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We describe here the main pathological conditions of freshwater fish recently introduced for intensive rearing (open ponds and recirculating freshwater systems) in Greece. Sturgeon were susceptible to skeletal abnormalities of the spine (scoliosis and lordosis) of unknown aetiology. Horizontal transmission of nodavirus from infected sea bass to sturgeon was detected for the first time. This caused serious pathology and clinical signs, such as lethargy and imbalance, leading to secondary infections with Aeromonas hydrophila and Trichodina sp. and chronic, but steady, mortality. Sea bass were very susceptible to nodavirus infection, monogenean infections and gas bubble disease. Mullet reared under recirculated and open-flow conditions were very sensitive to Chilodonella sp. infection, whereas catfish were susceptible to infection with Ichthyophthirius sp. leading to secondary infections with A. hydrophila, Saprolegnia sp. and Myxobacteria spp. Tilapia were very susceptible to gas bubble disease and A. hydrophila. This bacterium was associated with management manipulations for all species and fully responsive to corrective hygiene methods. KEY WORDS: Freshwater aquaculture · Acipenser sp. · Mugil sp. · Recirculating systems · Nodavirus Resale or republication not permitted without written consent of the publisherDis Aquat Org 60: [247][248][249][250][251][252] 2004 MATERIALS AND METHODS Fish. Sea bass Dicentrarchus labrax weighing approximately 50 to 80 g, reared in both recirculating and open freshwater production facilities situated in western Greece, showed nervous signs during an outbreak of clinical disease in late summer 2000 (during the present project period) when water temperatures were 25 to 28°C. The fish from both facilities were originally obtained from the same marine hatchery and transported to freshwater conditions when they weighed 1.5 g. They were subsequently reared in well freshwater in both recirculating and open-flow system facilities at a salinity of 0 ‰ and pH of 7.8. Sea bass were fed on commercial pellets specific to this species and their size.Mullet Mugil cephalus was also reared in both recirculating and open freshwater production facilities. The fish from both facilities were originally obtained from the same marine hatchery and transported to freshwater conditions when they weighed 1.5 g. After transfer to the farm, all fish were reared in freshwater until the end of their production cycle. Mullet were fed on commercial pellets containing approx. 40 to 45% protein and 12 to 18% lipids, depending on fish size. Stocking densities for sea bass and mullet in the recirculating system was 45 kg m -3 and the temperature ranged from 18 to 21°C. In the open-flow system densities were 15 and 20 kg m -3 for sea bass and mullet, respectively. Water temperature ranged from 16 to 18°C in winter and 19 to 21°C in the summer (Table 1).The other species examined in the present study were reared in freshwater in a commercial hatchery and transported to the farm when they weighed 2 to 5 g. ...

A systemic infection of a Rickettsia-like organism (RLO) in cultured sea bass is described for the first time. In hatcheries, clinical signs were lethargy, inappetence and discoloration. Twenty days after transfer to sea cages from hatcheries where the disease existed, fish showed erratic and abnormal swimming behaviour, loss of orientation, and lethargy. Cumulative mortality in colder months of the year reached 30% in hatcheries and 80% in cages. Surviving fish in cages did not show any clinical signs of RLO infection in the subsequent year. Evidence for a systemic distribution of RLO was supported by histolopathological lesions in both infected hatchery and caged fish, where the lesion profile included cranial sensory, central nervous, integumental and alimentary organ systems. Intracranial lesions were primarily characterized by an ascending histiocytic perineuritis and necrotizing congestive meningoencephalitis, with evidence for transfer of infective agents across the blood-brain barrier confirmed by the presence of RLOs within capillary endothelium and histiocytes in inflamed regions of the optic tectum and the cerebellum. In the most severe cases, infection spread to the statoacoustical (semicircular) canal system and the ependymal lining of ventricles, with marked rickettsial-laden histiocytic infiltration of the canal lumen. Integumental lesions were restricted to the oral submucosa, nares and integumental dermis of the cranium. Alimentary lesions were noted in both the liver parenchyma and mucosa/submucosa of the stomach. In all affected organs the RLOs were found by immunohistochemistry to be related to Piscirickettsia salmonis.