The first epizootic of edwardsiellosis, caused by Edwardsiella tarda, is described. The epizootic occurred in the Chesapeake Bay, Maryland (USA) during the summer and autumn of 1994, and affected wild adult striped bass (Morone saxatilis). Clinical signs included numerous irregular coalescing hemorrhagic ulcers on the body and fins that were distinctly malodorous. Internally, the body cavity was filled with abundant yellowish or sanguinous mucoid fluid, and the visceral organs had multiple tiny white foci. The intestines contained thick white opaque mucus. Histopathological lesions included ulcerative dermatitis, cardiac endothelial hyperplasia, and necrotic foci and granulomata in multiple organs. A bacterium isolated in pure culture was characterized taxonomically and serologically as the wild-type or classical biotype of E. tarda: In infectivity trials, it was pathogenic for striped bass, gilthead seabream (Sparus aurata), and turbot (Scophthalmus maximus) with an LD50 of about 10(5) cells; however, the isolate was non-virulent for mice (LD50 > 10(8) cells). The isolate also was resistant to the bacteriolytic activity of normal fish skin mucus.

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