Occurrence of the Oyster Parasite, Haplosporidium nelsoni, in the Damariscotta River Estuary, Maine

Haplosporidium nelsoni is a protozoan parasite that causes the devastating disease MSX in the Eastern oyster, Crassostrea virginica, along the East coast of the United States. Until recently, H. nelsoni had not reached epizootic levels in Maine. However, in the summer of 2010, H. nelsoni was responsible for significant mortalities in cultivated Eastern oysters in the Damariscotta River Estuary. Since the initial outbreak, surveys performed by the Department of Marine Resources (DMR) show that MSX is still prevalent in the Damariscotta River Estuary. The industry remains concerned about the parasite levels with each new grow out season. In this study, MSX prevalence in C. virginica was surveyed at a commercial site and two natural oyster beds in the Damariscotta River Estuary. Using the results from the prevalence study, organisms found in and around the commercial farms in the river were screened for the parasite in search of an intermediate or reservoir host(s) of the parasite. Over a 3-month period (August-October 2012), a total of 316 oyster samples were surveyed using histology and a PCR-based assay. H. nelsoni was prevalent at every site. At some sites prevalence was as high as 50%. Using a qPCR assay developed for H. nelsoni, the local organisms found in and around the oyster farms were screened for the presence of the parasite. The majority of organisms tested were negative for the parasite, although weak signal (mean Cq 26-33) of H. nelsoni was observed in some tunicates, gastropods and arthropods. Intermediate levels of infection were observed in a very small percentage of tunicate species (3.4%)