DOAG: Oyster & Clam Diseases

Bureau of Aquaculture

Oyster and Clam Diseases

The Bureau of Aquaculture monitors oyster and clam diseases during annual surveys, develops disease management guidelines, develops disease-resistant oyster strains in selective breeding programs and performs scientific research in collaboration with universities and government laboratories.     

Reseach projects target all bivalve species of interest to CTís shellfish industry; the eastern oyster, Crassostrea virginica, northern quahog, Mercenaria mercenaria, the blue mussel, Mytilus edulis, the soft-shell clam, Mya arenaria and the bay scallop, Argopecten irradians. 

The following economically-important infectious oyster diseases are present in CT; MSX (Multinucleated Sphere Unknown, due to Haplosporidium nelsoni), SSO (Seaside Organism, due to Haplosporidium costale), Dermo (due to Perkinsus marinus) and ROD (Roseovarius Oyster Disease, due to Roseovarius crassostreae).  The clam disease QPX (Quahog Parasite Unknown, due to a Labyrinthomorpha) is detected in quahogs. 



         MSX                    SSO                 Dermo                    ROD                      QPX   


MSX occurs in cyclic prevalences (%) and has caused high mortalities in CT in the past.  The last significant mortality event was recorded in 1997.   During recent years, the activity of MSX has declined in Long Island Sound, as well as in other large east coast estuaries. In 2012 not a single MSX-positive oyster was detected.  MSX occurs in CT as a co-infection with another haplosporidian parasite, SSO.

{MSX Prevalence}


Table 1.  MSX prevalence in CT

The intensities (number of parasites in an oyster) of Dermo are low and not associated with significant mortalities in market size oysters.

  {Dermo prevalence}

Table 2.  Dermo prevalence in CT     

{Dermo intensity in CT}                        

 Table 3.  Dermo intensity in CT

ROD has affected hatchery-raised oyster seed in the past however,  resistant oyster seed is available.  QPX has been detected in 0.3% of CTís quahogs (six positive cases) and is not considered to pose a threat to the clam industry.              

Oyster Disease Fact Sheets (Click the title in blue to open a complete .pdf Fact Sheet):

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 ďDermoĒ

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 aĖproteobacterium 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.