The Relationship of Intra-Articular Corticosteroid Administration with Infectious Disease Susceptibility in Horses

This proposal investigates whether intra-articular corticosteroid (IAC) administration increases susceptibility to equine influenza in previously vaccinated horses. Although corticosteroids are widely used in equine practice to manage musculoskeletal conditions, they are known to exert systemic immunosuppressive effects. Prior work demonstrates that a single intra-articular dose of triamcinolone or betamethasone can suppress endogenous cortisol and ACTH for multiple days, indicating measurable suppression of the hypothalamic-pituitary-adrenal axis. However, no studies have examined how such treatments influence susceptibility to infectious disease in horses, representing a significant knowledge gap. The project hypothesizes that administering triamcinolone acetonide 48 hours prior to viral challenge will increase clinical disease severity, viral shedding, and impair immune responses compared to saline-treated horses. The study uses equine influenza as an ethically appropriate and relevant model. Seventeen horses will be enrolled, with ten vaccinated horses assigned to treatment or control groups based on hemagglutinin inhibition titers, and three unvaccinated horses included to verify challenge virulence. All horses undergo standardized clinical monitoring, nasopharyngeal swab collection for viral load quantification, and daily scoring of clinical signs for ten days post-challenge. Immune responses will be assessed through repeated blood sampling, RNA extraction, and quantitative PCR targeting cytokines associated with cell-mediated immunity. Statistical analyses include mixed-effects models and nonparametric comparisons. Expected outcomes include measurable differences in disease severity and immune markers between groups; however, absence of differences will still provide meaningful insight for equine health management. Results aim to inform evidence-based guidelines for scheduling corticosteroid joint injections relative to travel and competition to minimize infectious disease risk in horses.