annual Review of Fish Diseases 3: 259-276.
Elston, R. 1997. Special topic review: bivalve mollusc viruses. World Journal of Microbiology and Biotechnology 13: 393-403.
Elston, R.A. 1999. Health management, development and histology of seed oysters. Chapter 9. Oyster Velar Virus Disease (OVVD). pp. 67-69. World Aquaculture Society, Baton Rouge, Louisiana, USA. 110 pp.
Elston, R.A. and M.T. Wilkinson. 1985. Pathology, management and diagnosis of oyster velar virus disease (OVVD). Aquaculture 48: 189-210.
Bower, S.M. (2001): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Oyster Velar Virus Disease (OVVD).
Date last revised: March 2001
Comments to Susan Bower
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