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

Physical exercise represents a physiological stressor capable of activating the acute-phase response (APR) in horses. However, the relative contribution of exercise intensity versus duration to acute-phase protein (APP) dynamics remains incompletely defined. This study compared the effects of short, high-intensity gallop exercise (2400 m flat race; n = 12) and prolonged, low-intensity endurance exercise (40 km; n = 13) on serum amyloid A (SAA), haptoglobin (Hp), and ceruloplasmin (Cp) in Thoroughbred racehorses. Blood samples were collected before exercise and at defined post-exercise time points. Between-group comparisons were performed at shared time points (72 h and 96 h post-exercise) using mixed-effects modeling and effect size analysis. The significant Group × Time interaction for SAA indicates that exercise duration plays a key role in determining the magnitude and persistence of the late-phase systemic inflammatory response. Serum amyloid A emerged as the most sensitive biomarker of cumulative physiological stress following prolonged exercise. These findings support the use of SAA monitoring during recovery to assist training management in equine athletes. Because early post-exercise sampling was not performed in the gallop group, conclusions primarily reflect differences in late-phase (72-96 h) APP kinetics.