Viral Nervous Necrosis (VNN) with absence of cellular vacuolation in juveniles of sea bass (D. Labrax L.)

From the years eighty, the viral encephalopathy-retinopathy (VER, OIE 2003), neuropathy caused by a virus belonging to genus Betanodavirus, family Nodaviridae (Ball et al., 2000), is responsible for severe losses in many species of marine farmed fish (more than 31 species distributed 16 different families) throughout the world (Munday et al., 2002). In the Mediterranean basin the VER has been quickly diffused from years ninety, affecting above all seabass (D. labrax) and, later on, shi drum (U. cirrosa ). To the contrary, spontaneous infection reports of VER in gilthead seabream (S. aurata) have been very rarely documented (Comps and Raymond 1996; Castric et al., 2001). Although the gilthead can be experimentally infected (Novoa et al., 2001; Aranguren et al., 2002), it thought that it is a resistant species, documented from the fact that, during serious VER outbreaks in seabass larvae or in other species, no gilthead larvae infection is seen. Some authors documented, through experimental infection, as this species represent rather an asymptomatic carrier of betanodavirus (Castric et al., 2001; Aranguren et al., 2002), more pathogenic for seabass. The descriptive attempt of this surveying is to document a massive mortality outbreak in 15 day-old gilthead larvae, ascribable to viral encephalopathy-retinopathy on the basis of the histopathological and immunohistochemical analyses. Gilthead larvae, reared in tanks at density of 150ind/l and supplied to marine filtered and sterilised water (temperature 20-21°C and salinity 40‰), showed swim bladder tympanism, abdominal distension, lethargy, anorexia and alteration swim arrangement. During the acute phase of mortality, two samples of moribund larvae have been collected (19 and 29 days of age) and fixed in 4% neutral buffered formalin. The immunohistochemical tests, using two monoclonal antibodies anti-VER (supplied from the Lombardia and Emilia IZS), were performed. The histopathological observations have put in evidence the presence of the characteristic vacuolar lesions of the central nervous system and peripheral ascribable to VER, localized above all in optic lobe, medulla oblongata, spinal verve chord and in the retinal layers (outer and inner nuclear layer). The immunoperoxidase has confirmed the presence of the virus in the tissues, showing a strongly positivity close to the lesions, characterised by the intense brown diaminobenzidine staining. After the existing literature analysis, this report seems to be the first massive mortality outbreak in gilthead larvae caused by a spontaneous infection of viral encephalopathy-retinopathy in Italy.