Experimental transmission of a salmonid Rhodococcus sp. isolate to juvenile Atlantic salmon, Salmo salar L.

Abstract. This paper provides further characterization of a 'Rhodocoecus' -like pathogen of Atlantic salmon, Salmo salar L., and describes its experimental transmission. An isolate from naturally infected fish was determined to be 'Rhodococcus' -like based on its biochemical profile and from gas liquid chromatography assessment of its cellular fatty acid methyl esterase profile in comparison to a data base. This isolate was used in an infectivity trial in which Atlantic salmon juveniles received an intraperitoneal injection of known concentrations of the bacteria. Fish receiving 0.5 ml of a 10⁹bacteria ml⁻¹ solution developed severe peritoneal granulomatous reactions, and bacteria were recovered from the peritoneum and the kidney. In contrast to the natural disease, in which pathological changes are often most dramatic in the renal interstitium, in our study, the lesions were restricted to the injection site—the peritoneal cavity. However, in this location, the large colonies of bacteria and the nature of the host response were both characteristic of the changes noted in the renal interstitium of naturally infected fish. Lesser concentrations of injected bacteria resulted in either no or minimal pathological change, and subsequent recovery of bacteria from injected fish was poor. Horizontal transmission to non-injected co-habitants was not achieved.