2012 Summer/Fall MSX Prevalence Report in the Damariscotta River Estuary, ME

The Eastern oyster, Crassostrea virginica, makes up an important commercial fishery along the East and Gulf coast of North America. Consumer demand has permitted the development of the oyster aquaculture industry. Specifically, about 70% of the total production of harvested oysters in Maine came from the Damariscotta River Estuary in 2010(DMR). Unfortunately, disease issues have become a major concern. One particular disease affecting the eastern oyster is known as MSX (multinucleated sphere unknown). MSX is caused by the protozoan parasite, Haplosporidium nelsoni, and has devastating effects on cultured and wild oysters, killing 90-95% of the oysters it infects. Until recently, MSX had not managed to reach epizootic levels in Maine. However, in the summer of 2010 MSX was responsible for significant mortality among cultured oysters in the Damariscotta River estuary and mortalities have not abated since. This study investigated the current prevalence of MSX in one commercial oyster site and two natural oyster beds located in the Damariscotta river estuary by using histology and PCR-based assay specific for H. nelsoni. Over a 3-month period (August-October 2012), a total of 316 oysters were surveyed. H. nelsoni was prevalent at every site. At some sites prevalence was as high as 50%. These results are similar to previous prevalence surveys and suggest that MSX disease has not subsided in the Damariscotta River estuary