Analgesic and Gastrointestinal Effects of Methadone in Horses Undergoing Orchiectomy

A multimodal approach is recommended to optimize perioperative pain control in animals, although opioid use in horses remains limited due to the risks of central nervous system (CNS) stimulation and reduced intestinal motility. A group of 19 healthy, male, mixed-breed horses were divided into two groups and medicated with acepromazine (0.05 mg kg^-1) and detomidine (10 µg kg^-1), with methadone (0.05 mg kg^-1) (ADM) or saline (ADS) administered intravenously (IV). Physiological variables, intestinal motility, gastric distention, and facial pain (EQUUS-FAP) were evaluated one day before (DB), before the surgical procedure (BS), and at 1, 2, 4, 6, and 8 h (T1h-T8h) after administration (ADM-ADS). Results are presented as means with standard deviation or medians with an interquartile range. Analysis of variance, the Mann-Whitney, and Durbin tests were applied (p < 0.05). Intestinal motility was reduced at T1h and T2h, returning to baseline by T6h and T8h in both groups. Ultrasonographic examination revealed reduced motility, with less significant changes in the left ventral colon (LVC), right ventral colon (RVC), and cecum. Gastric dilatation was more pronounced in the ADM group at T1, 4, 6, and 8h. EQUUS-FAP scores were significantly lower in ADM at T2, 4, and 6h. ADM protocol may aid chemical restraint and analgesia without increasing hypomotility.