[Effects of morphine, butorphanol and levomethadone in different doses on thermal nociceptive thresholds in horses].

OBJECTIVE: Various opioids are available for use in equine medicine. Studies directly comparing their analgesic effects and side effects are rare. Therefore, the aim of this study was to compare the antinociceptive effect and the duration of analgesia of two different doses of morphine, butorphanol and levomethadone in horses.

MATERIAL AND METHODS: Eight adult, healthy horses were used for this randomized, placebo-controlled, blinded cross-over trail. Each horse received placebo (P = 0.9% saline) and morphine (M0.1  = 0.1 mg/kg; M0.2  = 0.2 mg/kg), butorphanol (B0.1  = 0.1 mg/kg; B0.2  = 0.2 mg/kg) and levomethadone (L0.1  = 0.1 mg/kg; L0.2  = 0.2 mg/kg) in a low and a high dose and with a wash-out period of 14 days. Thermal thresholds were determined by incremental contact heat applied to the skin at the withers. Single stimulations were performed 15 minutes prior and 10, 30, 60, 90, 120, 180, 240, 300, 360, 420, 540 and 1350 minutes after treatment. Threshold values, gastrointestinal auscultation score and horses' behavior were recorded. Data were analyzed with analysis of variance for repeated measurements (p < 0.05).

RESULTS: In group M0.1 , changes in thermal thresholds did not reach significance. Thermal threshold increased significantly in the groups M0.2 , B0.1 , B0.2 , L0.1 and L0.2 for 240, 90, 90, 60 and 300 minutes, respectively. Behavioural changes, increased locomotion and decreased bowel sounds as well as delayed time until defecation were noticed in all groups.

CONCLUSIONS: Levomethadone induced a dose-dependent increase and prolongation of analgesia, whereas with butorphanol there was no difference between dosages regarding duration and intensity of analgesia. Morphine provided detectable analgesia only in the high dose of 0.2 mg/kg.

CLINICAL RELEVANCE: Levomethadone and morphine in the low dose (0.1 mg/kg) produced only minor and short lived anti-nociception and further studies are necessary to give a profound dose recommendation for the use of these drugs in horses.