Dermo disease is caused by a pathogenic marine parasite Perkinsus marinus. It was the source of massive mortalities in Delaware Bay's oyster population in 1990 and 1991 and continues to be a problem at present. It was first discovered in the Gulf of Mexico in the late 1940s after oil companies were blamed for high mortalities in marketable oysters. It is prevalent in the Gulf of Mexico and along the Atlantic coast. Dermo persists as far north as Cape Cod, MA.

    Dermo is greatly affected by both temperature and salinity. The disease exhibits its lowest intensity in Delaware Bay around the month of May, and increases intensity as temperatures rise. Proliferation of the parasite is greatest when warm summer temperatures exceed 20°C. Peak periods of infection and mortality occur in August and September. The highest infections and mortalities tend to be in higher salinity water with prevalence and intensity decreasing as salinity decreases. Abundant rainfall in spring and summer decreases the infection intensity and prevalence of the parasite. Warm, dry winters along with hot, dry springs and summers can have a major impact on the mortality of market size oysters.

    The monthly Dermo Monitoring program has been in place since 1999. The program tracks the infection and mortality associated with dermo disease on Delaware Bay oyster seedbeds. Each month from March through November, selected seedbeds across a salinity gradient are sampled to provide information on dermo infection and intensity, as well as oyster mortality and size range. The information is entered into a database so that long-term trends can be evaluated, giving us a greater understanding of the effects of dermo disease in Delaware Bay.