- Perkinsus marinus is a warm-season protozoan pathogen (Phylum Apicomplexa) that parasitizes eastern oysters Crassostrea virginica from Chesapeake Bay to the Gulf of Mexico. In years of normal rainfall in Chesapeake Bay, P. marinus is limited by low salinities in the estuaries and is dormant during winter and spring. A few subpatient infections persist to initiate new generations of infections from June to October. During 6 years of drought in the 1980s, the disease spread throughout Chesapeake Bay, killing eastern oysters at record rates. In 1987, it was more widely distributed and more destructive than the disease caused by Haplosporidium nelsoni (MSX). The invasion of P. marinus of normally low-salinity seed ares in Chesapeake Bay, which brought the pathogen to an abundance of hosts and accelerated dispersal of the disease, was a critical turning point. Control depends upon return of normal rainfall and low estuarine salinities to suppress the disease, which remains active during summer at levels of 12-15%. Perkinsus marinus is widely distributed in the Gulf of Mexico with very high prevalences. There, P. marinus maintains infections throughout the year, though intensities decline during the winter. Warm year-round water temperatures and few low-salinity refuges make management of the disease difficult in the gulf. Eastern oysters planted in late fall have been harvested by the following summer to avoid mortalities. This practice has been aided by reduction of legal size limits in Texas. Removal of diseased oysters and planting of only disease-free oyster will also limit destruction by P. marinus.