The protozoan parasites Haplosporidium nelsoni, popularly known as MSX, and Perkinsus marinus, popularly known as Dermo, are serious pathogens of oysters in the Chesapeake Bay. MSX first appeared in Chesapeake Bay in 1959 and in the early 1960s killed millions of bushels of oysters on lower Bay oyster grounds. The continued presence of the parasite has discouraged use of these prime-growing areas since that time.
Because of the detrimental effect of these diseases on the Virginia oyster industry and the significant variation in disease abundance that can occur from year to year, the Virginia Institute of Marine Science has been monitoring the prevalence of both parasites since 1960. Each year disease surveys of the major oyster populations in Virginia are sampled in the fall. More intensive monthly surveys have been made on four oyster reefs in the upper James River since 1987. These surveys have helped us gain important information about the epizootiology of these oyster pathogens. Information on disease severity and distribution each year is provided to management agencies and the oyster industry through publications and special advisories of the Marine Advisory Service office. The results of disease monitoring are can be found in our annual monitoring reports.