Clinicians familiar with anaesthetizing horses will find many basic similarities between the species, but knowledge of some of the differences with donkeys will ensure a better approach.
Background: Donkeys make up a small but important percentage of patients seen by equine and mixed practices, but, while similar to horses in many respects, they have different management needs. Many of these differences are evident in the anaesthesia setting, and differentiating the treatment of donkeys from ‘small horses’ can lead to better outcomes for patients, owners and practices.
Aim of the article: This article highlights the differences between donkeys and horses with regards to induction and maintenance of anaesthesia and provides practical tips about performing safe anaesthesia of donkeys.
The number of donkeys and mules throughout the world is stable, and awareness of their use and concern for welfare, pain recognition and treatment are receiving increasing veterinary interest. Therefore, accurate information about anaesthesia and analgesia in donkeys and mules is important to ever more equine practitioners. Since donkeys are physiologically and pharmacologically different from horses, knowledge on species specific aspects of anaesthesia and analgesia are very important. Mules combine elements from both donkey and horse backgrounds, leading to great diversity in size, temperament and body type. Physiologically, they seem to resemble horses more than donkeys. This review highlights the current knowledge on various anaesthetic and analgesic approaches in donkeys and mules. There is still much information that is not available about donkeys; in many circumstances, the clinician must use available equine information to treat the patient, while monitoring carefully to observe for differences in response to therapy compared to the horse.
Nutrition
A donkey foal should be weaned gradually from 6 months of age and able to graze and eat supplementary straw feed. Barley straw is the forage of choice for healthy donkeys with good dentition, as it is low in calorie content while high in fibre, which aids slow digestion and reduces the risk of gastric ulceration. Straw can also be supplemented with hay in cold weather or if extra energy is required. If excess calories are provided to young donkeys, there is a risk of development of orthopaedic conditions including flexor tendon contractures leading to club foot. To balance the high-fibre diet, a low-calorie vitamin/mineral balancer ration is needed until the foal is at least 2 years old or up to 3 years in the larger breeds of donkey. Top Spec provide a donkey specific forage balancer that is appropriate for young donkeys.
Donkeys have lower nutritional requirements compared with horses. Aim to feed 1.3–1.7% of bodyweight in dry matter, the amount dependent on the weather and the individual animal. The donkey’s body condition score should be measured at least four times a year, while weigh tapes and donkey weight normograms help to monitor for slow, steady weight gain. Weaning the foal can be a stressful time for the jenny, so she needs similar careful management and monitoring.
Castration
Castration is a valuable tool to reduce the population of unwanted donkeys and encourage responsible ownership. The optimum time to castrate a donkey is between 6 and 18 months, although some reports suggest that a jack may be sexually mature by 12 months. Donkeys castrated after 18 months are more likely to retain stallion-like behaviours, and to have complications from surgery due to extensive fat deposits in the scrotum, larger testicles and associated blood vessels. Ensure a thorough preoperative check; many donkeys will have had no veterinary contact until castration. Check for heart murmurs and subclinical lung disease. Discuss vaccination and worming programmes: at a minimum ensure tetanus protection. For most young donkeys, a field castration is adequate. Use a weigh tape or weight estimator to calculate weight. Take a qualified assistant vet or nurse to administer the anaesthetic and top-up doses, as the procedure takes longer than a standing castration. Owners are not suitable assistants. For field anaesthesia, remember to take equipment to protect the donkey’s face and eyes: towels, eye drops, padded head collar, etc.
The Donkey Sanctuary vets prefer to use a standard closed technique for castration of donkeys. The donkey is placed under general anaesthesia, and local anaesthesia is used in the testicle (5–10 mL depending on size). The upper hindlimb is held or roped out of the way. The area may need to be clipped as the scrotum is frequently covered with hair. In the young donkey there should be minimal swelling post-operatively; we use analgesia at donkey doses, for 3–5 days, and depocillin intramuscularly usually for 3 days. Encourage exercise daily; it is useful to cold hose the inguinal area to reduce swelling, avoiding saturating the wound with water. Monitor appetite, faecal output, and demeanour for a week post-surgery. Complications include haemorrhage and infection. If blood is dripping faster than 1 drop/second and not slowing, consider external pressure or re-anaesthetise to locate the source. Infection manifests as a slow-healing wound, discharge, and a painful thickening of the remaining cord tissue. Surgical investigation is often required. In cryptorchid donkeys the anti-Mullerian hormone (AMH) test is proven to detect retained testicular tissue. If this test is not available, a human chorionic gonadotrophin (hCG) stimulation test is required as the oestrone sulfate test is unreliable in donkeys.
Sedation and anaesthesia
Many young donkeys are not well handled, and we must provide good sedation in a welfare-friendly manner. Aim to keep bonded companions together to reduce stress, and consider oral or i.m. sedation if required before attempting i.v. access.
Young colts can have thick coats and a well-developed ventral neck muscle – clip the vein, elevate the head and aim for the jugular above or below the mid third of the neck. A catheter is needed to top up anaesthesia for castrations. Donkey skin is relatively thick so use a scalpel to nick the skin before inserting the catheter; using a bleb of local anaesthetic makes this easier. Typical equine doses of alpha-2 agonists work for donkeys, but be prepared to increase the dose if the donkey is stressed and do not induce anaesthesia until the head has dropped below the withers. Ketamine at a dose of 2.2–3 mg/kg is typically used together with diazepam at 0.1 mg/kg for induction. Multimodal analgesia is provided with the use of an NSAID, an opioid (typically butorphanol) and local analgesia. Donkeys metabolise ketamine faster than horses so be prepared to top up at timed 10-minute intervals with one-third of the induction dose.
If a triple drip is used for anaesthesia, use a recipe appropriate for donkeys, for example: 300 mL saline, 225 mL 10% guaphenesin, 225 mg xylazine and 900 mg ketamine. Avoid doses of guaphenesin above 150 mg/kg (1.5 mL/kg of 10% solution), as this can cause respiratory and cardiovascular depression. Decreased depth of anaesthesia is often preceded by increased rate and depth of respiration before movement occurs; monitor carefully.
Recovery from anaesthesia is usually good in donkeys, unless multiple ketamine top ups have been used. Be prepared to re-sedate with an alpha-2 agonist. A typical 180 kg donkey requires a size 16 mm endotracheal tube, but have a range of sizes between 14 and 18 mm available. Donkeys can be difficult to intubate due to the narrow epiglottis and caudally angled larynx.