Stages of Dermo seen in Crassostra viginica. From The Eastern Oyster: Crassostrea viginica, Edited by V.S. Kennedy, R. I. E. Newell and A. F. Eble

MSX first appeared in Chesapeake Bay in 1958, after it ravaged oyster beds in Delaware Bay the year before. Though this single-celled organism has been identified as a protozoan, Haplosporidium nelsoni, its life cycle and means of infecting oysters remains as mysterious now as it did then. Unlike Dermo, MSX cannot be transmitted from oyster to oyster - new molecular tools, however, are now making it possible to hunt for carriers of microscopic MSX cells, in zooplankton, in fish and in bottom-dwelling organisms.

Dermo - also a protozoan, Perkinsus marinus - was first observed in Gulf Coast oysters in the 1940s, and in the lower Chesapeake in the early 50s. With the widespread movement of oysters to different areas throughout the Chesapeake, Dermo itself has inadvertently been spread as well to most harvesting areas. While recent studies have shown a higher virulence of Perkinsus in the mid-Atlantic than in Gulf waters, genetic research has revealed differences between Perkinsus in two mid-Atlantic locations - Delaware and Mobjack bays. Knowing areas where Perkinsus strains are more virulent could help state and private aquaculturists better manage around disease.

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