Mass mortalities associated with a virus disease in Japanese pearl oysters Pinctada fucata martensii

The annual mortality of cultured Japanese pearl oysters Pinctada fucata martensii in all western regions of Japan was over 400 million in both 1996 and 1997. The main pathological signs of the diseased oysters were atrophy in the adductor muscle, the mantle lobe and the body accompanied by a yellowish to brown coloration. Histological studies revealed necrosis and degeneration of muscle fibers of the adductor, pallial and foot musculatures as well as the cardiac muscle. Electron microscopy revealed the presence of small round virions approximately 30 nm in diameter within intrasarcoplasmic inclusion bodies in necrotized muscle fibers of the adductor and pallial musculatures, and the heart. The causative virus was isolated and cultured in EK-1 (eel kidney) and EPC (epithelioma papilosum cyprini) fish cell lines. Marked mortalities occurred in pearl oysters that had been experimentally inoculated with the cultured virus; these oyster displayed the same pathological signs of the disease as oysters in natural infections. These results inbcate that a previously undescribed virus caused the mass mortalities in cultured pearl oysters.