Myopathies in dressage horses: Latest research and why nutrition matters
In this article, Dr Marga Mas MRCVS, Head of Veterinary, Equine Health & Performance at D&H, explores the latest understanding of equine muscle disorders and the key role nutrition plays in their management. From muscle-supporting proteins to the importance of vitamin E, she explains how targeted feeding strategies can help support health, recovery and performance in sport horses.
Dr Marga Mas MRCVS – D&H Head of Veterinary, Equine Health & Performance
In recent years there has been a significant paradigm shift regarding equine myopathies, especially those affecting Warmblood sport horses. Whilst the several muscle disorders that are relevant to dressage horses have different causes and disease mechanisms, nutrition is central to the management of all of them. In this article, we explore the latest evidence about these conditions and also look at the key role of vitamin E in muscle health and function.
Exertional rhabdomyolysis
This is what we typically recognise as tying up: muscle pain, stiffness, elevated muscle enzymes in blood samples and, in acute cases, discoloured urine due to the presence of broken-down muscle proteins. These episodes may be sporadic (also known as exertional rhabdomyolysis, ER) or repetitive (recurrent exertional rhabdomyolysis, RER) and they are mostly the result of several management factors, combined with a genetic predisposition in some cases. Most sporadic tying up episodes are likely linked to training, fitness and nutritional factors, whilst horses that go onto develop repetitive episodes of RER (typically horses with Thoroughbred bloodlines) often have abnormalities in the regulation of calcium within the muscle cells. Both forms of rhabdomyolysis are typically worsened by high-starch diets, electrolyte imbalances (especially during and after effort) and stress factors.
Polysaccharide storage myopathy type 1 (PSSM-1) and type 2 (PSSM-2)
PSSM-1 is most commonly described in Quarter Horses and related breeds, and draft horses. These horses will tie up even after only light effort because they have a genetic mutation which causes excessive storage of muscle glycogen (its preferred fast-release energy source). This condition can be diagnosed through a combination of muscle biopsy (which shows glycogen accumulation within muscle fibres) and genetic testing from a hair or blood sample (which identifies a mutation in a gene called GYS1). Horses with PSSM-1 often have an abnormal insulin response to starch and sugar in their diet, which is why a low-starch, low-sugar diet sits at the cornerstone of keeping this condition under control. The ideal hard feed should have a combined starch and sugar level <10-12%, with fat as a preferential source of energy for exercise. However, because many horses with PSSM-1 tend to be overweight, calorie-restricted diets are often indicated in which case a forage-based diet (using a hay low in sugar and fructan, or soaked hay if required) combined with a balancer can a good solution.
PSSM-2 affects primarily Quarter Horses which present with tying up episodes after light work, abnormal glycogen accumulation within their muscle but without the mutation of gene GYS1. These horses can broadly be managed in a similar way as horses with PSSM-1.
Myofibrillar myopathy (MFM) in Warmbloods
Warmblood horses with MFM do not typically ‘tie up’ (and muscle enzyme levels in blood can often be within normal levels) but rather present with exercise intolerance, difficulty or reluctance to collect, poor engagement of hindlimbs or the horse becoming disunited, and overall poor exercise performance. In dressage horses, even subtle deficits in strength, coordination and tolerance to effort can limit training progression and raise the alarm bells early on. MFM causes abnormal protein aggregates within the muscle cell which lead to structural damage to the muscle fibre. This condition used to be mistakenly diagnosed as PSSM-2 in non-Quarter Horses (sometimes mistakenly referred to as muscle integrity myopathy), and since there is no validated genetic test, the only way to diagnose it is with a muscle biopsy. MFM affects Warmblood between 6 and 10 years of age and in early training. These horses have a low pool of antioxidants, which are required to offset the oxidative damage that happens as a result of muscle contraction during effort – hence they are a key essential nutrient to support muscle function and recovery. Horses affected by MFM benefit from additional supplementation with antioxidants – particularly vitamin E and amino acids that are precursors of cysteine, like methionine.
Contrary to previous advice, Warmblood horses with MFM do better with diets with a more balanced combination of energy sources with a moderate (10-20%) combined level of starch and sugar in their diets especially when they are in work, and a fat level of 6-8%. What matters most in the nutritional management of these horses is the provision of muscle support in the form of key amino acids (particularly lysine and methionine) from high-quality protein sources, as well as antioxidants. Warmblood horses with MFM benefit from additional supplementation with vitamin E, selenium and amino acids that are precursors of cysteine, like methionine, to help offset the effect of free radicals on the muscle.
Vitamin E deficiency myopathies
Vitamin E and selenium are key nutrients for correct muscle development and function, so much so that broodmares fed a diet deficient in these key nutrients are at risk of having foals with nutritional muscular dystrophy or white muscle disease. Although foals affected by this condition can be treated successfully with injectable selenium and daily supplementation with vitamin E, it is far better to prevent this situation by ensuring the diet of pregnant broodmares supplies adequate levels of these nutrients. Vitamin E is also a key nutrient in the diet of young horses in training: certain Lusitano bloodlines with a low vitamin E status may be at risk of a condition called degenerative myeloencephalopathy, affecting the function of the nervous system and causing incoordination and poor mobility. Optimising vitamin E intake through diet can be effective at preventing the onset of this condition, especially since damage to nervous system structures is very difficult to reverse. Finally, in mature adult horses particularly when kept without access to pasture for prolonged periods of time, long-term vitamin E deficiency can cause equine motor neuron disease – a debilitating condition causing muscle weakness and wasting. Once again, early stages of this condition can be reversed by supplementing high levels of vitamin E every day for 3 months, yet ensuring adequate vitamin E intake on a regular basis is a far better approach.
Take home message
- Nutrition plays a decisive role in the management of muscle conditions that may affect dressage horses, and it can be the determinant factor to keep them healthy, fit and in training.
- What the correct balance of energy sources looks like will depend on each type of muscle condition: research shows consistent benefits from using controlled starch feeds which provide steady-release energy from a blend of fibre sources and quality oils.
- Protein quality should not be overlooked, and recent research demonstrates that a balanced amino acid profile, rather than simply the total protein in the horse’s diet, can improve muscle fibre development and function and support recovery. For horses with a tendency to gain excess weight, concentrated sources of protein and micronutrients such as in Balancers can be the perfect solution whilst keeping starch intake at a minimum.
- Micronutrient supply is also critical: electrolytes, particularly sodium, potassium and calcium must be steadily supplied in the horse’s diet to prevent imbalances which can trigger muscle disorders. Maintaining a healthy vitamin E and selenium status is also essential for muscle health, as well as for supporting muscle recovery in dressage horses on regular training.
Early recognition, appropriate veterinary diagnosis, and targeted nutritional management are critical to keep muscle conditions under control in dressage horses. Nowadays it is possible to align feeding strategies with current veterinary evidence – focusing on controlled starch, balanced energy sources, high-quality protein and consistent supply of electrolytes and antioxidants can significantly improve outcomes for dressage horses at all levels.
D&H are committed to delivering excellence in nutrition for happy, healthy, horses. For free feed advice, please call the D&H helpline team on 01270 782 223 or email helplineenquiries@dodsonandhorrell.com