Association of Aeromonas sobria with mortalities of adult gizzard shad, Dorosoma cepedianum Lesueur

Abstract. This report describes an epizootic in a natural population of spawning females of gizzard shad, Dorosoma cepedianum Lesueur, which occurred in the Potomac River (Maryland, USA). Microbiological analysis of moribund shad revealed the presence in pure culture in all the organs examined of one bacterium which was identified as Aeromonas sobria by conventional tube and plate tests. The different API-systems used to attempt for a rapid diagnosis of the disease yielded false negative and positive reactions in important differential tests such as ornithine decarboxylase, arabinose fermentation, gelatinase, urease, indole and production of H₂S which gave an erroneous identification of the shad isolate. The virulence assays in rainbow trout revealed that A. sobria was pathogenic with a mean LD₅₀ of 2 × 10⁵. Bacterial extracellular products also displayed fish lethality and were cytotoxic for poikilothermic and homeothermic cell-lines. Although the source of shad infection is unknown, the physico-chemical conditions of the river, together with the stress associated with the spawning could have been contributing factors in the proliferation of A. sobria inducing this epizootic.