Haplosporidiosis is caused by two protistans of the phylum Haplosporidia: Haplosporidium nelsoni (= Minchinia nelsoni )(10) and H. costale (= Minchinia costalis )(11). H. nelsoni is commonly known as MSX (multinucleate sphere X). Similarly, H. costale is commonly known as SSO (seaside organism).
Crassostrea virginica is infected by both H. nelsoni and H. costale (4). H. nelsoni also infects Crassostrea gigas (8).
The geographical distribution of H. nelsoni (1, 6) is north Florida to Massachusetts and Maine, United States of America (USA) (1, 6). Enzootic areas are described as limited to Delaware Bay with occasional epizootics in Chesapeake Bay, Long Island area, and Cape Cod. H. nelsoni was also reported from Crassostrea gigas in California and Washington State, USA (8) and was detected in Korea (7). H. costale is reported in Long Island Sound, New York to Cape Charles, Virginia, USA.
H. nelsoni infects haemocytes, connective tissue and digestive gland epithelia. It is often accompanied by brownish-red discoloration of gills and mantle. Sporulation of H. nelsoni is prevalent in juvenile oysters and sporadic in adult oysters (2). Spores of MSX occur in the epithelial cells of the digestive tubules; spores of SSO occur in the connective tissues. H. nelsoni infections appear in summer and cause a gradual disruption of digestive tubule epithelia. Unlike H. costale , there is no spore formation in the connective tissue and sporulation is not associated with any marked seasonal peak in mortality. In the Northern Hemisphere, infection by H. nelsoni takes place between mid-May and the end of October. Mortalities may commence early in the spring. However, mortalities from new or recurrent infections occur throughout the summer and peak in August-September. The disease (MSX) is restricted to salinities over 15 ppt (parts per thousand); rapid and high mortalities occur at 20 ppt (9). There is some evidence that water temperatures exceeding 20°C may cause the disease to disappear; 2 weeks in water of 10 ppt salinity at 20°C kill the parasite but not Crassostrea virginica . H. costale causes a seasonal mortality between May and June.
It has not been possible to transmit the disease experimentally in the laboratory. The Haplosporidium spp. life-cycle is unknown, but an intermediate host is suspected (5).