Gut Microbiota and Selected Systemic Biomarker Profiling in Dogs with Distinct Osteoarthritis Phenotypes

Osteoarthritis (OA) is increasingly recognized as a heterogeneous disorder involving metabolic and inflammatory alterations beyond the affected joint. This study evaluated gut microbiota composition and selected systemic biomarkers in client-owned dogs with distinct OA phenotypes. Fecal microbiota profiling was performed using near full-length 16S rRNA Nanopore sequencing in clinically healthy controls (CG), obese OA dogs (OOA), and thin OA dogs (TOA). Alpha and beta diversity analyses were performed across multiple taxonomic levels, together with evaluation of circulating lipopolysaccharide (LPS), lipopolysaccharide-binding protein (LBP), and tumor necrosis factor alpha (TNF-α). OA phenotypes exhibited modest but detectable microbial ecological differences. OOA dogs showed reduced richness and lower Firmicutes/Bacteroidota ratios relative to CG dogs. PERMANOVA identified significant phylum-level compositional differences among phenotypes (R² = 0.121, p = 0.002), which remained significant after adjustment for age, sex, and body weight (R² = 0.095, p = 0.009). Lower-level taxonomic associations became non-significant after multivariable adjustment and false discovery rate correction. LPS and LBP concentrations did not differ among groups, whereas TNF-α differed significantly between OOA and TOA dogs after adjustment for covariates (p = 0.014), with back-transformed estimated marginal means (95% CI) of 27.5pg/mL (23.0-32.8) in OOA dogs and 38.5pg/mL (32.3-45.8) in TOA dogs. Associations between microbial ecological variables and circulating biomarkers were weak and did not remain significant after false discovery rate correction. These findings suggest that naturally occurring canine OA is associated with subtle and phenotype-dependent microbial ecological restructuring rather than marked global dysbiosis.