Abstract

Mycobacteriosis was diagnosed in a population of intensively cultured striped bass (Morone saxatilis) maintained in an aquacultural system for research purposes. Two fish were presented two months apart for listlessness, anorexia and erratic circling at the surface of the tank. At necropsy, a thick, brown exudate was present in the swim bladder. Numerous to coalescing granulomas were present in the spleen, liver, gonads and kidneys. Granulomas were observed grossly in the gills of one fish. Impression smears of affected tissues showed the presence of acid-fast organisms and a pure culture of Mycobacterium marinum was obtained from the swim bladder. Aerobic cultures grew on Lowenstein-Jensen agar at 25 C in five days. Species identification was confirmed using polymerase chain reaction (PCR) analysis. A survey of 20 fish randomly selected from the same tank showed that 70% of these animals had granulomas similar to those found clinically ill. The atypical clinical presentation of this organism warrants further investigation of the etiology and pathogenesis of this disease.