Perkinsus marinus (dermo) is a protozoan pathogen of the eastern oyster, Crassostrea virginica. Infection is often fatal and can result in mass mortality. Temperature and salinity are the primary drivers of disease patterns. The disease extends from the Gulf of Mexico to the coast of Maine, which contains latitudinal variations in temperature and salinity that influence the seasonal patterns of infection. In the northern end of the range, dermo prevalence and intensity are depressed during winter with a large surge in late summer and fall, whereas south Atlantic and Gulf populations show flatter seasonal patterns with disease prevalent throughout the year.
Dermo became endemic in the Delaware Bay in the 1990s following a period of extended warm temperatures. Long-term patterns indicate the infection and remission season begins and ends as temperatures reach then fall below about 15°C. The number of days above 15°C has increased by approximately twenty days since 2004. Temperatures above this threshold are moving earlier into spring at a faster rate than they are moving later into fall, indicating an asymmetrical widening of the dermo infection season. Interestingly, weighted prevalence (ie, infection intensity) has not increased over that time period, which defies what is expected from the typical positive correlation between temperature and dermo infections. This raises questions about whether mortality has become decoupled from infection intensity, and whether dermo infections in the Delaware Bay are shifting to a seasonal cycle similar to that found in lower latitudes.