Abstract

Summer flounder, Paralichthys dentatus, maintained in recirculating aquaculture systems for over one year developed oral masses, opercular masses, head swelling, exophthalmia, coelomic distention and had a low level chronic mortality rate. Some of the affected fish became lethargic and emaciated. The discrete masses in the head region were generally white-yellow in color and encompassed the rostral portion of the mandible and nasal area. Masses were fast growing and eventually disrupted the normal architecture of the region affected. It was estimated that at least 40 percent of the entire population displayed one or more of these clinical signs.

Impression smears and histopathology of these lesions (stained with Hematoxylin-Eosin and Ziehl Neelson) revealed a dense population of acid-fast bacilli. All affected tissues had marked effacing and coalescing nodular granulomatous inflammation primarily composed of epithelioid macrophages. In affected renal tissue, 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.