A comparative study of xylazine-induced mechanical hypoalgesia in donkeys and horses.

To compare the effects of xylazine on mechanical nociceptive thresholds in donkeys and horses. Randomized, controlled, crossover, Latin-square, operator-blinded design. Six 3.1 ± 0.89 year old standard donkeys weighing 145.0 ± 30.5 kg and six 9.6 ± 4.4 year old Thoroughbred horses weighing 456.0 ± 69.0 kg. Each animal received one of four doses of xylazine (0.5, 0.7, 0.9, and 1.1 mg kg(-1) ), or acepromazine (0.05 mg kg(-1) ) or saline solution (0.9%) intravenously and mechanical nociceptive thresholds were assessed over 90 minutes. The areas under the threshold change versus time curve values for 60 minutes (AUC(0-60) ) post-drug administration were used to compare the effect of treatment. A 1-week interval was allowed between successive trials on each animal. All doses of xylazine, but not acepromazine or saline, increased mechanical thresholds for up to 60 minutes. Xylazine-induced hypoalgesia was dose-dependent and corresponding AUC(0-60) values for each treatment were not significantly different between donkeys and horses (p ≥ 0.0697). The hypoalgesic effects of xylazine at four different doses were not different between donkeys and horses. Xylazine induced a similar degree of mechanical hypoalgesia in donkeys and horses suggesting that similar doses are needed for both species with regard to analgesia.