Unknown specific aetiological agent
Summary from the online OIE Diagnostic Manual
Three Irido-like virus infections have been reported in bivalve molluscs. Although the viruses have not yet been purified and studied in detail, they have been classified as iridoviruses because of the presence of DNA and the shape and size of their virions - 380 nm in diameter. The first cases of infection described were in the Portuguese oyster, Crassostrea angulata , in which the virus elicited necrosis of branchial tissues and infections of haemocytes. Virions develop from inclusion bodies, present in infected cells.
Iridoviruses can affect both C. angulata and C. gigas (larvae). Lethal iridoviral infections in C. angulata produce lesions of the gills, or lesions of haemocytes and connective tissues. In C. gigas , the infection is particularly responsible for the death of larvae, and it is known as oyster velar virus disease.
The geographical distribution of these diseases, which are no longer being reported, was Spain, France and Portugal. Larval mortality has affected hatcheries in Washington State on the western coast of the United States of America.