Extrapallial abscesses associated with chronic bacterial infections in the intensively cultured juvenile Pacific oyster Crassostrea gigas

During a detailed survey of Pacific oyster Crassostrea gigas juvenile health at intensive rearing facilities, an episode of persistent morbidity and mortality occurred over an 8 wk period in 1997. Affected oyster seed were typically between about 1.0 and 2.4 mm in shell height. Abscesses were formed in the extrapallial space resulting from invasion by straight bacterial rods along the inner shell surface. The abscesses contained host cells and bacteria. The infection appeared to be chronic, was associated with relatively low numbers of bacteria cells, and caused alterations of the underlying mantle and abnormal shell deposition. No signs of any other type of infectious agent were found associated with the lesions. After a chronic time course, 2 outcomes were possible: either the mantle was breached, leading to an overwhelming terminal bacterial infection or, in some cases, host cell debris and bacteria were sequestered by new shell deposition and the infection was resolved. The condition can cause mortality and significant loss of growth in intensively cultured juvenile oysters.