Severe Renomegaly Associated With An Acid-Fast Bacterial Infection In Summer Flounder, Paralichthys dentatus     Kathleen P. Hughes, Robert B. Duncan, Jr. and Stephen A. Smith   Aquatic Medicine Laboratory, Department of Biomedical Sciences and Pathobiology, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 USA     A small population of commercially reared juvenile summer flounder (28 cm, ~180 grams) was housed at the Virginia-Maryland Regional College of Veterinary Medicine Aquatic Medicine Laboratory for over a year in a recirculating saltwater (16-20 ppt) system.  Historically, fish were exposed to different experimental salinity manipulations, but not intentionally exposed to any pathogens. Over a three to six month period all fish in the population developed severe abdominal distention.  No other external clinical signs were noted. Fish gradually became anorexic and lethargic.  A fish from the affected population was humanly euthanized and a necropsy was performed.  Gross examination revealed a large multilobulated retroperitoneal mass in the region of the kidney. Small multifocal yellow lesions were also noted on the liver, spleen and heart tissues.  Normal renal tissue could not be identified. There was a moderate amount of pale yellow fluid in the coelom, which was later characterized as a transudate. Samples of the mass and liver were cultured on 2% NaCl TSA agar and Middlebrook agar. Small raised yellow colonies of Gram-positive acid-fast bacilli grew after ten days at 25oC. Histopathologically, the mass was identified as renal tissue that had severe granulomatous nephritis with marked infiltration of macrophages and multinucleated giant cells, which obliterated preexisting normal renal architecture.  Sections of heart, brain stem, mesentery, stomach and intestine also had multifocal granulomatous inflammation.  Special stains revealed Gram positive, acid-fast bacilli distributed in multiple areas or multiple tissues