Effect of transdermal melatonin on circulating cortisol and blood chemistry in horses exposed to transport stress

Our objective was to evaluate the effect of transdermal melatonin on physiological stress biomarkers in horses, with some subjected to transport stress. Sixteen horses were assigned to a 2 × 2 factorial design assessing treatment (melatonin vs. no melatonin) and transport status (transported vs. non-transported), creating four groups: Control (CON), Transport only (TRS), Melatonin only (MEL), and Melatonin plus Transport (MELTRS). Horses in melatonin groups received 24 mg/day transdermally for 21 days and transported groups underwent a 3-hour trailer ride on day 21. Day 21 blood samples were analyzed for cortisol, melatonin, and standard blood chemistry parameters. There was no interaction of Transport and Melatonin. Fixed effect of transport significantly elevated cortisol, glucose, direct bilirubin, and sodium-to-potassium ratios (P ≤ 0.039). Fixed effect of melatonin significantly reduced cortisol levels (P = 0.029). Results suggest transdermal melatonin may reduce cortisol and potentially mitigate transport-induced stress, though further studies are needed.