Postweaning diarrhea remains a major challenge in swine production, and bacteriophages are explored as antibiotic alternatives. This meta-analysis quantified the effects of bacteriophage therapy on bacterial load in piglets and examined the factors contributing to variation in treatment outcomes. A multi-database search identified 85 effect sizes from 11 challenge studies involving 198 piglets. Standardized mean differences (SMDs) were calculated using multilevel random-effects models, and heterogeneity, publication bias, and moderator effects were assessed. Among moderators, only challenge pattern significantly explained variation in effect sizes (Q = 11.72, df = 5, p = 0.039), while all other moderators were not significant (p > 0.05). Within moderators, some trends emerged. Prophylactic dosing produced a moderate and highly consistent reduction (SMD = −0.64, p < 0.001; I2 = 19.41%), whereas therapeutic dosing produced a larger but more variable effect (SMD = −1.29, p < 0.001; I2 = 66.77%). Reductions were significant for both Escherichia coli (SMD = −0.79, p = 0.014) and Salmonella (SMD = −0.90, p < 0.001) models. Encapsulated phages produced larger reductions than non-encapsulated formulations (SMD = −1.51, p = 0.026; SMD = −0.70, p < 0.001), although moderator test was not significant. Single-phage produced larger reductions (SMD = −1.13, p < 0.001) than cocktails (SMD = −0.59, p < 0.001), whereas cocktails showed more consistent responses (I2 = 33.47% versus 55.39%). Infeed delivery yielded more consistent outcomes than oral gavage (I2 = 21.98% versus 63.91%). Assessment of publication bias did not indicate small-study effects (Egger’s test: p = 0.77). Slight funnel plot asymmetry was not sufficient to indicate reporting bias. Sensitivity analyses showed that the pooledestimate was stable across diagnostics. Overall, the evidence indicates that phage treatment reduced bacterial load in piglets, while practical application will likely require further standardization of formulation, delivery, and dosing procedures.
