Epidemiology of Salmonid Rickettsial Septicemia (SRS) in Farmed Salmon: The Role of Sea Lice Infestations in Mortality Risk.

Piscirickettsia salmonis, the causal agent of salmonid rickettsial septicemia (SRS), is the main pathogen affecting farmed salmonids in Chile. Outbreaks of SRS lead to substantial economic losses for producers. Many determinants related to SRS outcomes are still poorly understood. Here, we conducted a retrospective cohort study from 2014 to 2021 to investigate the epidemiology of SRS at the farm level in southern Chile, employing historical monitoring data. Using time series analysis of weekly SRS mortality risk and sea lice (Caligus rogercresseyi ) infestation levels, we found that SRS mortality risk had a strong seasonal component, with mortalities being significantly higher in the warmer seasons. While Caligus infestation levels have increased significantly over the years, SRS mortality risk has remained constant. Using mixed effects regression models, we identified that a key predictor for both increased weekly SRS mortality risk and higher hazard of reporting the first SRS outbreak of a production cycle was the level of female egg-laying sea lice. We hypothesise that the interaction between sea lice, P. salmonis and rising water temperatures may produce synergistic stress on salmon that accelerates disease progression and prompts overuse of antimicrobials. This calls for an urgent integrated pest management approach in aquaculture practice.