Bacteriophage Therapy against Bacterial Pathogens of Farmed Finfish: Mechanistic Evidence, Experimental Outcomes and Translational Constraints

Bacterial disease remains a major constraint on intensive finfish production, while antimicrobial resistance and environmental stewardship create incentives to investigate pathogen-specific interventions. Lytic bacteriophages can reduce bacterial abundance through receptor-dependent infection and can sometimes alter the virulence of surviving bacteria. Their therapeutic value nevertheless depends on whether active particles reach susceptible pathogens at the right anatomical site and time. This critical narrative review evaluates evidence concerning bacteriophages against clinically important bacterial pathogens of freshwater and marine finfish, distinguishing bacterial killing, experimentally improved fish survival and practical disease control. Peer-reviewed studies and relevant methodological literature published through 12 August 2026 were identified from accessible scholarly resources and critically compared by host–pathogen pairing, experimental challenge design, administration route, resistance, environmental persistence and health outcomes. Experimental successes have been reported against Aeromonas, Flavobacterium, Vibrio, Edwardsiella, Pseudomonas, Lactococcus and Streptococcus infections, but the results are neither uniform nor readily transferable across species. Particularly informative comparisons demonstrate that bath administration can protect against an externally transmitted columnaris infection, whereas the same broad delivery category fails to establish reliable protection against some systemic infections. A negative furunculosis study and route-dependent results in rainbow trout directly challenge the proposition that in-vitro lysis reliably predicts clinical benefit. Treatment timing, accessible dose, bacterial population structure, phage resistance and water-system hydraulics emerge as recurring effect modifiers. Evidence for reduced disruption of rearing-water microbiota is encouraging but still context-dependent. The review proposes a staged translational framework linking local surveillance, genomic qualification, route-specific pharmacology, adequately replicated tank trials and controlled farm evaluations. Bacteriophages are credible experimental tools and potentially valuable adjuncts to aquatic animal health management; routine replacement of antibiotics is not yet justified by the available finfish evidence.