Using Diet to Manage Neuromuscular Disorders in Horses

Horse grazing in a pasture adjacent to red barn.
Horse grazing in a pasture adjacent to red barn. Barry Blackburn

Newsdate: Wednesday, November 22, 2023 – 11:30 am
Location: DAVIS, California

Equine nutritionist Dr. Clair Thunes and Dr. Carrie Finno, CEH director and expert in equine neuromuscular diseases, share dietary recommendations for horses with neuromuscular diseases.

orse grazing on fresh green grass - Source of vitamin E for horses.

Horse grazing on fresh green grass - Source of vitamin E for horses.

Veterinarians caution against adopting dietary changes without a confirmed veterinary diagnosis since clinical signs displayed by horses with neuromuscular disorders are not necessarily unique to the described conditions.
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What it is – Equine Motor Neuron Disease (EMND) is a degenerative condition that affects nerve cells (neurons). Neurons transmit impulses to muscle fibers that make them contract, resulting in movement. The neurons of horses with EMND are damaged which causes them to send incorrect signals. Affected horses may display muscle wasting, weight loss, muscle twitching, and sweating.

Horse with EMND showing neurologic signsWhat causes it – Dietary vitamin E deficiency over 18 to 21 months can lead to clinical signs. However, not all horses with prolonged vitamin E deficiencies develop the disease.

How diet can help – Ensuring that horses receive adequate vitamin E reduces the risk of developing EMND. For affected horses, treatment involves changing the vitamin E level in the diet. The natural source of vitamin E for horses is fresh green grass. Horses that do not have adequate access to pasture may require supplementation.

The only biologically active form of vitamin E is α-tocopherol. The natural form, d-α-tocopherol, is more bioavailable, more readily absorbed, and has greater antioxidant activity than the synthetic form, dl-α-tocopherol (also called all-rac-α-tocopherol). It is not recommended to use the synthetic product at any dosage as it is ineffective at raising blood vitamin E levels.

Determine baseline vitamin E levels through bloodwork before starting a supplementation program,as it may be necessary to fine-tune the dose.For horses that do not have adequate access to fresh green grass, recommendations are made based on baseline blood vitamin E levels and α-tocopherol formulation (i.e. natural liquid product or natural powder/pellet product).Perform follow-up bloodwork to determine if the dose needs to be adjusted.

Equine Neuroaxonal Dystrophy and Equine Degenerative Myeloencephalopathy

A horse with eNAD showing neurological signsWhat it is – Equine neuroaxonal dystrophy (eNAD) and equine degenerative myeloencephalopathy (EDM) result from abnormalities of certain neurons. Research suggests that eNAD is the underlying basis for EDM. Clinical signs may begin with performance issues at a young age and progress to incoordination, abnormal circling, and a horse not knowing where to place its feet.

What causes it – Cases in closely related horses suggest a genetic basis for eNAD/EDM, but a causative mutation has not been identified. Vitamin E deficiency during specific developmental stages can lead to clinical signs.

How diet can help – Since newborn foals get vitamin E in the colostrum, pregnant mares must receive adequate vitamin E during the last trimester. Foals born to mares that previously produced affected foals should be supplemented around weaning. Baseline vitamin E levels are essential to determine if the supplementation plan is working. The timing of supplementation is important; once a foal starts to display signs of eNAD/EDM, the disease is irreversible.

Polysaccharide Storage Myopathy Type 1

What it is – Polysaccharide storage myopathy type 1 (PSSM1) results in an abnormal accumulation of glycogen, the form of sugar stored in muscle. This can cause episodes of muscle stiffness and pain after exercise, also known as “tying up” or exertional rhabdomyolysis. Affected horses may be reluctant to move and may display sweating, lameness,and muscle tremors.

What causes it – A genetic mutation in the glycogen synthase 1 (GYS1) gene causes PSSM1. Since it is an autosomal dominant trait, only one copy of the mutation is needed for a horse to be affected. However, environmental factors, namely diet and exercise, play important roles in the onset of clinical signs.

How diet can help – Dietary nonstructural carbohydrate (NSC) intake in affected horses should be limited to less than 12% of the diet to keep blood insulin levels low and reduce glycogen storage in muscle. Horses must exercise daily to maximize the muscles’ ability to burn glycogen. Affected horses are often easy keepers and management through a low-NSC grass hay and a good-quality ration balancer is usually sufficient. If additional calories are needed, a low-NSC and/or high fat feed source should be incorporated.

Polysaccharide Storage Myopathy Type 2

What it is – Polysaccharide storage myopathy type 2 (PSSM2) also results in abnormal glycogen storage in muscle, but horses do not have the GYS1 mutation. Clinical signs are the same as PSSM1.

What causes it – The cause of PSSM2 remains unknown; there may actually be multiple causes. There is currently no scientifically validated diagnostic test for PSSM2.

How diet can help – Affected horses can improve with the correct diet and exercise regimen. Since small management changes can have a big impact, fine-tuning the diet overtime may be necessary. Ensuring an adequate source of quality protein in the diet appears to be important.

Drs. Finno and Thunes caution against adopting dietary changes without a confirmed veterinary diagnosis. The clinical signs displayed by horses with neuromuscular disorders are not necessarily unique to the described conditions. For example, clinical signs seen under saddle in PSSM2-affected horses are similar to those seen in horses with gastric ulcers. Without an accurate diagnosis, a horse could be treated for the wrong condition.

Horse owners should work closely with their veterinarian to achieve an accurate diagnosis to enact management changes for the best outcome. Seeking the expertise of a qualified equine nutritionist may also be needed.

Article and Press  release by Amy Young


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