Mortality outbreak in turbot (Psetta maxima) farmed in Italy.

Introduction: Turbot (Psetta maxima) is one of the most important fish species in European aquaculture. The increase of its production should pass through the overcoming of some zootechnical and health issues, among which several transmissible diseases strongly limiting its expansion. During 2021-22 in a farming trial of turbot in inland tanks in Italy, some disease outbreaks were recorded, starting from juveniles and progressively reaching a cumulative mortality of around 95% in a few months. Methodology: Fish samples were submitted to the lab at three different times coinciding with as many severe disease outbreaks that occurred first in juveniles (33g), then in subadult and adult fish (155g and 489g respectively) from the same batch. All fish were subjected to anatomopathological, bacteriological and parasitological examinations according to standard laboratory methods. Results: The first outbreak involved juveniles showing erosive and hemorrhagic lesions in the mouth and fins; Aeromonas salmonicida was isolated from brain and kidney. The second outbreak allowed to observe the onset of extensive necrotic-ulcerative skin lesions and enteritis with ascites; massive infections of the skin and gills by scuticociliates referable to Philasterides dicentrarchi and of the gut by the myzozoan Enteromyxum leei were found in the affected fish. In the third outbreak, turbot showed enteritis with ascites and the presence of nodular lesions in spleen, liver and intestine; in this case, in addition to observe a severe intestinal E. leei infection, bacteriological exams allowed to isolate Vibrio harveyi and molecular analyses detected Mycobacterium marinum from the nodular lesions. Conclusion: All pathogens detected during the outbreaks described here are already well known in farmed turbot, with the exception of M. marinum infection of which very few cases were reported several years ago. The frequency and severity of transmissible disease outbreaks recorded during the production cycle of the turbot batch considered here raise concerns about a possible sustainable production of Psetta maxima in the Mediterranean area.