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

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