EPIZOOTIOLOGY OF VIRAL NERVOUS NECROSIS ( VNN) IN STRIPED JACK

The present paper describes epizootiological features of VNN in larval striped jack Pseudocaranx dentex in the seed production process. This disease, caused by a nodavirus named striped jack nervous necrosis vi rus (SJNNV), occurred repeatedly at the two Japan Sea-Farming Associat ion stations, Goto (Nagasaki Prefecture) and Kamiura (Oita Pref.) from 1989 to 1992. The outbreaks of the disease were observed in the larva e from 2 to 20 days old. The noticeable signs of this disease were los s of appetite and skinny body. All of the affected larvae younger than 10 days old died within 2 to 4 days after they showed clinical sips. When the larvae older than 11 days old were affected, they showed enla rgement of swim bladder, and vertebral deformity, but some of them sur vived. ELISA tests revealed that SJNNV increased rapidly in the younge r larvae, but slowly in older larvae. VNN occurred at any water temper atures from 20-degrees-C to 26-degrees-C at which seed production of s triped jack is generally conducted, and SJNNV increased most rapidly a t 24-degrees-C. Striped jack were induced to spawn repeatedly in one s pawning season. In each spawning season, VNN rarely occurred in the la rvae obtained from the early period of spawning, but the incidence of VNN became higher in the larvae obtained from late period of the seaso n.