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Definition

Opioids can produce excitement, seen as box walking, restlessness and dysphoria when administered alone in pain‐free horses.

Risk factors

 Administration of high doses of opioids without the concurrent administration of a sedative drug

 Administration of opioids in pain‐free horses

Pathogenesis

Horses possess a unique opioid receptor profile and density compared to other species and are sensitive to opioid‐induced CNS stimulatory and locomotor effects. Excitement may result indirectly from increased release of norepinephrine and dopamine. This may explain the mechanism why noradrenergic and dopaminergic blocking drugs like phenothiazines suppress the opioid induced excitement [12]. However, increased locomotor activity produced by opioids seems to be associated with opioid receptors. The propensity of an opioid analgesic to promote locomotion may be greater with mu (e.g., morphine) than with kappa agonists (e.g. butorphanol) [27]. Kappa agonism more commonly causes ataxia and staggering [28]. Opioids were studied in varying numbers of pain‐free horses in one of the most commonly cited references on opioid‐induced locomotion in horses [32]. It is important to note that there is marked individual variation in responses. The median effective dose of morphine that causes an increase in locomotion activity in pain‐free animals is 0.91 mg/kg, which is considerably higher than the doses used clinically to produce analgesia [30].

Prevention

Using appropriate clinical doses of opioids in combination with a sedative drug will prevent this excitement [30].

Diagnosis

Increased locomotor activity, box walking, head jerking, disorientation and/or ataxia

Treatment

The use of sedative agents like acepromazine or apha‐2 adrenergic agonists can calm and sedate the horse, solving the excitement and increased locomotor activity. The use of the opioid antagonist naloxone (0.015 mg/kg) entirely prevented locomotor responses to morphine and fentanyl [31]. Naloxone will revert the analgesics effects of any opioid, therefore it should be used with caution in painful horses and only in severe cases or overdose.

Expected outcome

The outcome is good as these effects are usually mild and easy to control with the administration of a sedative.