Nodavirus infection of juvenile white seabass, Atractoscion nobilis, cultured in southern California: first record of viral nervous necrosis (VNN) in North America

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Abstract
The viral aetiology of mass mortalities of white seabass, Atractoscion nobilis, cultured in southern California, USA was examined. Disease outbreaks occurred in juvenile fish reared at two culture facilities from June to December 1999, with clinical signs such as anorexia and erratic swimming motion. Microscopic lesions observed in moribund fish included marked vacuolation of brain, spinal cord and retina. The piscine nodavirus (Betanodavirus), the causative agent of viral nervous necrosis (VNN), was detected in the affected tissues by electron microscopy, indirect fluorescent antibody test (IFAT), reverse transcription–polymerase chain reaction (RT–PCR), and isolation in cell culture. The agent was identified as one of the four known genotypes of piscine nodavirus. In addition, a similar nodavirus was also detected in fish samples from disease outbreaks at the same facility in 1992. In the last decade, VNN has been reported among cultured populations of marine fish worldwide and this paper is the first record of the agent in North America.