History and Impact of MSX and Dermo Diseases on Oyster Stocks In the Northeast Region

Abstract

Two other infectious oyster diseases, however, commonly known to oyster growers as “MSX” and “Dermo”, have had a much more significant and chronic effect on oyster populations in the northeastern region of the United States. The combined impact of both diseases has caused hundreds of millions of dollars in losses over the last 35–45 years in mid-Atlantic states (especially Chesapeake and Delaware Bays) and seriously threatens both natural and cultivated oyster populations in the New England states. Although both parasites are lethal to eastern oysters, they are not harmful to humans. This publication provides a current overview of both diseases by summarizing information on their natural history and distribution, methods of transmission, diagnostic techniques, environmental influences, control measures, and recommended management practices. MSX Disease Natural History and Distribution MSX disease was first recognized as the cause of massive oyster mortalities (90–95%) in lower Delaware Bay in 1957. Two years later, the disease was discovered in the lower Chesapeake Bay. The causative agent, a singlecelled parasite, Haplosporidium nelsoni, was originally given the acronym “MSX” because it was observed as a Multinucleated Sphere with unknown affinity (“X”).