My first encounter with the fact that there are differences between donkeys, mules and horses occurred in my first few months out of vet school. I tried to castrate a mule colt using the standard dose of xylazine and ketamine I’d used on horse colts. The mule just stood there, still upright on all four feet, looking at me as if to say, “What are we doing here?” It struck me that I needed to learn more about those differences before proceeding further.
Another experience early in my veterinary practice career left a lasting impression while working a few days a week at the Palomino Valley Wild Horse and Burro Center in Northern Nevada. Newly admitted wild equids were “processed” upon entry into the facility. Standard protocol was to obtain dental age of the horses using tongs to open the mouth, gather blood for Coggins testing, immunize and deworm, freeze brand, and take care of any injuries or illnesses of individual animals. One morning I was called in to help process a large group of wild burros, which are essentially a smaller version of a donkey. Their behavior was completely contradictory to how the wild mustangs behaved. Looking down the lane into the chute, all I could see were ears. I had no idea what I was in for. When faced with a stress situation like being shuttled through a narrow lane into a holding chute, burros tend to plant their feet and brace, back up or sit down rather than run amok like the behavior displayed by many of the horses. The foal burros behaved the same as the adults.
It was an eye-opener that was preparatory for the many veterinary calls I had to make to treat wild burros adopted from the facility into the community.
The more obvious differences stem from variations on an anatomical theme.
Anatomical features of the limbs vary between donkeys/mules and horses. The dorsal hoof wall of donkeys and mules is more vertical, and the hoof-pastern axis is more upright. The frog extends more toward the heel, and the sole is U-shaped. The distance between the proximal limit of the hoof wall and the extensor process used to help identify laminitis on radiographs is wider in donkeys (10-13 mm) compared to 3-5 mm in horses. Donkeys and mules are prone to laminitis and hoof abscesses, the same as horses.
Attempting to do an intravenous stick to take blood or give medication can be a bit challenging with donkeys since a thick muscle (cutaneous coli) runs alongside the neck. It is best to capitalize on better venous access in the upper or lower one-third of the jugular groove.
Passing a nasogastric (NG) tube also requires some thought. The nasal passages of donkeys and mules are narrower than those of horses, requiring a smaller bore NG tube. Mules and donkeys have a less sensitive cough reflex than horses, so you can’t always rely on a cough to indicate inadvertent passage of the NG tube into the trachea. And, the nasal puncta of the nasolacrimal duct, while situated on the inside floor of the horse’s nostrils, is located more on the dorsal outside flare of the nostril in donkeys and in the lateral wall of the external nares in mules.
Terminology for Donkeys and Mules
It helps to know the terminology when describing crosses between donkeys and horses:
Mule = offspring between a male donkey and a female horse
Hinny = offspring between a horse stallion and a female donkey
For the sake of this article, any cross between a horse and a donkey is referred to as a mule.
A notable characteristic about donkeys and mules is that they can thrive on lower-quality forage and less water than horses. That makes them invaluable to people and economies in developing countries where they are often part of a family’s livelihood as beasts of burden. They do like to have access to clean water and can be picky about that.
However, this “thriftiness” quality can also cause problems when living as companion animals in affluent areas due to the high quality and dense caloric value of foodstuffs. They tend to become obese and are at risk for laminitis and fatty liver disease. Practitioners should be on the lookout for hyperlipemia that can occur both as a primary and secondary disease.
Donkeys are efficient at storing lipids and mobilizing fat stores rapidly. Predisposing factors to development of hyperlipemia include anorexia, fasting, late pregnancy, lactation, stress, endotoxemia, parasitism, liver or gastrointestinal disease, and laminitis. Triglyceride evaluation is not always helpful in diagnosing hyperlipidemia in donkeys and mules unless the levels are unusually high.
Doney Metabolic Syndrome
Donkey metabolic syndrome (DMS), similar to equine metabolic syndrome (EMS) in horses, is prevalent when food is plentiful. Not every obese donkey has insulin dysfunction (ID), and even lean and/or young donkeys can develop DMS. A cut-off value >50 uIU/ml for insulin is more appropriate for donkeys and mules to determine the presence of ID. A specific donkey body condition score has been developed ranging from 1 to 9, and a neck crest system ranges from 0 to 4.
Testing for insulin dysfunction takes a different form for donkeys and mules: Baseline glucose and insulin levels tend to be higher than in horses, and glucose stays elevated for longer despite the return of insulin to baseline. When pursuing dynamic testing due to inconclusive baseline insulin concentrations (20-50 uIU/ml), rather than fasting donkeys pre-testing, they should be offered a flake of hay the night before a dynamic metabolic test done in the morning. This is intended to prevent the risk of hyperlipidemia.
Commercial diets intended for control of ID in horses might not be appropriate for donkeys, especially because it is best to eliminate concentrates from the diet altogether.
PPID in Donkeys
To test for PPID (pars pituitary intermedia dysfunction), the TRH-stimulation test is used in the same way as for horses. However, the validation of ACTH results in donkeys needs more sampling. At this time, a donkey is PPID-positive if plasma ACTH ≥110 at the 10-minute time point following IV injection of TRH.
Treatment with oral pergolide (2 ug/kg/day) has good bioavailability in donkeys. Plasma concentrations are similar to or higher than those achieved in horses on the same dose. The donkeys in the study tolerated this well with no adverse effects [Xue, C.; Davis, J.; Berghaus, L.J.; et al. Pharmacokinetic properties of pergolide mesylate following single and multiple-dose administration in donkeys. Equine Veterinary Journal Dec 2022; DOI: 10.1111/evj.13917].
A study of 138 domestic donkeys demonstrated that none of 15 hematologic values and only four of 20 biochemical values compared well between donkeys and horses [Burden, F.A.; Hazell-Smith, E.; Mulugeta, G.; et al. Reference intervals for biochemical and hematological parameters in mature domestic donkeys (Equus asinus) in the UK. Equine Vet Educ 2016; 28:134–139].
Coagulation testing is one issue for which coagulopathies could be underdiagnosed in donkeys since partial thromboplastin and prothrombin times are shorter in donkeys than in horses. Thyroid hormone concentrations in donkeys are higher than in horses.
As noted, donkeys/mules tend to need higher doses of sedatives and anesthetics while also metabolizing drugs more rapidly than horses. Anesthesia wears off quickly, so drugs are often increased by 25-50%. Xylazine duration is less than detomidine. Butorphanol enhances the sedative effect of alpha2-adrenoreceptor agonists. The half-life of guaifenesin takes longer to abate than in horses, leading to prolonged recovery times following anesthesia. Using propofol as an adjunct to ketamine or to guaifenesin-ketamine-xylazine combination might provide smoother induction and recovery for donkeys/mules.
NSAIDs (flunixin and phenylbutazone) and antimicrobial drugs are metabolized faster in donkeys and mules than in horses, necessitating higher doses or shorter dosing intervals.
Surgery always has some peculiarities between individuals, no matter the breed or whether one is dealing with donkeys/mules compared to horses.
Castration is a common procedure in male equids, but it can be challenging in donkeys and mules because of their tendency to bleed. Extra care needs to be taken to ligate the vascular cord with both sutures and hemostats rather than or in addition to using an emasculator.
Vaccine doses and frequency are administered the same in mules and donkeys as for horses.
Deworming medications administered based on results of fecal egg counts and fecal egg count reduction measures are similar to strategies for horses and efforts to mitigate worm resistance. Caution is recommended when deworming with macrolides, especially for donkeys in poor body condition—too great a dose can lead to neurotoxicity. To manage lungworms, moxidectin has greater efficacy than fenbendazole.
Dental care is similarly important as with horses, especially if a bit is being used in the mouth. Miniature donkeys often have genetic abnormalities of their dentition. Donkeys with poor dentition often are also in poor body condition.
Gastric ulcers, when they occur in donkeys/mules, are identified 95% of the time as non-glandular mucosal lesion—equine squamous gastric disease (ESGD). This is mostly found around the cardia and along the lesser curvature of the stomach. While prevalence might be as high as 50%, donkeys tend not to show clinical signs of gastric ulcer disease.
Respiratory disease identified in a study of 1,444 geriatric donkeys commonly includes pulmonary fibrosis (35%), tracheal collapse (9%) and guttural pouch disease (5%). In addition to potential infection with EHV-1 and EHV-4, donkeys and mules can acquire asinine herpesviruses (AHV-1 through 6), for which there are no vaccines.
Lungworm can be prevalent in donkeys (68–87%) and mules (29%) without showing clinical signs. Yet they can serve as an infective reservoir. With this in mind, it is best to separate horses in different grazing areas apart from donkeys and mules.
Donkeys and mules experience similar problems of the eye as horses, but corneal ulcers, kerato-conjunctivitis, uveitis and eyelid trauma are the most common issues. Recurrent anterior uveitis occurs less prevalently than seen in horses, but it is still quite prevalent in donkeys. A thorough ocular exam is important.
Due to a larger conjunctival sac and periocular hair, foreign bodies can lodge more easily in donkey and mule eyes than in horse eyes. It is noted that not only is the eyeball more sunken in donkeys and mules than in horses, but the lateral aspect of the frontal bone that forms the upper portion of the orbit extends more laterally. These features make it more challenging to directly examine or ultrasound the eye. It often is necessary to sedate, nerve block and apply local anesthetic to enable opening the eye for examination since their palpebral muscles are so strong.
Cataracts and lens luxation can occur more often than in horses simply due to the long lives of donkeys/mules.
Horses, donkeys and mules develop Culicoides hypersensitivity, but donkeys and mules have lesions more often on the legs and head rather than the withers, rump and tail as in horses. Also, bilateral alopecia doesn’t tend to develop along the trunk midline.
Mutual grooming is common in donkeys, but owners and practitioners should be aware that this could cause traumatic alopecia, as well as stimulate stereotypic behavior in others in the herd.
A jennet (female donkey, also called a jenny) enters estrus at a young age, 8-12 months. Ovulatory follicles are comparable to horse follicles, i.e., 25-35 mm diameter. Jennets that are protective of their foals might not show estrus while nursing.
Live cover of a jennet needs some precautions to people handling her as she often backs up to the jack, makes jawing motions with her mouth, and kicks him. While this might seemingly indicate that the jenny is not receptive to breeding, some jennies display this behavior even when in estrus.
For some jacks, this is considered courtship in order to get an erection. It can take 10-15 minutes (or longer) for full erection. Some donkey studs are extremely aggressive and might require a breeding muzzle. Handlers need to take caution. A jennet in estrus might be mounted by other jennets.
Because the donkey’s or mule’s pelvis is tilted down and back more than a horse, reproductive exams, ultrasound and managing dystocia can be somewhat challenging.
Gestational length is longer for donkeys than horses, ranging from 12–13 months.
A well-described Horse Grimace Ethogram has been established to help detect pain in horses. Similarly, a donkey pain ethogram is in development.
One study indicates that a donkey’s overall appearance and body stance have the most predictive value for pain. Other signs of discomfort or pain include orbital tightening of the eyes, muzzle and nostril tension, and specific ear movements. Researchers recommend careful observation of a donkey’s head and facial expressions and looking at the donkey as a whole for other behavioral signs of pain.
Donkeys are quite stoic in their presentation of pain or discomfort. This can be challenging because a serious problem might go undetected until somewhat well advanced—colic and laminitis, as examples. The hard, narrow hoof can make it difficult to elicit pain with hoof testers, so it is prudent to pursue diagnostic radiology with any suspicion of laminitis.
Subtle Pain Behaviors
Even with severe colic, the only clinical signs a donkey or mule might demonstrate are dullness, isolation, reduced appetite and increased time lying down. Some might stand still, reluctant to move, with heads carried low.
With careful observation, other subtle pain behaviors can be noted—changes in ear position; eye and nostril shape and tension; and head, neck and tail movement. A thorough physical exam might identify more obvious signs of a problem.
The stoicism of donkeys and mules can also make it tricky to assess post-operative pain. One study evaluated pain behavior after castration in 40 donkeys and found that the only specific pain indicator was the lifting of the pelvic limbs. Additionally, some donkeys shook their heads and turned, and moved their ears. The authors concluded that insects confound those as indicators of pain [de Oliveria, M.G.C.; Luna, S.P.L.; Nunes, T.L.; et al. Post-operative pain behavior associated with surgical castration in donkeys. Equine Veterinary Journal May 2020; DOI: 10.1111/evj.13306]. The authors further noted that lifting of the hind limbs is also observed in horses after surgical castration and is associated with a high specificity and sensitivity for pain.
While it is tempting to extrapolate experience and data from horses and apply that to other equids, research strongly advises against doing so. More studies are necessary to formulate appropriate clinical signs and treatment modalities for donkeys and mules. For now, these reported differences provide a starting point of consideration for equine practitioners involved in donkey and mule care.