Viral encephalopathy and retinopathy of farmed marine fish species in Italy.

Viral encephalopathy and retinopathy, otherwise known as fish encephalitis or viral nervous necrosis (VNN), is an emerging problem in several farmed marine fish species in various geographic areas all over the world. Since summer 1995, heavy losses affecting mainly juvenile and adult sea bass (Dicentrarchus labrax) have been observed in several on-growing facilities in Italy. Dying fish show abnormal swimming behaviour and, at temperatures higher than 20-22 degrees C, mortality rates range between 15 and 50%. Neither significant external nor internal gross pathological signs, except frequent abnormal swim bladder hyperinflation, were detected. Histological investigations reveal vacuolations in the grey matter of the brain and spinal cord and in the granular layers of the retina. Serial tissue sections examined by an immunohistochemical method carried out with antisera against fish nodaviruses showed a positive reaction. Additionally, spherical virus-like particles 22-25 nm in diameter were detected by electron microscopy in negative stained preparations of brain tissues, and the same samples gave a positive reverse transcription-polymerase chain reaction (RT-PCR) for the T4 region of the fish nodavirus gene. These results indicate that both juvenile and adult sea bass subject to mass mortality in Italy since summer 1995 are infected with a fish nodavirus and strongly suggest that the identified virus is the cause of the observed mortality.