Morphine in donkeys: Antinociceptive effect and preliminary pharmacokinetics.
Authors: Maney Jill K, Dzikiti Brighton T, Escobar Andre, Knych Heather K, Bennett Rachel C
Journal: Equine veterinary journal
Summary
# Morphine Analgesia in Donkeys: Dose-Dependent Effects and Pharmacokinetic Profile Whilst morphine is well-established for pain relief in horses, its efficacy and behaviour in donkeys remained largely undocumented until this 2023 study. Researchers administered intravenous saline, low-dose morphine (0.1 mg/kg) or high-dose morphine (0.5 mg/kg) to eight castrated male donkeys and measured mechanical nociceptive thresholds at multiple timepoints over six hours, whilst simultaneously tracking plasma morphine concentrations. The high dose produced substantially greater antinociceptive effect, with mechanical nociceptive thresholds peaking at 60 minutes (25.0 N versus baseline 8.9 N) and remaining significantly elevated compared to placebo for up to five hours; the low dose showed more modest and briefer analgesia, reaching peak effect only at 45–60 minutes. Pharmacokinetic analysis revealed a terminal half-life of approximately 51 minutes and rapid plasma clearance, with metabolite profiling showing donkeys preferentially produce morphine-3-glucuronide over morphine-6-glucuronide—a pattern worth noting given the differing analgesic and adverse effect profiles of these metabolites. For equine practitioners managing donkey pain, the 0.5 mg/kg intravenous dose appears justifiable for sustained analgesia without observed adverse effects, though the short plasma half-life suggests consideration of repeat dosing or alternative routes if prolonged analgesia is required.
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Practical Takeaways
- •High-dose morphine (0.5 mg/kg IV) is effective for pain management in donkeys with analgesic effects lasting 4-5 hours, making it suitable for acute pain relief in clinical settings
- •The shorter half-life in donkeys (51 minutes) compared to horses suggests donkeys may require more frequent dosing intervals for sustained analgesia
- •Donkeys show poor analgesia with low-dose morphine (0.1 mg/kg), so practitioners should use at least 0.5 mg/kg IV to achieve clinically meaningful pain relief
Key Findings
- •High-dose morphine (0.5 mg/kg) produced significant mechanical antinociception in donkeys with peak effect at 60 minutes, maintaining elevated thresholds for up to 300 minutes compared to saline
- •Low-dose morphine (0.1 mg/kg) showed antinociceptive effects only at 45 and 60 minutes post-injection, with substantially shorter duration than high-dose
- •Terminal half-life of morphine in donkeys was 51.0 ± 10.7 minutes with morphine-3-glucuronide being the predominant metabolite throughout sampling period
- •No significant adverse effects were reported with either morphine dose in this study population