Epizootiology of Viral Nervous Necrosis (VNN) in Striped Jack

Abstract

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 virus (SJNNV), occurred repeatedly at the two Japan Sea-Farming Association stations, Goto (Nagasaki Prefecture) and Kamiura (Oita Pref.) from 1989 to 1992. The outbreaks of the disease were observed in the larvae from 2 to 20 days old. The noticeable signs of this disease were loss 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 signs. When the larvae older than 11 days old were affected, they showed enlargement of swim bladder, and vertebral deformity, but some of them survived. ELISA tests revealed that SJNNV increased rapidly in the younger larvae, but slowly in older larvae. VNN occurred at any water temperatures from 20°C to 26°C at which seed production of striped jack is generally conducted, and SJNNV increased most rapidly at 24°C. Striped jack were induced to spawn repeatedly in one spawning season. In each spawning season, VNN rarely occurred in the larvae obtained from the early period of spawning, but the incidence of VNN became higher in the larvae obtained from late period of the season.