The Musculoskeletal System

Donkeys have few anatomical differences compared to horses and ponies, but these may affect interpretation of imaging or the logistics of treatment.

• The pectoral muscles are relatively underdeveloped and are not suitable for use as an injection site.

• The donkey has five lumbar vertebrae.

• The donkey’s sacrum curves dorsoventrally.

• The first coccygeal vertebra is often fused to the sacrum, with occlusion of the sacro-coccygeal space.

• The first inter-coccygeal space is narrower than the second, so the preferred injection site for epidural anaesthesia is between C2 and C3.

• The coccygeal vertebrae are better developed in donkeys, the vertebral arches of the first three are complete and each has interarcuate ligaments.

THE EXAMINATION

Donkeys in pain tend to stand still or lie down. As a result, identification of lameness or the presence of pain may be difficult, and delayed presentation is common. Owners may report subtle changes in behaviour, including reduced activity or increasing periods spent lying down. Acute and chronic conditions are easily overlooked and may go undiagnosed for a considerable period of time.

Assessment of body condition

Use a donkey-specific body condition score chart to assess body condition.

Fat deposits, which may be fibrosed or calcified, occur at the crest and dorsally over the ribs. They often persist despite extreme weight loss and emaciation. Obesity or the presence of large fat deposits may indicate an endocrinopathy. Excess bodyweight may lead to chronic pressure on the joints of the limb.

Geriatric and non-working donkeys tend to have less muscle mass compared to horses and ponies. However, it should not be possible to feel individual spinous or transverse processes. Loss of muscle mass in this area may indicate an endocrinopathy, systemic disease, or lack of movement due to discomfort. Localised areas of muscle loss help localise sources of pain.

Many donkeys with chronic laminitis have poor development of the shoulder muscles. Donkeys with chronic hindlimb pain may have poor development of the biceps femoris, semimembranosus, and semitendinosus muscles.

Consideration of conformation

Donkeys have different conformation to horses and ponies and it is important to distinguish between the normal and a disorder.

• They have a narrower breast, with less defined pectoral muscles which can lead to a base-narrow stance.

• Mild toe-out conformation (less than 20 degrees) may be normal for donkeys, in both front and hind legs.

• The donkey foot is about 5-10 degrees more upright than the horse or pony foot.

• Donkeys’ ribs are wide, flat, and straight, creating a ‘boxy’ appearance to the thorax.

• The back is straight and short and the croup has a more sloped appearance.

• The points of the hocks turn in slightly when standing so a ‘cow-hocked’ appearance, where the points of the hocks are close to the median plane, may be normal in donkeys.

Assessment of gait

Donkeys walk with a ‘plaited’ motion, where the foot falls closer to the midline. It is important to differentiate between this and the crossing over seen in some neurological or orthopaedic conditions, such as sacro-iliac pain.

Donkeys are more likely than horses and ponies to attempt to hide their pain. Therefore, they may only show lameness at a trot, despite severe pathology. Unless donkeys are trained, it can be difficult to encourage a steady trot during lameness examination. Some donkeys may learn with minimal training and food rewards. Where possible, this is worth attempting in cases of subtle lameness. Sometimes the only way to assess a donkey’s gait is to spend time observing them in their usual environment.

Perform nerve and joint blocks as in the horse and pony:

• clip for all blocks, donkeys have a thick coat

• take care to avoid entry into synovial structures, donkeys have generally smaller anatomy

• aseptically prepare all blocks adjacent to synovial structures

• as in all equines, assess temperamental suitability for nerve and joint blocks because the number of blocks, or the time over which they take place, may need adjusting in some patients.

COMMON ORTHOPAEDIC CONDITIONS

Angular limb deformities

Angular limb deformities in donkeys will present in the first few months of life. Possible causes include maternal disease, prematurity, and inappropriate diet. The carpus, fetlock, and hock are the most commonly affected sites.

It is necessary to differentiate angular limb deformities from normal conformation. Treatment is the same as in horses and ponies, however there is more time available to perform corrective procedures because the donkey’s growth plates close when they are slightly older.

In many cases, because donkeys rarely undertake athletic work, these deformities remain untreated. This may present a problem later in life due to the development of secondary degenerative joint disease (DJD). At this stage, pain relief becomes the mainstay of therapy. Correction of the angular limb deformity will not correct the secondary DJD. Affected donkeys may also develop issues with abnormal hoof wear.
 

Flexural limb deformities

Flexural limb deformities can be either congenital or acquired. The distal interphalangeal joint is most commonly affected.

Congenital cases have been linked to intrauterine positioning, maternal disease, and potentially increased maternal body condition score.

Acquired cases can occur in donkeys of different ages and lifestyles such as:

• young, rapidly growing donkeys fed more than their required level of nutrition

• working donkeys, in association with foot injury and distal joint arthritis

• geriatric donkeys, linked to chronic foot pain and upper limb pain.

Due to the donkey’s naturally upright hoof-pastern angle, distal interphalangeal joint hyperflexion (‘ballerina syndrome’ or ‘club foot’) is often missed in the early stages. Mild cases have a raised heel but the dorsal wall is not beyond the vertical. In more severe cases, the dorsal hoof wall tips beyond the vertical, with a marked reduction in the heel to fetlock distance.

As in horses and ponies, mild cases may respond to a combination of dietary restriction, corrective farriery, extension shoes, and analgesia. More severe cases require surgical intervention. The inferior check ligament is underdeveloped in donkeys. Deep digital flexor tenotomy may be necessary to resolve the deformity.

Upward fixation of the patella

Due to their more upright hind limb conformation and relatively smaller muscle mass, donkeys are predisposed to upward fixation of the patella (UFP). Generalised muscle loss of the upper hind limb, due to hind limb osteoarthritis, may also lead to UFP, and therefore all donkeys should be evaluated for underlying causes. UFP must be differentiated from stringhalt; seen as a sporadic involuntary flexion of one or both hind limbs in ageing donkeys.

In mild cases, exercise and physiotherapy to improve muscle tone and strength can be effective. Walking and trotting up an incline and working in soft footing are effective ways to encourage lifting of the hind feet and flexion of the hock and stifle. Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) should be provided to prevent pain and inflammation during this process.

In more advanced cases, surgery may be necessary in the form of medial patellar ligament splitting or desmotomy. These procedures can be performed with the donkey either standing or under general anaesthesia. The donkey’s short stature and relatively pendulous abdomen mean surgical access while standing may be difficult, so the need for general anaesthesia should be assessed on a case-by-case basis.

Osteoarthritis

Osteoarthritis is a common problem in middle-aged and geriatric donkeys. It can be either primary or secondary to many of the conditions discussed in this chapter. Signs may be subtle and only noticed when lesions are chronic and severe. Common clinical signs of advanced disease include:

• shortened stride length
• difficulty getting up
• reluctance to lie down
• resentment when having limbs lifted for routine procedures (such as foot cleaning or farriery)
• pressure sores over joints; particularly the hocks and carpi.

Osteoarthritis frequently affects the upper limb joints, primarily the shoulders and the hips, but may develop in any joint. Working donkeys may show signs of osteoarthritis in the lower limb joints due to repetitive strain injury or trauma.

Ankylosing spondylitis of the cervical or, more often, thoracic spine can also occur as part of the ageing process.

Examination of a donkey with suspected osteoarthritis should include careful evaluation of the neck, back, and limbs. Reduced range of motion is common, and there may be pain associated with flexing or extending the affected joint.
 

Treatment of the donkey is similar to that of the horse and pony. Targeted therapies, such as joint injections, may be useful. However, the advanced stage of osteoarthritis in many cases may lead to decreased efficacy. Advanced osteoarthritis may also lead to difficulty entering the joint.

Systemic NSAID therapy is often necessary. Donkeys metabolise NSAID dosages differently to horses and ponies and the frequency of dosing will need to be adjusted accordingly.

Regular monitoring and assessment of quality of life is essential to identify changes to the severity of osteoarthritis, which tends to increase over time.

Trauma

Trauma occurs in donkeys; both as a primary insult and as a secondary consequence of overworking. Donkeys have relatively dense and compact cortical bone compared to horses and ponies and, a large amount of force is necessary for fractures to occur. Blunt force trauma is more likely to lead to injury to ligaments and bones of joints above and below the trauma site. Where fractures do occur, incomplete, partial fractures are common. Careful radiological evaluation is essential.

Fractures of the scapulohumeral joint appear to be over-represented, potentially due to the relatively exposed nature of this site.

Tendonitis and desmitis

Trauma, fatigue, abnormal loads, and poor farriery can cause, or exacerbate, chronic tendonitis and desmitis. These conditions occur in working donkeys and those with chronic limb pain and abnormal weight distribution.

Secondary effects of laminitis

Chronic laminitis leads to predictable secondary orthopaedic conditions, which may be confused as primary or other conditions.

Chronic forelimb pain leads to offloading of weight onto the hind limbs. Reduced forelimb weight bearing can cause disuse atrophy of the shoulder muscles, which may become pronounced. Over time, increased weight on the hind limbs can lead to osteoarthritis, particularly of the hocks.

As increased weight on the hind limbs becomes more uncomfortable, weight shifting between front and hind limbs can increase. This leads to a shortened stride in all limbs and development of tension and pain of the epaxial musculature. Abnormal weight bearing may also lead to suspensory desmitis, particularly of the branches. Affected donkeys may become reluctant to move, leading to generalised muscle wasting. In some cases, wasting of the hindlimb musculature can lead to UFP.

Treatment of any one of these conditions is inadequate for resolution of pain. A full examination is essential to identify and treat all the conditions. In addition to treatment of the primary laminitis, this may include:

• treatment of hindlimb arthritis with joint injections or systemic NSAID therapy

• massage therapy for chronically overloaded muscles

• gentle physiotherapy to encourage muscle formation.

Under-represented conditions

The following conditions have not been described, or appear to be under-represented, in donkeys:

• podotrochlear syndrome

• osteochondrosis dissecans

• subchondral cysts

• ossification of collateral cartilages

• hyperkalaemic periodic paralysis.