First report of piscirickettsiosis in farmed sea bass fingerlings (Dicentrarchus labrax, L. 1758) in Italy

Piscirickettsia salmonis is a pathogenic bacterial agent causing a septicaemic disease in salmon. Since its isolation in Chile in 1989, P. salmonis has continually produced high levels of mortality in salmon farms (Manuel and Miller, 2002). In the last few years disease outbreaks caused by Rickettsia-like organisms (RLOs) have been reported in non-salmon species such as seabass (Comps et al., 1996; Athanassopoulou et al., 1999; Athanassopoulou et al., 2004). RLOs have been found in the nervous tissue of farmed juvenile seabass (from France and Greece) exhibiting abnormal swimming and whirling behaviour and which exhibit medium to high levels of mortality (20-80 %). From 2002 to 2004, three disease outbreaks have been recorded in sea bass fingerlings (140-180d) in an Italian hatchery, characterised by low levels of mortality (1 %) and abnormal behaviour, with infected fish showed signs of spiral swimming and lethargy. The outbreaks lasted for a few days and were always recorded during the spring-summer period (April-July), when water temperatures ranged from between 16 and 19°C. The hatchery was supplied with filtered sea water. During disease outbreaks a number of fish were collected for histopathological and immunohistological examination. The fish showed no signs of macroscopic lesions, but histological lesions were observed in the brain, liver and pancreas. Meningitis and nervous tissue necrosis were observed in the brain, where strongly basophilic coccoidal organisms, which stained red with Macchiavello's stain, were located in the cytoplasmatic vacuoles of macrophages particularly within the meningis. Multifocal necrosis and inflammation were present in the liver, and macrophages and hepatocytes appeared to be filled with RLOs. All fish sampled during the disease outbreaks were positive for P. salmonis by immunohistochemistry using a rabbit P. salmonis antiserum. The symptoms observed between the different disease outbreaks were all similar, and were comparable with the infection described in seabass by other authors (Comps et al., 1996; Athanassopoulou et al., 1999 and 2004). However the fish size and period of infection were not similar, with Italian outbreaks always characterised by very low levels of mortality. From an epidemiological viewpoint, the annual recurrence of this infection appears very interesting and warrants future investigation. On the basis of this first report of Piscirickettsiosis in Italy and the other RLOs disease outbreaks in Mediterranean areas, it would be favourable to carry out a differential diagnosis for diseases with nervous symptoms (like encephalo-retinopathy).