Clinical Presentation, Detection, and Immunopathogenesis of Mycoplasma hyosynoviae Field Isolates in Experimentally Inoculated Pigs

<i>Mycoplasma hyosynoviae</i> is a significant pathogen in swine populations, contributing to polyarthritis and lameness in growing pigs. This study characterizes the clinical presentation, pathogen detection, immune response, and lesion development following experimental inoculation with two distinct <i>M. hyosynoviae</i> strains. Pigs were inoculated with either a low- or high-virulence strain and monitored for 18 days. Lameness was observed throughout the study, with affected pigs exhibiting mild to moderate clinical signs. <i>M. hyosynoviae</i> was often detected in the tonsils, while detection in oral fluids was transient. Serum IgG levels increased significantly in the inoculated groups. IL-1β, IL-6, and TNF-α cytokines were elevated only at 7 DPI, whereas IL-8, IL-10, and IFN-γ levels were unchanged in both inoculated groups. Notably, only pigs inoculated with the high-virulence strain developed lesions, and <i>M. hyosynoviae</i> was detected only in the synovial fluid by PCR from this group. These findings highlight strain-dependent differences in the pathogenesis of <i>M. hyosynoviae</i>. The pathological differences between these strains suggest variations in adherence factors, immune evasion capabilities, or metabolic adaptability. Further research is warranted to elucidate the genetic determinants of virulence and the protective role of humoral and cellular immune responses in <i>M. hyosynoviae</i> infection.