Clinical Presentations Of Mycobacterium sp. In Summer Flounder, Paralichthys dentatus, Held In Recirculating Systems   Kathleen P. Hughes 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   A population of 1000 commercially-reared juvenile summer flounder was housed in recirculating systems at the Aquatic Medicine Laboratory at the Virginia-Maryland Regional College of Veterinary Medicine for approximately one year. After approximately 6 months, fish began to develop oral masses on the lower mandible.  These discrete masses were generally white-yellow in color and encompassed the rostral portion of the mandible.  In addition, other fish developed head swelling, exophthalmia, coelomic distention and opercular masses.  It was estimated that at least 40% of the entire population displayed one or more of these clinical signs.  Some of the affected fish became lethargic and emaciated.   Impression smears and histopathology of these lesions (stained with Ziehl Neelson acid-fast stain) revealed a dense population of acid-fast bacilli.  All affected tissues had marked effacing and coalescing nodular granulomatous inflammation primarily composed of epithelioid macrophages. There were rare mineralized granulomas delimited by fibrous capsules with central calcification. This tissue reaction was not the typical granuloma response seen in fish to Mycobacterium sp.  Bacterial cultures of the affected tissues grew on Lowenstein-Jensen and Middlebrook media and were confirmed to be acid-fast positive with Ziehl Neelson staining.  Further identification of the bacteria is being conducted.