Sedatives - Donkey

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Current best practice aspects of sedation in donkeys are described in detail in Anaesthesia and Sedation. A brief outline of the pharmacology of some of the agents used commonly in donkeys for sedation and analgesia is given below. These agents should never be administered without appropriate preanaesthetic evaluation and preparation and adequate monitoring is essential (Matthews and Van Dijk, 2004).

α2 agonists

The α2 agonists (xylazine, detomidine and romifidine) produce their clinical effects by binding to α2 adrenergic receptors centrally, to produce sedation, analgesia and decreased output from the sympathetic nervous system, or peripherally. The latter produces a number of effects, including arterial hypertension followed by hypotension, decreased heart rate, often with second degree atrioventricular block, increased total respiratory resistance and respiratory effort, slowed gastrointestinal motility, hyperglycaemia and increased urination.

Animals with severely impaired hepatic function may exhibit prolonged effects due to impaired metabolism of the α2 agonist. Some donkeys, particularly those not used to being handled, and miniature donkeys, may require higher doses of xylazine (and ketamine).

Acepromazine

The phenothiazine acepromazine (ACP) produces a general calming effect with decreased spontaneous motor activity via antagonism of dopamine at excitatory dopamine D2 receptors in the basal ganglia and limbic forebrain. Animals retain sensitivity to noise and visual stimulation.

The phenothiazines are also antagonists at α1 adrenergic receptors, histamine H1 receptors, muscarinic cholinergic receptors and serotonin 5-HT2 receptors, explaining a number of prominent side effects, including decreased hematocrit (α1 adrenoceptors in the splenic capsule), decreased total peripheral resistance (hypotension, α1 adrenoceptors in the vasculature) and penile prolapse (priapism). Hypotension can be profound, particularly in animals with high levels of circulating catecholamines (stress).

ACP does NOT produce analgesia and has a relatively long duration and relatively slow onset of effect, e.g. 20 to 30 minutes. ACP should be used with caution in male donkeys (priapism) and in animals with impaired hepatic function (potentially prolonged effects). ACP should not be used in breeding stallions. The neuroleptanalgesic combination of ACP with the potent opioid analgesic etorphine should NOT be used in donkeys.

Diazepam

The major pharmacological effects (mild sedation, muscle relaxation, ataxia, anticonvulsant effect) of the benzodiazepines (diazepam, zolazepam) are mediated via effects on chloride channels in the CNS. The primary metabolites of these agents, e.g. N-desmethyldiazepam, are active, have similar potency to the parent compounds and are of potential clinical significance, e.g. following repeated administration of the parent drug (accumulation) or in foals less than 21 days of age (lower clearance).

These agents are highly protein-bound and should be used with care in hypoalbuminemia and concomitantly with other highly protein-bound drugs. Benzodiazepines should also be used with caution in animals with impaired hepatic function and concurrently with agents that also undergo hepatic metabolism, e.g. erythromycin, cimetidine. Zolazepam is only available in a fixed combination with the dissociative anaesthetic tiletamine.

References

• Horspool, L. (2008) Clinical pharmacology In Svendsen, E.D., Duncan, J. and Hadrill, D. (2008) The Professional Handbook of the Donkey, 4th edition, Whittet Books, Chapter 12
• Matthews, N.S., Van Dijk, P. (2004). Anesthesia and analgesia for donkeys In Veterinary Care of Donkeys. N.S. Matthews, T.S. Taylor (eds). International Veterinary Information Service [www.ivis.org], New York. A2902.0904.

PHARMACOLOGY - DONKEY

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