Analgesic Effect of Butorphanol during Castration in Donkeys under Total Intravenous Anaesthesia

SIMPLE SUMMARY: Despite the growing interest in donkey’s welfare, there is little scientific evidence in this species about the analgesic effect of the drugs most used in horses. Total intravenous anaesthesia is a valuable option for field surgery such as castration. Moreover, it is often preferred in small donkeys due to the anatomical features of their proximal airways. The aim of this study was to evaluate the effect of a low dose of butorphanol, in donkeys undergoing castration, after sedation with detomidine before total intravenous anaesthesia with a mixture of guaifenesin, ketamine and detomidine. The addition of butorphanol to detomidine produced more muscle relaxation, and reduced mean heart rate and blood pressure. This resulted in a reduced requirement for rescue analgesia, a more superficial anaesthetic plan and a shorter surgical time. It can be concluded that butorphanol allowed a more stable and superficial anaesthetic plan, probably due to its analgesic effect and synergistic action with detomidine. Due to the low dosages of drugs used and the absence of adverse effects, this protocol can be considered valuable in field conditions and for short surgical procedures, where safety equipment is not available. ABSTRACT: Pain management is necessary for all surgical procedures. Little scientific evidence about drug efficacy in donkeys is available. The aim of this study was to evaluate the analgesic effect of butorphanol in donkeys undergoing orchiectomy under total intravenous anaesthesia with guaifenesin-ketamine-detomidine. A randomized blinded prospective clinical trial (Protocol n. 2021/0000338), was carried out on 18 clinically healthy donkeys undergoing bilateral orchiectomy. Patients were assigned to Group D (n = 8) or Group DB (n = 10) if receiving intravenous detomidine or detomidine-butorphanol respectively, before induction of general anaesthesia with ketamine-diazepam. Intraoperative muscle relaxation, nystagmus, palpebral reflex, heart and respiratory rate, and non-invasive blood pressure were evaluated every 2 min; time to prepare the patient, duration of surgery and anaesthesia and recovery score were recorded. Group D had significantly longer surgical time, higher heart rate, higher systolic and mean blood pressure (p < 0.05; repeated measure ANOVA), increased muscle rigidity and expression of palpebral reflex (p < 0.05; Mann–Whitney U test) than group DB. Top-ups with thiopental were statistically higher in Group D. Butorphanol and detomidine together produced a more stable anaesthetic plan. The low dosage of opioid and alpha-2-agonists and reduced rescue anaesthesia are responsible for a safer and more superficial anaesthesia, which is mandatory under field conditions.