Effects of Exercise Intensity and Duration on Acute-Phase Proteins in Thoroughbred Racehorses

Physical exercise represents a relevant physiological stressor capable of activating the acute-phase response (APR) in horses. However, the relative influence of exercise intensity and duration on the dynamics of acute-phase proteins (APPs) remains incompletely defined. This study compared the effects of short, high-intensity exercise and prolonged, low-intensity exercise on serum amyloid A (SAA), haptoglobin (Hp), and ceruloplasmin (Cp) concentrations in Thoroughbred racehorses. Two groups of clinically healthy horses were evaluated: one group completed a 2,400 m gallop race (n = 12), and the other completed a 40 km endurance race (n = 13). Blood samples were collected before exercise and at multiple time points post-exercise, up to 144 h in endurance horses. Acute-phase proteins were quantified using validated assays, and data were analysed using mixed-effects models. Prolonged endurance exercise induced a significant and time-dependent increase in SAA concentrations, with peak values observed at 96 h post-exercise (p < 0.001), accompanied by a moderate but significant increase in Cp at 72 h (p < 0.05). In contrast, no statistically significant changes in SAA, Hp, or Cp were detected following the gallop race. Haptoglobin concentrations remained relatively stable in both groups. These findings indicate that exercise duration plays a key role in modulating the systemic inflammatory response in horses. Serum amyloid A emerged as the most sensitive biomarker of exercise-induced APR, particularly following prolonged endurance exercise. Monitoring APPs may provide a useful tool for assessing recovery status and supporting evidence-based training management in equine athletes.