Pathogenicity of Striped Jack Nervous Necrosis Virus (SJNNV) Isolated from Asymptomatic Wild Japanese Jack Mackerel Trachurus japonicus

We isolated striped jack nervous necrosis virus (SJNNV), a causative agent (betanodavirus) of viral nervous necrosis (VNN), from asymptomatic wild Japanese jack mackerel Trachurus japonicus, which is one of the most popular feed fish for marine aquaculture in Japan. The five selected SJNNV isolates and a reference SJNNV strain (SJNag93) isolated from larval striped jack Pseudocaranx dentex were examined for their pathogenicity in hatchery-produced larvae of Japanese jack mackerel and striped jack by an immersion challenge method. All of the SJNNV isolates and SJNag93 were able to infect larvae of both species, although the infective titers in the dead larvae of striped jack were significantly higher than those of Japanese jack mackerel. One isolate (05SaiJJM-3) was particularly virulent to striped jack larvae and was comparable to SJNag93. However, phylogenetic analysis on a genotype specific sequence region (T4) of the coat protein gene (RNA2) showed that the 05SaiJJM-3 isolate was clustered differently from not only the other four isolates of Japanese jack mackerel but also from SJNag93. The present study revealed that SJNNV in subclinically infected wild fish was pathogenic, suggesting that wild fish can be a potential infection source for cultured fish via water- or food-borne transmission.