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Document Type

Book Chapter


Virginia Institute of Marine Science


Fisher, William S., ed.

Publication Date


Book Title

Disease Processes in Marine Bivalve Molluscs


American Fisheries Society


Bethesda, Maryland


American Fisheries Society Special Publication 18


For 30 years, the pathogen Haplosporidium nelsoni (MSX) has been causing serious mortalities of eastern oysters Crassostrea virginica in the Delaware and Chesapeake bays of the eastern USA. Its life cycle is largely unknown, and methods for control are wanting. Breeding of resistant eastern oyster strains, at this time, offers the best hope for some degree of control of the disease. Although haplosporidians are known by their spores, controlled transmission, with one possible exception, has not been achieved in any of the 30 recognized species. Haplosporidium nelsoni rarely sporulates in eastern oysters, and this and other observations led to early speculation that another host probably exists. Based on apparent effects of environment on H. nelsoni abundance, particularly in Delaware Bay, we are attempting to profile a hypothetical alternate host. Recent increases in abundance and activity of MSX in Chesapeake and Delaware bays and in Virginia rivers are associated with droughts, but this is not true elsewhere. Distribution of H. nelsoni along the Atlantic coast appears to have spread slowly southward from Chesapeake and Delaware bays, but is not identified with eastern oyster mortalities in southern localities. To the north of the original epizootics, H. nelsoni has been scattered along the Long Island, Connecticut, and Massachusetts coasts for 30 years or more, usually without occurrence of serious mortalities of eastern oysters. Foci of increasing H. nelsoni activity with mortality, over the past 5 years, are of great concern in these areas. Careful examination of changes in these northern areas may contribute significantly to our understanding of the relationships between H. nelsoni and its environment.

Uncertainties and speculations about the life cycle of the eastern oyster pathogen Haplosporidium nelsoni (MSX)