Mortality due to viral nervous necrosis in zebrafish Danio rerio and goldfish Carassius auratus.

Heavy mortality was observed in an experimental lot of 2 ornamental fish species, zebrafish Danio rerio (F. Hamilton, 1822) and goldfish Carassius auratus (Linnaeus, 1758). The fishes showed typical symptoms of viral nervous necrosis before death. Gross morphological examination revealed no visible lesions except in the brain, visible as a creamy opaque patch through the dorsal side of the head. Parasitic and bacteriological analysis revealed no pathogenic agents. Histopathological analysis revealed severe vacuolation in the brain and spinal cord of the samples. A fragment within the variable region of genomic RNA2 of betanodavirus was amplified from the samples by reverse transcription polymerase chain reaction using specific primers designed previously. The analysis suggests that the observed mortality in the fishes was due to betanodavirus infection. This is the first report of natural infection of betanodavirus in laboratory fishes causing viral nervous necrosis leading to mortality. The observation is alarming, as the ornamental fish culture and trade is being popularized in India where the fatal disease may cause severe setbacks in the industry. It emphasizes the need for quarantine and control strategies to prevent the spread of the virus and outbreak of the disease.