EXPERIMENTAL TRANSMISSION OF A SALMONID RHODOCOCCUS SP ISOLATE TO JUVENILE ATLANTIC SALMON, SALMO-SALAR L

This paper provides further characterization of a 'Rhodococcus'-like p athogen of Atlantic salmon, Salmo salar L., and describes its experime ntal transmission. An isolate from naturally infected fish was determi ned to be 'Rhodococcus'-like based on its biochemical profile and from gas liquid chromatography assessment of its cellular fatty acid methy l esterase profile in comparison to a data base. This isolate was used in an infectivity trial in which Atlantic salmon juveniles received a n intraperitoneal injection of known concentrations of the bacteria. F ish receiving 0.5 ml of a 10(9) bacteria ml(-1) solution developed sev ere peritoneal granulomatous reactions, and bacteria were recovered fr om the peritoneum and the kidney. In contrast to the natural disease, in which pathological changes are often most dramatic in the renal int erstitium, in our study, the lesions were restricted to the injection site - the peritoneal cavity. However, in this location, the large col onies of bacteria and the nature of the host response were both charac teristic of the changes noted in the renal interstitium of naturally i nfected fish. Lesser concentrations of injected bacteria resulted in e ither no or minimal pathological change, and subsequent recovery of ba cteria from injected fish was poor. Horizontal transmission to non-inj ected co-habitants was not achieved.