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The article focuses on a study conducted to investigate the outbreak of viral nervous necrosis (VNN) in the farmed Atlantic cod fishes in Norway during 2006. It discusses the genetic factors causing VNN in farmed juvenile and adult marine fish species, and gives an overview of the investigations done by the study on the infected fishes. Also discussed are the disease symptoms found in fishes during the study, and explores its findings on the pathogenesis and etiology of VNN.

Viral nervous necrosis (VNN) caused by betanodavirus affects many species of farmed marine fish, in particular juveniles. Apparently healthy, normally feeding, adult farmed Atlantic cod, Gadus morhua, were sampled in a farm 14 months after an outbreak of VNN with clinical signs. Following necropsy, brain and eye tissues were examined by histology, immunohistochemistry and polymerase chain reaction (PCR). Nodavirus-provoked cell death and inflammation was detected in eye and brain, particularly in the retina and cerebellum and differed from that previously described in Atlantic cod during clinical stages of VNN. Virus was detected both by PCR and immunohistochemistry. This is, to the best of our knowledge, the first description of pathological changes associated with chronic subclinical nodavirus infection in Atlantic cod. Our observations suggest that severe infection and pathological changes may go undetected if investigations are restricted to clinical examination and macroscopic evaluation at necropsy. [ABSTRACT FROM AUTHOR]