Vaccine-associated systemic Rhodococcus erythropolis infection in farmed Atlantic salmon Salmo salar

In 7 instances between 2000 and 2003, clinical investigation of populations of fresh- and seawater-reared, vaccinated, Atlantic salmon Salmo salar suffering total losses of between 0.1 and 35 % revealed infection with a Gram-positive rod-shaped bacterium. The isolations were geographically widespread, occurring in both Norway and Scotland. In all cases, a Gram-positive bacterium, subsequently identified as Rhodococcus erythropolis, was isolated in pure culture. Infections, although systemic, were focused within the peritoneal cavity. While initial attempts to reproduce the disease by intraperitoneal injection of unvaccinated Atlantic salmon failed, Koch's postulates were subsequently fulfilled in fish vaccinated with a commercially available oil-adjuvanted vaccine.