Opioids: An Underutilised Option?

Opioids play a prominent role in analgesic protocols for people and small animals. In contrast, the administration of opioids for analgesia in horses has been, until recently, less common. Reasons for this include concerns over excitement, colic and a lack of data on efficacy. Over the last 10 years publications have reported no adverse, and some beneficial, effects of perioperative opioid administration to horses and this has stimulated renewed interest in using opioids in horses.

Butorphanol was the most commonly administered opioid in the UK 2002 (Price et al. 2002) possibly because it has a UK marketing authorisation and its administration with detomidine has a synergistic effect on sedation (Clarke and Paton 1988). The analgesic effects of butorphanol are short lived and this means that for longer-term pain relief a continuous rate infusion should be considered (Sellon et al. 2001). If a single dose is used the analgesic effects may wane shortly after recovery from anaesthesia or sedation and this could account for the lack of analgesia observed after administration of butorphanol to colts undergoing castration (Love et al. 2009). It is also likely to be insufficient in horses suffering from severe pain as butorphanol administration (0.1 mg/kg bwt) to horses with colic did not produce satisfactory analgesia in any animal (Jochle et al. 1989). Despite the lack of evidence either way regarding the analgesic efficacy of butorphanol, administration during anaesthesia has been recommended by some authors since it appears to blunt sympathetic stimulation during surgery (Hofmeister et al. 2008) and may improve the subjectively assessed ‘quality’ of anaesthesia (Corletto et al. 2005). [...]