Equine motor neurone disease (EMND)

Equine moter neurone disease was first described in 1990. The number of cases is greater in the north east of the USA, even though it is widespread throughpout the world. It affects adult horses (2 to 25 years old) of both genders.

Acute form

In the acute form of the disease, affected horses will suddenly show signs of trembling, adopt unusual postures (limbs bunched up beneath the body, weight on the hind quarters), more frequent lateral recumbency periods. There may also be other signs such as : head carriage  lower than normal, sweating, elevated tail-carriage, and muscular fasciculations or involuntary movements. The appetite is ravenous and transit normal.

A clinical examination will show symetrical muscle wasting around the neck, shoulders, back and sacrum. The neurological examination is usually normal, with no signs of ataxia, and cranial nerves which are only slightly affected.  Abnormalities of the retina are often observed, with pigmented deposits at the tapetal-nontapetel junction or on the entire fundus surface in the most advanced cases. The colour of these deposits varies from light brown to black. Hyperreflexivity areas form mosaic patterns in the tapetal area. The acute phase of the disease lasts from 2 to 8 weeks. Most horses then appear to regain some muscle mass, but rarely enough to be ridden again. The resurgence of clinical signs may occur several years later and generally lead to the horse being euthanized.

Chronic form

Following the acute phase, the horse’s general state will stabilise, or evolve much more slowly. The reasons for consulting the veterinarian then become lack of weight gain or intolerance to exercise, linked to muscle atrophy. Lateral recumbency is prolonged, and weight relieving postures can be noted (leaning against the wall or against a manger). Occasionally the muscle mass may improve. More often than not, the clinical signs reamin stable for several months or years, then a new episode of trembling and a progression of the disease can lead to the horse being euthanized. Stringhalt (involuntary hyperflexion) in one or both the hind lims may be noted during the chronic phase.

Clinical signs are the result of the detetioration of the motor neurones of the ventral horns of the spinal cord. The most affected regions are the cervico-thoracic and lumbar-sacral areas of the spinal cord.

The neurones appear swollen and frequently contain karyolytic abnormal nuclei. These « ghost neurones » contain thickened neutraliszers. Punctiform eosinophilic inclusions often appear. Neurofilaments and lipograms condense as the cell bodies retract and shrink. Neuronophagia is also noted during the active phase. During the non-active phases, clusters of  glial cicatricial cells (astroglia, microglia and oligodendroglia) can indicate prior loss of motor neurones. The death of these neurones is accompanied by degenerative changes to axons and to peripheral nerves (demyelination) as well as neurogenic muscle atrophy. Studies have shown that posture muscles are primarily affected (atrophy of type I fibers). Associated changes are lipogram deposits in the capillaries of the spinal cord, and in the epithelial layer of the retina.

It is thought that the degeneration of the motor neurones could be attributed to an oxidative stress mechanism. The affected neurones are primarily those which have the greatest oxidative activity (alpha motor neurones). Horses seem to have a predisposition for the disease through a lack of anti-oxidants in their diet, and a higher consumption of enzymes reducing free radicals such as super oxide dimutase. For the disease to become apparent, it is estimated that a loss of over 30 % of the motor neurones is necessary.

Laboratory diagnosis

Routine blood tests and biochemical analyses generally do not show up any significant abnormalities, except slight increases in muscle enzymes (CK, AST) during the active phase of the disease. These can then drop back down to normal levels during phases when the horse’s condition improves. Sometimes an increase in IgG in the cerebrospinal fluid may be noted. Glucose absorption tests are abnormal in 30 % of the cases.

The most noticeable abnormalities are low values of plasma α-tocopherol concentration (vitamin E | <1,0g/mL in 90 % of the cases | tissue level in the muscle, peripheral nerves, and spinal cord is also extremely low), a drop in the activity of super oxide dimutase in the red blood cells and in the spinal cord. Vitamin A,  ß-caroten, and ascorbic acid are modified to varying degrees. Serum concentration of selenium is normal.

Clinical diagnosis

Diagnosis is easier with horses presenting an active form of the disease. Sudden weakness, trembling with no ataxia, depression, or loss of appetite, are very distinctive clinical signs.

Moderate elevation of the enzyme level can complete the diagnosis.

Muscular biopsies, preferably of the  sacrocaudalis dorsalis medialis muscle, can be sent to the pathologist for an immunohistochemical test, and for differentiation of the different types of fiber.

Biopsy of the ventral horn of the accessory nerve can be carried out ante mortem. These biopsies will help to confirm the diagnosis.

There is to date no evidence to support the effectiveness of a treatment.

Supplementing with vitamin E (5000 to 8000 IU/day/horse, unassociated to selenium to avoid intoxications) is recommended on the basis of low plasma or tissue concentration, and on the assumption of an oxidative cause to the disease.  Prednisolone can also be administerd (0,5-1mg/kg, PO SID)

Differential diagnosis includes equine polyneuritis, botulism, organophosphorous poisoning or lead poisoning, exercise-induced rhabdomyolysis, myopathies, Equine protozoal myeloencephalitis.