DOAG: Oyster & Clam Diseases

Bureau of Aquaculture


Oyster and Clam Diseases

“There are several diseases that can have an impact on Connecticut’s oyster and clam populations, none of which are known to be harmful to humans.  In 1997 and 1998 two of these diseases, Dermo and MSX, resulted in a major oyster die-off.”



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Oyster Diseases:

Factors Affecting the Health of Oysters Oysters are long-lived, sessile animals, which feed by filtering large quantities, up to 100 gallons, of seawater per day. They accumulate hundred folds of micro-organisms and pollutants.  These characteristics make them susceptible to diseases. Several factors can cause pathological changes in oysters. Different factors affect them during their planktonic, larval stage. The sum of environmental stimuli, together with the genetic make up of the oysters, will determine their likelihood to get ill.


Dermo Disease - Dermo disease is caused by a single-celled Protozoan parasite, Perkinsus marinus. Originally, it was thought to be caused by a fungus and named Dermocystidium marinum. Even after the reclassification the disease is commonly called “Dermo”


MSX - (Multinucleated Sphere Unknown) disease is caused by a single-celled Protozoan parasite, Haplosporidium nelsoni.  MSX is lethal to the eastern oyster (Crassostrea virginica), but it is not known to be harmful to humans.


Roseovarius Oyster Disease (ROD), previously known as Juvenile Oyster Disease (JOD), affects hatchery-raised seed of eastern oysters, Crassostrea virginica, on the east coast of the U.S. from Maine to New York. The disease is caused by a marine a–proteobacterium Roseovarius crassostreae, a member of the Roseobacter clade.



Clam Diseases:

QPX (Quahog Parasite Unknown) is a Protozoan parasite of hard clams, Mercenaria mercenaria. DNA analysis places the QPX in marine fungus-like protists (Labyrinthomorpha, Thraustochytriales). Organisms from this group occur commonly in marine and estuarine environments.