Formulation-Level Tolerability of Repeated Drinking-Water Administration of a Commercial Multi-Component Bacteriophage Product in Ross 308 Broiler Chickens: A Pilot 27-Day Dose-Escalation Study

Background: Bacteriophage-based interventions are increasingly considered antibiotic-sparing tools in poultry production, but target-animal tolerability should be documented before efficacy-oriented field application. Methods: This pilot study evaluated the formulation-level tolerability of the repeated drinking-water administration of Fagovet, a commercial multi-component bacteriophage product, in Ross 308 broiler chickens under non-challenged conditions. According to the manufacturer’s product documentation, the formulation contains a declared mixture of 23 bacteriophages at a concentration of not less than 1 × 106 plaque-forming units per milliliter. However, the concentration of active bacteriophages was not independently confirmed immediately before administration or after dilution in drinking water. A total of 120 birds were allocated to 12 pens, with three replicate pens per treatment, and received untreated water or product-volume-based administration rates corresponding to 1×, 3×, or 5× the manufacturer-recommended rate for 27 days. Body-weight gain and pen-level feed intake were analyzed using linear mixed-effects models with Dunnett-adjusted comparisons against controls. Clinical observations, mortality, diagnostic necropsies, terminal gross postmortem examination of all surviving birds, and descriptive histopathology in eight randomly selected birds, comprising two birds per treatment group, were used as supportive endpoints. Results: Body-weight gain did not differ from controls at 1× (ratio 0.99; 95% CI 0.88–1.10; p = 0.9856), 3× (1.03; 0.92–1.15; p = 0.8273), or 5× (1.01; 0.91–1.13; p = 0.9902). Feed intake showed no significant treatment-related reduction, and no consistent dose-related adverse clinical or pathological pattern was identified. Conclusions: Repeated drinking-water administration of the tested commercial formulation was well tolerated under the conditions of this pilot study. Because individual phage components, genomic safety, actual PFU-based exposure, and drinking-water stability were not independently characterized, the findings apply to the administered formulation and should not be interpreted as a component-level safety or quantitative phage dose–response assessment.