Abstract

The protozoan blood-cell parasite Bonamia ostreae of the European flat oyster Ostrea edulis has caused extensive mortalities in France, the United Kingdom, The Netherlands, Spain, and Denmark. The discovery of the disease in 4 oyster populations in North America is the first documentation of the disease outside Europe. Ultrastructure of the parasite, inflammatory lesions in the affected oysters, and transmissibility patterns of the disease demonstrate the identity of the disease in Europe and North America. Current studies trace the origin of the disease to California (USA) and possibly an Atlantic North American site, followed by its spread within North America and to Europe.