Epizootiology of Perkinsus marinus disease of oysters in Chesapeake Bay, with emphasis on data since 1985

Since 1987 Perkinsus marinus has been the most important pathogen of the eastern oyster, Crassostrea virginica, in Chesapeake Bay because of its widespread distribution and persistence in low salinity areas. The pathogen became established on all oyster beds in the Chesapeake Bay as a result of natural spread during the consecutive drought years from 1985 to 1988 or by movement of infected oysters during the same period. Elevated salinities resulting from drought conditions and concomitant warm winters allowed P. marinus to proliferate in what were historically low salinity areas. Oyster mortality was high on most beds and landings of market oysters declined to record low levels in both Maryland and Virginia during the late 1980s and early 1990s. The seasonal periodicity of P. marinus is primarily controlled by temperature. Both prevalence and intensity of infections begin to increase in June as temperature increases above 20 degree C and overwintering infections begin to proliferate. Maximum values of prevalence and intensity occur in September immediately following maximal summer temperatures. Infection regression occurs during winter and spring as temperature declines resulting in minimum prevalence and intensity values in April and May. Prevalence and intensity of P. marinus infections in oysters from the James River, VA, over a five year period were significantly correlated with temperature when temperature data were lagged three months. Temperature explained 39% of the variability in prevalence and 46% of the variability in intensity. The relationship between temperature and annual variability in P. marinus abundance is somewhat obscure, in part because of the difficulty separating salinity and temperature effects. Nonetheless, data from 1988 to 1994 from the James River, VA, suggest that abnormally warm winters have a more significant impact on summer P. marinus abundance than abnormally cold winters. Salinity is the primary environmental factor that controls local distribution and intensity of P. marinus infections. Long-term oyster disease monitoring along a salinity gradient in the James River, VA, revealed a statistically significant relationship between salinity and P. marinus prevalence and intensity. P. marinus infections remain light in intensity and no oyster mortality results if salinity is consistently less than 9 ppt. However, infections may persist for years in low salinity areas. If summer/fall salinities range from 9 to 15 ppt some in