Oyster Disease
Since the 1950’s, two infectious and fatal oyster diseases caused by parasitic protozoans, MSX (Haplosporidium nelsoni) and Dermo (Perkinsus marinus), have been plaguing Gulf Coast and mid-Atlantic regions and have been steadily moving north since that time. In 1957, MSX caused the die-off of 90-95% of the population of oysters in lower Delaware Bay and was detected as far up as the Damariscotta River, Maine by 1984 (Ewart and Ford 1993). In similar fashion to other regions, MSX has been responsible for major dieoffs in New Hampshire oysters, with the first documented kill occurring mainly in the Piscataqua River in 1995. At present, this die-off has left less than 10 reefs in the estuarine system, with the largest reef totaling approximately 35 acres (0.14 km2).

The life cycle and transmission of MSX has remained a scientific mystery. When tissue from infected oysters was first examined, the protozoan was not yet identified and called “MSX” or “Multinucleated Sphere X”. Later, it was identified as Haplosporidium nelsoni, a single-celled protozoan. The disease proliferates during periods of high salinity and temperature between the months of May and September, depending on geographic location. Oysters pump water through a filter in the gill chamber, where early infections are found, and scientists have concluded that the infective stage of this parasite is water-borne. Interestingly, the infective stage has not been determined and laboratory transmission of MSX from infected oyster to uninfected oyster has never been accomplished (Ewart and Ford 1993). Because of the severity of the problem, scientists from various institutions, like Rutgers University and the Virginia Institute of Marine Science, have been searching for answers about the connections between the two parasites and oysters.

References
J.W. Ewart and S.E. Ford. History and Impact of MSX and Dermo Diseases on Oyster Stocks in the Northeast Region. Northeastern Regional Aquaculture Center. NRAC Fact Sheet No. 200-1993.
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