Risk factors associated with piscirickettsiosis outbreaks in farmed Atlantic salmon (Salmo salar L.) in Chile

Piscirickettsiosis (SRS) is the infectious disease responsible for the highest mortality rates in Chilean marine salmon farms. Since 2012, the country has implemented a national fish health program for surveillance and control of SRS (PSVC-SRS). Identifying risk factors and quantifying their impact on SRS outbreaks is critical for improving disease characterization and strengthening risk-based surveillance and control strategies. We conducted a retrospective longitudinal study of 571 randomly selected production cycles of farmed Atlantic salmon ( Salmo salar L.) between January 2013 and June 2019, with outbreaks classified according to PSVC-SRS criteria. Time to SRS outbreak was estimated using Kaplan-Meier survival analysis and compared across attributes with the log-rank test. A parametric Weibull model was applied to identify independent predictors of outbreak onset. The overall median time to outbreak was 41 weeks (w) post-stocking (95% CI: 38–43 w). Fish stocking during spring was associated with delayed outbreak onset (median 47 w, 95% CI: 43–51 w), whereas initiation of antibiotic treatment when weekly SRS-related mortality ≥0.03% was associated with earlier outbreaks (median 26 w, 95% CI: 24–28 w). In the final parametric Weibull model, two independent variables remained significant: stocking season and timing of SRS treatment initiation. Specifically, farm cycle stocked in spring and initiating antibiotic treatment when SRS-related mortality <0.01% exhibit a median time to outbreak of 51.5 w post-stocking (95% CI: 48.3–54.8 w). This study contributes to a better understanding of SRS outbreak dynamics and offers an epidemiological assessment of the disease within the framework of an official national health program.