Abstract

A commercial aquaculture facility producing black sea bass, Centropristis striata, reported a two-week duration of increasing mortality with associated clinical signs that included abdominal distention and exophthalmia, but no apparent gill or skin lesions. Two fish (approximately 450 grams) were presented live to the Virginia-Maryland Regional College of Veterinary Medicine for diagnostic evaluation. At the time of presentation, the fish were being fed three times a day with a commercial extruded pelleted diet (9.5 mm, Silver Cup Fish Feeds, Skretting, Norway) consisting of 50% protein and 18% lipid. Water quality parameters monitored at the facility included temperature (24.3°C), salinity (30 ppt), pH (6.8), ammonia (1.0 mg/L), nitrite (0.5 mg/L), nitrate (120 mg/L), DO (5.8 ppm), and alkalinity (125.3 ppm). Gross external examination revealed abdominal distention in both fish, but did not reveal any other abnormalities. Gill and skin biopsies did not reveal any obvious external parasites, fungal or bacterial elements. Internal examination revealed a small amount of clear fluid free in the ceolomic cavity, a swollen pale liver with multiple variably-sized clear cystic vacuoles scattered throughout the hepatic parenchyma, enlarged swollen kidneys, and a single large mass in the abdominal cavity of one fish. Bacterial samples were acquired from the posterior kidney and liver, and tissue samples of the gills, liver, anterior and posterior kidney, heart, spleen, intestine, and abdominal mass were taken and preserved in 10% neutral buffered formalin. Following fixation, tissues were routinely trimmed, processed, sectioned and stained with either hematoxylin and eosin (H&E) or a modified Fite-Faraco stain. None of the bacterial samples from the liver or posterior kidney grew any bacteria after five days of culture on BHI or TSA. Histopathology of the tissues revealed moderate multifocal granulomatous inflammation of the liver and kidney, as well as the heart and intestines. The vacuoles in the liver were filled with an eosinophilic, acellular material surrounded by granulomatous inflammation. In addition to the granulomatous hepatitis, prominent hepatic lipidosis was also present. There was moderate to severe granulomatous branchitis of the gills. The multinodular mass in the ceolomic cavity was also composed of granulomatous inflammation. Within the mass, there were numerous rod-shaped, Fite Faraco-positive bacteria. Based on the pathology and the presence of positive Fite-Farco staining bacteria in the granulomatous inflammation, a diagnosis of mycobacteriosis (Mycobacterium sp.) was made.