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

An isolate from naturally infected Atlantic salmon 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. This isolate was used in an infectivity trial in which Atlantic salmon juveniles received an intraperitoneal injection of the bacteria. Fish receiving 0.5 ml of a 109 bacteria/ml 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, the lesions were restricted to the injection site. The large colonies of bacteria in the peritoneal cavity and the nature of the host response were both characteristic of changes seen in the renal interstitium of naturally infected fish. Lower concentrations of injected bacteria resulted in minimum pathological change, and subsequent recovery of bacteria from injected fish was poor. Horizontal transmission to non-injected co-habitants was not achieved..