Chile's salmon farms face antibiotic crisis with no easy exit

Aquaculture has expanded rapidly over recent decades, positioning salmon farming as a major contributor to global food security while intensifying concern over antimicrobial use and the emergence of antimicrobial resistance (AMR). Chile, the world’s second-largest producer of farmed salmon, represents a critical case study because of its historically high dependence on antibiotics, particularly florfenicol and oxytetracycline. This dependence is driven largely by the endemic burden of salmon rickettsial syndrome (SRS), caused by Piscirickettsia salmonis , for which currently available vaccines have shown limited and inconsistent effectiveness under commercial farming conditions. In this review, we examine antimicrobial use patterns, resistance dynamics, environmental dissemination, and regulatory frameworks associated with Chilean salmon aquaculture within a One Health perspective. We show how intensive production systems, persistent disease pressure, and operational constraints have favored a predominance of metaphylactic treatments delivered through medicated feed. Although operationally feasible, this strategy entails major biological and ecological drawbacks, including heterogeneous drug exposure, unnecessary treatment of clinically healthy fish, and sustained selective pressure on microbial communities associated with fish, sediments, and surrounding aquatic environments. We further argue that antimicrobial dependence in Chilean salmon aquaculture is sustained by multiscale drivers that extend beyond pathogen burden alone. These include the structure of an export-oriented production model, the mismatch between long production cycles and prolonged disease susceptibility, incomplete incorporation of host resistance into preventive strategies, and the destabilizing effects of environmental stressors such as harmful algal blooms and low-oxygen conditions. Although acquired resistance in major salmon pathogens remains limited and, in some cases, mechanistically unresolved, aquaculture-associated microbiota constitute important reservoirs of antibiotic-resistant bacteria and resistance genes. Mobile genetic elements linked to aquaculture environments have also been detected in opportunistic and clinically relevant human pathogens, highlighting ecological connectivity and broader public health relevance beyond farm boundaries. We conclude that reducing antibiotic dependence in Chilean salmon aquaculture will require a transition toward preventive, biologically informed, and data-driven health management, supported by improved vaccine performance against SRS, integrated genomic and environmental surveillance, and regulatory thresholds grounded in robust biological evidence.