Effectiveness of vaccination for the control of salmonid rickettsial septicaemia in commercial salmon and trout farms in Chile

Salmon rickettsial septicaemia (SRS, or piscirickettsiosis) is a major disease of the Chilean salmonid aquaculture industry. Although numerous vaccines for SRS are currently registered for use in Chile, there is little evidence of their effectiveness under field conditions. In this study, we aimed to quantify the effectiveness of different vaccination strategies in delaying the time from stocking to the first SRS outbreak and reducing SRS-attributable mortality over the full production cycle for Atlantic salmon, rainbow trout and coho salmon. We analysed data from 5446 cage-level production cycles across 248 farms using linear mixed-effects models and Cox regression within an information-theoretic framework. In Atlantic salmon and rainbow trout (but not in coho salmon), we found some freshwater vaccination regimens to be significantly better than others. In contrast to some previous studies, we did not observe a beneficial effect of booster vaccinations in the seawater phase of production. Our study highlights that the occurrence of SRS is influenced by multiple factors, and illustrates opportunities and challenges in using routine industry-generated data for epidemiological research.