Over the years there have been lots of different names to describe episodes of horses experiencing muscle pain and cramping. While this usually happens during or after exercise, it can also be noticed in some horses after a period of time in the stable, or even when out in the field in particularly susceptible horses.
Symptoms include reluctance or even complete inability to move at the extreme end of the scale to more subtle signs including a horse being unable to lengthen the stride when asked, slight dragging of the hind toes during exercise, difficulty picking up hooves, and tense muscles that are sore when touched. When experiencing an attack, the horse may show signs of pain such as sweating, increased heart and breathing rates and in severe cases the urine may turn a shade of red, which is due to a protein called myoglobin leaking out of damaged muscle cells.
Equine Rhabdomyolysis Syndrome
While the symptoms can be similar, the reasons can be different between horses and all of these conditions are grouped under the umbrella term Equine Rhabdomyolysis Syndrome (ERS).
ERS can be broadly split into two different categories, intrinsic also known as chronic, and extrinsic which can be referred to as sporadic. These terms refer to whether the issue originates from an underlying condition within the muscle or a different factor such as management, environment, nutrition, or even a condition that doesn’t happen within the muscle itself, but does still affect another body system.
Intrinsic ERS would include Polysaccharide Storage Myopathy (PSSM) and Recurrent Exertional Rhabdomyolysis (RER), that occur due to an issue with the way the muscle contraction happens, or how carbohydrate is stored and used within the muscle. Both of these are genetic predispositions, but nutrition and management also have an effect. Symptoms are often apparent from the beginning of exercise and when warming up, and in severely affected horses who have been out on grass, can even be apparent in the field. PSSM and its variants tend to be mainly found in Quarter Horses and related breeds, Warmbloods, Morgans, Anglo-Arabs, Andalusians and UK Cob types, while RER is more commonly found in Thoroughbreds, Standardbreds and Arabians.
Extrinsic ERS can affect any breed, age and discipline. The most common triggers for an episode are:
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Hard exercise after a period of light or no work along with overfeeding.
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A diet that provides too much starch and sugar and not enough fibre.
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Electrolyte imbalances in the diet, or an imbalance in the way the horse absorbs or excretes electrolyte minerals.
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Potentially inadequate dietary Vitamin E and Selenium, although this remains a theoretical link at present.
Whether intrinsic or extrinsic ERS is responsible, other factors that can have an effect on frequency and severity of symptoms may include stress, infection, hormone fluctuations, and weather / ambient temperature.
Management and prevention
Unfortunately, there isn’t one universal strategy that can guarantee a horse won’t have any future ERS episodes, but there are various management and nutritional practises that can be put in place to try to limit the number and severity of episodes and so improve the horse’s muscle comfort and quality of life.
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Regular movement and exercise adapted to the horse’s fitness level (after vet approval following an episode).
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Free paddock or arena access to facilitate gentle movement.
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Keeping increases to workload and exercise gradual, with plenty of warm-up time for PSSM or RER horses.
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Regular, predictable routines to support stress management for horses with RER.
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Appropriate rugging horses with intrinsic ERS to prevent them getting cold, especially after exercise when they have been sweating.
Nutrition strategies
While dietary strategies are likely to vary between horses depending on the underlying reason for the ERS and severity, there are several useful principles that can help reduce both the frequency and severity or episodes, as well as support recovery.
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Providing forage as the main source of dietary energy, particularly during a recovery period, alongside an appropriate source of vitamins and minerals.
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For horses with PSSM in particular, try to keep the total diet below 10% Non-Structural Carbohydrate (NSC) content.
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If the horse struggles to maintain weight, adding oil to the ration can increase the calories provided without risk of increasing symptom severity. Oils in the diet have been found to support maintenance of healthy muscle function, although the type and amount to be fed have not been studied in enough detail to give exact guidelines yet.
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Keep meal sizes small; even if the feed has a relatively low starch and sugar percentage, giving a large meal will provide all of the NSC from that quantity of feed in a short period of time, resulting in increased blood sugar and subsequent glycogen formation in the muscle. For example, a feed containing 14% combined starch and sugar will give 350g per 2.5kgs. If this were fed in one meal, that would all be absorbed in a short space of time giving a large blood sugar spike. Splitting that between 3 small feeds would give just over 116g per meal, resulting in much smaller glycaemic, or blood sugar, responses.
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Restricting grass intake for PSSM horses: As grass can contain a very high level of sugar at certain times of the year, restricting grass intake while still allowing turnout and movement can be helpful. Grazing muzzles, arena turnout or a bare paddock (similar to managing a laminitis-prone horse) can all be useful strategies.
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Ensuring adequate selenium and Vitamin E intakes for the individual horse are beneficial to avoid deficiency and maintain a normal antioxidant balance within the muscle. Exact intake levels will vary between horses depending on the type of ERS and severity, with the Natural (RRR-alpha-tocopherol) having higher bioavailability.
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Providing sufficient levels of electrolyte minerals supports normal muscle contractions.
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Adding oil to the feed, with the amount fed depending on the horse’s workload and body condition, can help support normal inflammatory levels although there is currently not much evidence exploring the exact rations and types of fatty acids that are most beneficial.
Conclusion
While ERS can be distressing for both horse and owner, there are practical, simple steps we can take with both management and nutrition to help support horses who are prone to these painful symptoms. Most horses who have experienced extrinsic ERS and those who have chronic intrinsic ERS are able to live healthy, pain-free and happy lives with the correct strategies in place. If you would like any help adapting your horse’s diet our nutrition team are always happy to help, and can be reached by email at [email protected], on our webchat, through social media, or by phone on 0800585525.
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