- Perkinsus marinus is a protozoan parasite that causes a major disease of eastern oysters Crassostrea virginica from Chesapeake Bay south along the Atlantic coast of the USA and throughout the Gulf of Mexico. It is a warm-season disease that kills eastern oysters at temperatures above 20°C. The pathogen requires salinities of at least 12-15% to be active, but it persists tenaciously when low temperatures and salinities occur during winter and spring. .Prolonged droughts that increase salinities cause extensions of the range of disease. In the Chesapeake Bay, mortalities begin in June and end in October, and up to 50% of native susceptible eastern oysters are killed each year. Most infections are acquired by eastern oysters in proximity to disintegrating dead eastern oysters. Massive populations of prezoosporangia are released into marine waters and eventually produce thousands of zoospores, which are infective when ingested by eastern oysters. The disease is controlled by isolating new beds from infected eastern oysters and by early harvest before the pathogen becomes established. Seed areas in low-salinity waters usually provide disease-free eastern oysters, but beds must be monitored regularly to avoid transplanting infected eastern oysters. In the Gulf of Mexico where warm temperatures persist through much of the year, control is much more difficult. Eradication is difficult unless introductions of infected eastern oysters are avoided and summer warm seasons are short and relatively cool to prevent the pathogen from multiplying in eastern oysters. The presence of P. marinus in Chesapeake Bay has been monitored for 37 years; the disease is established in most eastern oyster-growing areas of Virginia and in many tributaries of Maryland. The disease thrives on densely planted private beds of eastern oysters but persists through wet periods of weather on sparsely populated public beds and artificial structures along shores where there is recruitment of eastern oysters. Unfavorable temperatures essentially eradicated it in Delaware Bay after importations of infected seed eastern oysters from Virginia were discontinued.