The comprehensive examination of bile acids is of paramount importance across various fields of health sciences, influencing physiology, microbiology, internal medicine, and pharmacology. While enzymatic reaction-based photometric methods remain fundamental for total BA measurements, there is a burgeoning demand for more sophisticated techniques such as liquid chromatography–tandem mass spectrometry (LC-MS/MS) for comprehensive BA profiling. This evolution reflects a need for nuanced diagnostic assessments in clinical practice. In canines, a BA assessment involves considering factors, such as food composition, transit times, and breed-specific variations. Multiple matrices, including blood, feces, urine, liver tissue, and gallbladder bile, offer insights into BA profiles, yet interpretations remain complex, particularly in fecal analysis due to sampling challenges and breed-specific differences. Despite ongoing efforts, a consensus regarding optimal matrices and diagnostic thresholds remains elusive, highlighting the need for further research. Emphasizing the scarcity of systematic animal studies and underscoring the importance of ap-propriate sampling methodologies, our review advocates for targeted investigations into BA alterations in canine pathology, promising insights into pathomechanisms, early disease detection, and therapeutic avenues.

Veterinary metabolomics has redefined bile acids (BAs) from simple digestive surfactants to systemic endocrine signals within a microbial–host metabolic axis. This review aims to evaluate how BA dysmetabolism acts as a central pathogenic factor in canine and feline disease. We analyze the BA pool’s integrity, which depends on a specialized functional guild, primarily Peptacetobacter hiranonis, responsible for 7α-dehydroxylation. We delineate two principal pathological profiles: (1) microbial collapse, characterized by secondary bile acid (SBA) depletion and compromised farnesoid X receptor (FXR) and Takeda G protein-coupled receptor 5 (TGR5) signaling, which exacerbates inflammation in chronic enteropathy (CE), protein-losing enteropathy (PLE), and exocrine pancreatic insufficiency (EPI); and (2) hepato-biliary spillover, wherein host-induced dysfunction results in primary bile acid (PBA) excess. Recent data have linked these disruptions to skeletal health, feline renal fibrosis, cardiac remodeling in myxomatous mitral valve disease (MMVD), and neuroinflammation in epilepsy and hepatic encephalopathy. The discovery of microbially conjugated bile acids (MCBAs) and microbial extracellular vesicles (MEVs) reveals highly specific, vesicle-mediated communication pathways impacting systemic health. Diagnostic protocols should prioritize functional profiling, including the dysbiosis index (DI), serum conjugated BA analysis, and SBA/PBA ratios. Clinical management is moving beyond empirical fecal microbiota transplantation (FMT), towards precision synthetic microbial consortia (SynComs), neuroprotective BAs like tauroursodeoxycholic acid (TUDCA), and molecular postbiotics to restore the collaborative metabolome.

Extracellular vesicles (EVs) are key mediators of intercellular communication across biological kingdoms, with central roles in immune regulation and disease processes. Despite shared structural features, EVs derived from bacteria, plants, and mammalian cells differ substantially in their biogenesis, molecular composition, and immunological functions. EV formation pathways generate vesicles with distinct cargo profiles, including pathogen-associated molecular patterns (PAMPs) in bacterial EVs, regulatory small RNAs in plant-derived vesicles, and cytokines, microRNAs, and antigen-presenting complexes in mammalian EVs. Differences in cargo result in divergent immune outcomes. Bacterial EVs predominantly activate innate immunity via pattern recognition receptors such as Toll-like receptors, whereas plant-derived EVs exhibit low immunogenicity and mediate cross-kingdom RNA interference. In contrast, mammalian EVs primarily regulate immune responses by modulating antigen presentation and cytokine signaling. These findings support a framework in which EV origin determines immunological function and therapeutic applicability. This perspective highlights the importance of selecting appropriate EV sources for vaccine development, regenerative medicine, and targeted delivery strategies, while addressing current challenges related to heterogeneity, standardization, and safety.

Collagen, long regarded as the inert structural scaffold of the extracellular matrix, is now recognised as a source of bioactive signaling molecules: enzymatic hydrolysis releases low-molecular-weight bioactive collagen peptides that act systemically. This narrative review synthesises in vitro, animal-model, and veterinary clinical evidence with explicit differentiation of evidence levels; literature was identified through structured searches of PubMed, Web of Science, and Google Scholar, covering peer-reviewed, English-language reports published between 2000 and January 2026, without a formal systematic protocol. Across rodent, porcine, and human pharmacokinetic studies, orally administered collagen hydrolysate is efficiently absorbed, with a fraction reaching the circulation as intact prolyl-hydroxyproline and hydroxyprolyl-glycine through the conserved PEPT1/PEPT2 transporters; reported bioavailability varies with source, dose, and method, so no single value generalises across species. Native collagen and larger collagen fragments engage structure-dependent receptors in vitro (α2β1 integrins, DDR1/DDR2, GPVI, LAIR-1/2) that require triple-helical or Gly-Pro-Hyp presentation, whereas it is not established that the di- and tripeptides that reach the circulation after oral dosing engage them; their systemic actions are partly attributable to intracellular routes, including the Keap1–Nrf2 axis, HDAC/HAT modulation, and glycine-dependent glutathione synthesis. The gut–collagen peptide axis, a model derived from rodent and cell-culture data, links microbial bile acid remodeling and FXR/TGR5 signaling to systemic effects. Current evidence supports validated use in canine and equine osteoarthritis; species-specific bioavailability studies in dogs and cats remain the priority.

Bile acid (BA) dysmetabolism is implicated in canine chronic inflammatory enteropathy (CIE), but fasting serum profiles across treatment-response groups remain poorly characterized. This exploratory observational study analyzed pretreatment serum from dogs subsequently classified as food-responsive enteropathy (FRE; n = 8) or immunosuppressant-responsive enteropathy (IRE; n = 5), with four unmatched purpose-bred Beagle controls. Targeted LC–MS/MS, performed using a published canine serum method, provided results for 15 BA species. All samples were analyzed in one batch by an analyst blinded to subsequent group assignment. Eight species quantifiable in every dog and ten derived variables were compared using Kruskal–Wallis tests with Benjamini–Hochberg correction across 18 omnibus tests. Cholic acid and unconjugated primary BA concentrations were higher in IRE than in controls and FRE. The total primary-to-secondary BA ratio showed a monotonic group ordering (control < FRE < IRE; H = 13.76, p = 0.0010, q = 0.0092; epsilon-squared = 0.840), with all Bonferroni-corrected pairwise comparisons significant. A post hoc four-scenario sensitivity analysis comprising the submitted zero-substitution analysis and three LOD/LOQ-based alternatives reproduced the same 18-test FDR-significant set and the same ratio inference. This analysis tests robustness to censored-value handling but is not independent validation. The findings suggest group-associated serum BA patterns, but the small sample, clinical heterogeneity, and unmatched controls preclude diagnostic or treatment-predictive interpretation.