Effectiveness of antimicrobial treatment of salmonid rickettsial septicaemia in commercial salmon and trout farms in Chile

Salmonid rickettsial septicaemia (SRS) is the most important disease of farmed salmonid fish in Chile and the main driver of a high rate of antimicrobial use. This study evaluated the effectiveness of antimicrobial treatment of SRS outbreaks, using industry-generated data from 8318 cage-level production cycles stocked between 2003 and 2018. We defined SRS outbreaks by a specified level of SRS-attributed mortality over a 3-week period and calculated the mortality rate attributed to SRS and unknown causes during a follow-up period from the start of treatment until resolution of the outbreak. The post-treatment mortality rate was used as a proxy for assessing the effectiveness of the antimicrobial treatment on the magnitude of the SRS outbreak. After developing a causal diagram, we analyzed the data with generalized, mixed-effects regression models within an information-theoretic framework. For producers of Atlantic salmon, our results suggest that treatment should be provided to all infected cages on the farm, without interruption, as soon as possible after the start of the SRS outbreak. For producers of rainbow trout, our results suggest that treatment should be initiated as early as possible after the start of the SRS outbreak and with longer treatment durations if using in-feed florfenicol treatments. In the rainbow trout model, the large proportion of unexplained variance at the company and farm level indicates that lessons can be learned from the experience of other companies and farms. This study demonstrates the value of integrating aquaculture industry-generated health and management data to support applied epidemiological research.