Increased survival and reduced bacterial shedding following single but not repeated oral immunizations against <i>Piscirickettsia salmonis</i> in Atlantic salmon <i>Salmo salar</i>

Outbreaks of Piscirickettsia salmonis in cultured salmon populations result in high mortality and high economic loss. An efficacious vaccine against this pathogen is highly desired; however there has been limited success in this regard. In the current study, Atlantic salmon ( Salmo salar ) smolts were vaccinated by single, double, or quadruple immunization with a commercial oral vaccine developed using a Chilean isolate of P. salmonis . Fish were then challenged by immersion with a Canadian isolate of P. salmonis . Significantly greater survival was observed 300 degree-days (DD) following single immunization (30.9 relative percent survival - RPS) and 370 DD following quadruple immunization (21.1 RPS). Increased survival in the former group was associated with reduced bacterial shedding, and in the latter group, with increased mean days-to-death. Bacterium-specific serum antibodies were not influenced by vaccination in any treatment. No protection was elicited following double immunization. The relatively low protection observed following oral immunizations may be related to antigenic heterogeneity between the isolates used in vaccine manufacture and laboratory challenge. The results of this study highlight the difficulties in production of an efficacious vaccine against P. salmonis .