Seasonal change of infective state of marine birnavirus in Japanese pearl oyster Pinctada fucata

Summary.  This study examines the seasonal occurrence and infective state of marine birnavirus (MABV) in cultured Japanese pearl oyster (Pinctada fucata). Planted oysters were sampled monthly in 1997 and 1998. To detect MABV in the oysters, PCR and virus isolation were carried out. Also, the indirect fluorescent antibody technique (IFAT) was performed to know the organs expressing viral antigens. The detection rate of the MABV genome by PCR was low during July to October, but increased after November. This virus was isolated only after October, with a 10–40% isolation rate. Results of the IFAT showed that the specific fluorescence was observed in hemocytes in September. Fluorescence in hemocytes decreased in January, but increased in liver parenchymal cells. These results suggest that MABV persistently infected hemocytes in summer with a small amount of genome and protein, and then the virus spread in winter into the parenchymal cells. Received November 16, 1999 Accepted May 22, 2000