OCCURRENCE OF VIRAL NERVOUS NECROSIS (VNN) IN HATCHERY-REARED JUVENILE JAPANESE FLOUNDER PARALICHTHYS-OLIVACEUS

Mass mortalities of hatchery-reared juvenile Japanese flounder Paralichthys olivaceus were investigated. The disease initiated in fish of 17-18 mm in total length and heavy mortalities occurred when the fish reached about 25 mm, followed by total losses in 2 weeks. The disease was characterized by vacuolation in the retinal and brain tissues. Round-shaped, unenveloped virus particles, about 27 nm in diameter, were observed in the cytoplasm of retinal and brain cells. The virus was identified as a nodavirus based on the results of FAT using an anti-SJNNV (striped jack nervous necrosis virus) rabbit serum and PCR-based detection of a portion of SJNNV RNA2 gene. Transmission experiments revealed that the virus is the causative agent of this disease. In addition to the nodavirus, another virus (YAV-like birnavirus) was isolated in EPC and FHM cell lines from diseased fish. However, the latter virus is not thought to play an important role in this infection.