Animal Welfare-Friendly Noninvasive Oral Fluid in Swine Diagnostics: Current Applications in Disease Monitoring, Key Challenges and Future Directions.

The integration of oral fluid (OF) sampling into veterinary diagnostics represents a significant shift in animal herd health monitoring, offering a noninvasive alternative to traditional blood-based methods. OF, predominantly composed of saliva, mucosal transudate and plasma-derived components, contains immunoglobulins, acute-phase proteins and nucleic acids that mirror both local and systemic physiological processes. These components enable a thorough evaluation of immune responses and inflammatory status, as well as the detection of economically significant infectious pathogens in swine. OF can be collected noninvasively using cotton ropes or swabs, which is feasible for individual or pen-level herd health sampling with minimal animal handling and fosters an animal-friendly approach. Several studies have demonstrated pathogen antigen detection, particularly PCR-based detection of shedding pathogens, as well as antibody and biomarker measurements in OF; however, concentrations are often lower than those observed in other biological fluids, especially in serum. In addition, diagnostic interpretation is further constrained by poor pig participation, rope chewing due to dominance, low prevalence within pens, contamination with feed or environmental material, instability of analytes during transport and storage, and the scarcity of validated assays specific to OF. This review summarises OF production, composition and diagnostic analyte entry pathways from blood to OF and current applications for detecting pathogens and biomarkers in pigs and critically examines methodological factors that influence diagnostic performance, including sample collection procedures, handling and assay optimisation, such as dilution and threshold selection. Finally, it outlines priorities for future development, including standardised OF collection and handling protocols, OF-validated pathogen and biomarker assays, matrix-specific thresholds, regulatory validation and proteomic biomarker discovery to enable earlier detection and support continuous herd health monitoring in global swine production.