Endocrine disease in donkeys
There are significant numbers of donkeys kept as companion animals, both in the UK and abroad. Like their horse and pony compatriots, many kept donkeys are geriatric. Additionally, obesity is a common problem, irrespective of age. Diagnosis and management of endocrine disease in donkeys is challenging as many of the available tests for horses have not been specifically validated in donkeys. Furthermore, endocrinopathic laminitis in the donkey is not infrequently subclinical. The term ‘subclinical,’ is perhaps inappropriate, for the lack of clinical signs may not correlate with the severity of the laminitis episode but instead be a feature of the donkey’s notoriously stoic behaviour with respect to pain.
As a species, the donkey has evolved to be extremely energy efficient, with an inherent ability to maintain good body condition on lower energy diets. They have a slower intestinal transit time and greater efficiency of fibre digestion than the horse [1]. Donkeys appear able to accumulate adipose tissue with ease. Whilst these innate traits serve the working donkey or those kept in harsh environments where food resources are scarce well, they predispose the sedentary donkey to obesity, dyslipidaemias, insulin dysregulation and other metabolic disorders.
Donkey, or asinine, metabolic syndrome is increasingly recognised by equine veterinary practitioners. Clear and precise definitions are lacking and currently rely on extrapolations from equine metabolic syndrome. The clinician presented with an obese donkey should be suspicious of underlying metabolic derangements. The donkey may be generally obese, or have defined regional adiposity, with deposits along the neck, dorsum and rump being common. There are usually substantial subcutaneous fat deposits on the ventral abdomen in obese donkeys. Awareness of the obese donkey may only arise when the otherwise healthy donkey is presented for routine preventative care, such as vaccination or dental care. This provides an ideal opportunity to discuss the donkey’s body condition and diet, with the intention to reduce development of metabolic disease. Initial clinical assessment must include an evaluation of any indicators of laminitis, whether subtle or overt. The donkey that is described as unwilling to walk may actually be in discomfort rather than being unused to handling. It is imperative that provided diets are appropriate for the donkey’s life stage. For the adult, healthy donkey, with good molar function, the majority of forage intake should be provided by straw, with hay/haylage or restricted grazing making up the remainder of the diet. Owners should be advised to aim for straw to provide 75% of intake in summer and 50% in winter. Supplementation with a vitamin and mineral balancer is recommended. The conscientious owner may be willing to have their forage sugar content analysed, to further guide dietary management.
If a donkey remains obese, despite sufficient time and appropriate diet for expected weight loss to ensue, further investigation for metabolic disease should be undertaken. Basal insulin concentrations may be elevated in obese donkeys. The established reference range at The Donkey Sanctuary for a non-fasted insulin sample in the adult donkey is 0-15.1 uIU/ml (TOSOH AIA, using two-site immunoenzymometric serum assay). As for horses, a normal insulin concentration does not rule out insulin dysregulation and dynamic tests are recommended for further investigation. Protocols have been developed for donkeys, with intravenous glucose tolerance test, combined glucose-insulin test and oral glucose tolerance tests described in the literature [2] but insulin cut-off values for these tests have not been validated for donkeys. In the author’s clinic, the oral glucose tolerance test with corn syrup is used as this provides an accessible and practical dynamic test. The major limitation being that the protocol used is that developed for horses and may not be directly transferable to donkeys and, again, there is no validated insulin cut-off value. Consequently, diagnosis of asinine metabolic syndrome is currently made based upon combinations of clinical observations, poor weight loss despite appropriate diet, evidence of hyperinsulinaemia or insulin dysregulation and history of laminitis episodes.
Management of asinine metabolic syndrome is as challenging as the diagnosis. Many owners struggle to appropriately diet their donkey, particularly if any companion is not obese and/or has different dietary requirements. It is now widely understood that donkeys form strong bonds with their companions and can become distressed and at risk of hyperlipaemia if separated. However, for short periods if one donkey needs supplementary feed companions can be kept in sight of each other, yet physically separated by a fence or stable door. Owners should also try to encourage their donkeys to exercise, which is admittedly no mean feat. Turn out onto a bare pasture, which donkeys can explore and exercise within, is likely to carry more benefit than stabling a donkey away from rich grass pastures. If a donkey is accepting of a head collar and being led, many owners enjoy taking their animals for short walks. Providing enrichment activities that encourage the donkey to be mobile and explore are also helpful.
It goes without saying that exercise should not be advised until active laminitis has been ruled out. An association between obesity, insulin dysregulation and recurrent laminitis in donkeys has been reported but in a recent cross-sectional study of 707 donkeys in the UK [3], the presence of basal hyperinsulinaemia or PPID (indicated by seasonally adjusted elevations in ACTH) was not associated with increased odds of laminitis. However, this study acknowledged the lack of validated dynamic testing for either PPID or AMS and the potential errors in classification of cases. In the absence of definitive knowledge, clinicians should err on the side of caution and be alert to the potential for endocrinopathic laminitis. Management of laminitis in the donkey has been covered elsewhere [4].
The use of medication to treat asinine metabolic syndrome has not been extensively evaluated. As for horses, therapeutic protocols should only be used as an adjunct to dietary and exercise management. The use of levothyroxine in the donkey at standard equine doses has been reported [2]. Studies are needed to assess the safety and efficacy of ertugliflozin in the donkey, alongside any other medication proposed for treatment of metabolic syndrome in the horse.
References:
[1] Thiemann AK, Sullivan RJE. (2019). Gastrointestinal Disorders of Donkeys and Mules. Vet Clin North Am Equine Pract 35(3):419-432
[2] Mendoza FJ, Toribio RE, Perez-Ecija A. (2019) Metabolic and Endocrine Disorders in Donkeys. Vet Clin North Am Equine Pract 35(3):399-417
[3] Menzies-Gow NJ, Wakeel F, Little H, Buil J, Rickards K. (2021). Cross-sectional study to identify the prevalence of and factors associated with laminitis in UK donkeys. Equine Vet J. 54(4):757-765
[4] Thiemann A.K, Buil J, Rickards K, Sullivan R.J (2021). A review of laminitis in the donkey. Equine Vet Educ 34(10): 553-560