• A highly lethal parasite of the eastern oyster, Crassostrea virginica , but not at all harmful to humans.

• Introduced into the eastern United State sometime before 1957, when the first recorded outbreak occurred in Delaware Bay , resulting in massive oyster mortalities. Two years later, similarly heavy mortalities occurred in lower Chesapeake Bay. The parasite was originally referred to as MSX because, in histological section, it appeared as a Multinucleated Sphere of unknown (X) affiliation. The disease it causes in oysters is referred to as MSX disease.

• The method of introduction is unknown. Possibilities include the unauthorized movement of its presumed natural host, the Pacific oyster, Crassostrea gigas , from the Pacific. Alternatively, it could have been introduced via shipping from Pacific to Atlantic ports, which greatly increased after (and probably during) World War II.

• It has now been found from Nova Scotia , Canada to Florida in the United States , but mortalities of oysters predominate in the mid Atlantic and Northeastern US .

• The life cycle and means of transmission are unknown. Experimental transmission has not been successful and proximity to infected oysters is not needed for the acquisition of infections. The spore stage is presumed to be involved in transmission, but it is rare in adult oysters. Another host as been hypothesized, but never found.

• It is a water-borne parasite that infects oysters through the gills, then spread into the rest of the body.

• Salinity is the most important environmental control on the parasite. It is absent or inactive at less than 10 psu salinity and becomes very active at 15 psu or greater.

• The major infective period occurs between May and July, with a time delay from south to north, but some infections can be acquired as late as October.

• Selective breeding has produced strains of eastern oysters highly resistant to MSX-disease-caused mortality. Natural populations of oysters in some regions have also become resistant through natural selection.