Dermo - Perkinsus marinus

•  A highly lethal parasite of the eastern oyster, Crassostrea virginica, but not at all harmful to humans.

•  First discovered in the Gulf of Mexico and along the southeastern US coast in the late 1940s, but has probably been present there for much longer.   From the mid-1980s into the 1990s, outbreaks of Dermo disease occurred along an expanded range – from the northern Chesapeake Bay into southern Maine.  This range extension was associated with a warming trend that was especially pronounced in winter.

•  The parasite is transmitted directly from oyster to oyster.

•  It is a water-borne parasite that infects oysters through the gut, gills and mantle surfaces – perhaps carried into the body by blood cells.  Parasite replicate in the tissues, eventually becoming very abundant and killing the oyster.

•  The parasite grows most rapidly at temperatures above about 18 to 20o C and salinities above about 15 ppt.  Infection intensities typically diminish over the winter as many parasites are killed by low temperatures.  Nevertheless, some parasites can survive in oysters at very low temperatures and salinities.  They can multiply rapidly when conditions become more favorable.

• Infections are acquired during the warm months when oysters are feeding and the infective period is longer in southern regions.

•  Selective breeding and natural selection have improved survival, but not to the extent as that observed for MSX disease.