Pharmacology and Therapeutics
Author(s):
The Donkey Sanctuary
In: The Clinical Companion of the Donkey (2nd Edition) by Evans L. et al.
Updated:
JUL 03, 2022
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INTRODUCTION
Few veterinary products have market authorisation for use in the donkey. Although preparations designed for horses and ponies are often used for donkeys, there are recorded differences between these species in the pharmacokinetics of certain drugs. An understanding of these differences can enable the practitioner to make appropriate therapeutic decisions to achieve maximum efficacy of medication and reduce the risk of toxicity.
Medicines are included in this chapter only where it has been recorded that there is a notable difference between donkeys and the horse and pony.
Clinicians are advised that they must consult current equine formularies and datasheets. Owners should be informed accordingly and it is imperative that the appropriate national legislation is adhered to.
While we have made every effort to ensure that the content of this chapter is currently accurate, we accept no liability for any loss, damage or expenses arising as a result of relying solely on the information contained within the formulary in this book.
Accurate estimation of the weight of a donkey is essential for effective dosing regimes. A donkey-specific nomogram or a donkey specific weigh tape should be used to estimate weight where scales are not available, as those used for horses and ponies are not suitable.
This chapter will cover the following subjects:
Key points
- Estimating the weight of donkeys is acknowledged to be difficult where scales are not available. A donkey specific nomogram is advised to improve accurate dosing of medication.
- Pharmacokinetics of drugs for donkeys are often different to that for horses and ponies. The clinician is advised to check donkey specific dosage.
- Most drugs are not licensed for use in the donkey. Local legal requirements for prescribing unlicensed medication must be strictly followed.
PHARMACOLOGICAL DIFFERENCES OF THE DONKEY
The donkey’s semi arid environment is one of low value forage and limited water supply. The donkey has been reported to tolerate dehydration more effectively than the horse and pony, efficiently recycling water from the large colon and having a lower urine output, even when water is readily available. The donkey has a longer gut transit time. This is one factor leading to an increased digestive efficiency that could have an effect on drug absorption. These environmental adaptations may be relevant to the differences in pharmokinetics noted between the donkey and the horse and pony. In addition, there are variations within the species.
For example:
- miniature donkeys metabolise phenylbutazone at a faster rate than standard donkeys and therefore require more frequent dosing
- mammoth donkeys have a smaller volume of distribution of gentamicin than standard donkeys.
Hepatic metabolism is often faster in the donkey, with the volume of distribution lower, the clearance rate higher and the half-life shorter than in the horse and pony. In such cases shorter dosing intervals are generally advised rather than larger doses. However, there are important exceptions to this.
⚠ ALERT
Where metabolism of a drug is faster in the donkey than in the horse or pony, it is advisable to use shorter dosing intervals rather than larger doses.
In drugs with higher bioavailability and smaller volume of distribution, such as guaiphenesin, there is a higher likelihood of reaching toxic doses in donkeys if the weight is unknown or inappropriate drug doses are used.
PRESCRIBING FOR THE DONKEY
Very few drugs are licensed for use in the donkey. The clinician should be aware of the legal requirements of the country in which they are working with regard to prescribing.
In the UK, for example, unlicensed medication can currently be prescribed under the Cascade.
Where medication is prescribed under the Cascade in the UK, the owner must be informed that the medication is unlicensed. If the donkey has not been declared a non-food producing animal in their passport any medicine prescribed must be limited to one that has active substances that are included in the list of permitted essential substances for horses. A withdrawal period must be given and the animal or it’s products cannot go into the human food chain until the withdrawal period has ended.
ADMINISTRATION OF DRUGS AND WEIGHT ESTIMATION
In order to prescribe safe and effective drug doses, an accurate estimation of weight is essential. If no scales are available, the donkey specific weight estimator can be used to estimate the weight of the donkey. A line can be drawn on the nomogram from the heart-girth measurement to the height of the donkey, and a weight estimated from these two measurements:
- the heart-girth measurement is the circumference of the donkey’s thorax just caudal to the elbows, measured in cm
- height is measured to the withers in cm
- weight is estimated in kg.
See Appendix 2 for the donkey weight estimator and for a table of weights for donkeys under two years old.
⚠ ALERT
Horse and pony weigh tapes are not appropriate for donkeys.
Intravenous (IV) administration
Differences in anatomy which make intravenous access more challenging in the donkey are described:
Information
Procedure: Intravenous injection
- Donkeys may have a thick coat and clipping is advisable to give clear visibility of the vein.
- Donkeys have a prominent cutaneous colli muscle which can conceal the middle third of the jugular groove. Therefore the upper third or lower third of the jugular groove is often the best site for injection.
- The angle of needle introduction may need to be adjusted from that in the horse and pony and is typically steeper
Intravenous catheterisation may be appropriate for repeated medication.
Placing an intravenous catheter.
Intramuscular (IM) administration
Intramuscular injections can be given in the gluteal and neck musculature using the same landmarks as in the horse. However, the pectorals typically have a low muscle mass in the donkey and may not be suitable.
Oral administration
Oral medication can be hidden in feed or more easily administered in jam sandwiches or confectionary.
Drenching can cause stress in donkeys and is best avoided.
Epidural
Epidural anaesthesia in the donkey is performed at the second intercoccygeal space, advancing the needle at an angle of 30 degrees to the horizontal rather than a 45 degree angle at the first inter-coccygeal space as is used in the horse and pony.
See Chapter 17: Sedation, Anaesthesia and Analgesia for more information.
Topical administration
This is the same in donkeys as in horses and ponies.
Administering eye ointment.
DONKEY SPECIFIC DRUG DOSAGES
High quality evidence for appropriate drug regimes in donkeys is not available for many of the commonly used medications. The following table summarises specific differences and identifies drugs that are contra-indicated in the donkey. For all other medications, equine datasheets and current formularies should be consulted. Drug company veterinary advisers may also be a source of useful advice.
A note on antimicrobials
Clinicians are strongly encouraged to follow best practice when considering the use of antimicrobials to minimise the risk of promoting resistance.
See online toolkit at beva.org.uk/protectme for advice on responsible use of antimicrobials.
Drugs with donkey-specific key points
Mules & Hinnies
There is very limited information and evidence available for the mule and therefore nothing to add that is specific to the mule.
Further information
Factsheets, research and detailed information can be found online at: thedonkeysanctuary.org.uk/what-we-do/for-professionals
The Working Equid Veterinary Manual: thebrooke.org/for-professionals/ working-equid-veterinary-manual
Bertone, J.J. and Horspool, L.J. (2004) Equine Clinical Pharmacology. Saunders Ltd, Philadelphia, USA.
Grosenbaugh, D.A., Reinemeyer, C.R. and Figueiredo, M.D. (2011) Pharmacology and therapeutics in donkeys. Equine Veterinary Education 23(10), pp 523—530.
Lizarraga, I., Sumano, H. and Brumbaugh, G.W. (2004) Pharmacological and pharmacokinetic differences between donkeys and horses. Equine Veterinary Education 16(2), pp 102—112.
About
Affiliation of the authors at the time of publication
Sidmouth, Devon, EX10 0NU, UK.
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