Phage Display in Veterinary Infectious Diseases: From Molecular Recognition to Diagnostic Validation and Protective Efficacy.

Phage display generates defined binding reagents; however, successful selection does not establish diagnostic usefulness or protection in animals. This critical narrative review examines the veterinary applications that have progressed beyond binding assays and the biological and validation barriers that explain their limitations. The studies were compared based on the contribution of phage display, experimental setting, comparator, and endpoint, distinguishing antigen discovery, diagnostic evaluation, functional activity, and efficacy in the target host. Recombinant reagents for infectious bursal disease and African swine fever viruses demonstrated assay-specific improvements or agreement with comparator tests, whereas recent Senecavirus A results illustrated how apparent performance depends on the reference method. Vaccine and therapeutic studies provide a more heterogeneous picture: protective effects in selected animal models coexist with non-protective immune responses and unresolved delivery requirements. Chicken repertoire diversification and bovine ultralong antibody loops create opportunities but require host-adapted library construction and expression. Next-generation sequencing, phage immunoprecipitation sequencing, and artificial intelligence are discussed separately as enabling approaches whose veterinary benefits require direct validation. The principal contribution of this review is a comparison of what each experimental endpoint establishes and what remains necessary for deployment. Current evidence supports the development of application-specific reagents and selected experimental interventions rather than a general claim of superiority over established veterinary diagnostics or vaccines.