Abstract

Juvenile cobia, Rachycentron canadum, was presented to the Aquatic Medicine Laboratory of the Virginia-Maryland Regional College of Veterinary Medicine with the complaint of abnormal swimming behavior, skin lesions and chronic losses over a two-week period. Fish, approximately 15 to 20 cm in length, were representative of the affected population and presented with emaciation, lethargy, ulcerative dermal lesions, exophthalmia, hyper- and hypopigmentation. Necropsies were preformed on the fish and bacterial cultures were taken from the posterior kidney and skin for culture on TSA and BHI media. Representative tissues were obtained for histopathology, preserved in 10% neutral buffered formalin and processed for standard H&E staining. Bacterial cultures from the skin grew Aeromonas hydrophila, Citrobacter sp. and Vibrio sp., but bacterial cultures from the posterior kidney grew only Aeromonas hydrophila. Histopathology of the gill tissue demonstrated mild to moderate lamellar hyperplasia, while skin lesions showed epithelial ulceration and deeper dermal necrosis with a few well formed granulomas. Numerous granulomas were also observed throughout the spleen, liver, and anterior and posterior kidney. A modified acid-fast stain (Fites) revealed numerous acid-fast bacteria within the granulomas and granulomatous inflammation of the tissues, suggesting a Mycobacterium sp. infection. In addition, bacterial cultures grown on Middlebrook agar developed small, white colonies in seven days that stained acid-fast positive. Though the fish were externally and systemically infected with several other species of bacteria, it was thought that these were secondary to the underlying Mycobacterium sp. infection. The producers were immediately advised to depopulate and disinfect the systems holding the fish.