Learn information about equine muscle disorders and the management of horses with muscle issues such as equine polysaccharide storage myopathy.
The more common muscle disorders that horses suffer from are caused by an inability to release stored muscle energy, which can be the cause of the painful experience commonly known as tying-up.
Changing energy sources from carbohydrates high in starch or sugar to high-quality fats can greatly improve the degree and occurrences of tying-up, and provide an excellent alternative energy source for horses prone to these types of muscle disorders. Managing horses suffering from muscle disorders requires simple but thoughtful changes to their daily nutrition and activity levels.
Acronyms for Equine Muscle Disorders Explained
EPSM = equine polysaccharide storage myopathy
PSSM = polysaccharide storage myopathy
EPSSM = equine polysaccharide storage myopathy
These are all the same condition.
Types of Muscle Disorders by University of Minnesota diagnostic criteria:
Type 1 – associated with collections of abnormal glycogen in the muscle. Caused by a mutation in the GYS1 gene. Genetic testing is available.
Type 2 – associated with higher than normal amounts of glycogen in the muscle.
Currently, PSSM type 1 can be diagnosed with a genetic test, however, at present PSSM type 2 must be diagnosed with a muscle biopsy. Which test is most appropriate depends on the breed of your horse. EPSM is **not** synonymous with tying-up. There are many other causes of tying-up.
Decision tree for Diagnostic Testing of PSSM courtesy of University of Minnesota.
Breeds of Horse susceptible to muscle disorders
Type 1 is found in over 20 breeds and commonly affects Quarter Horses, Quarter Horse-related breeds, Morgans, some Draft breeds and some warmbloods. Type 1 is rare in Thoroughbreds, Standardbreds, Arabians, Morgans and nondraft pony breeds.
Type 2 is very common in draft breeds originating from continental Europe such as Belgians, Percherons and Trekpaard and their crosses. A high percentage of Continental European Draft breeds (62%) were found to carry the mutation responsible for Type 1 PSSM but Type 1 is rare in Shires and Clydesdales, which originated in Britain and Scotland. Type 2 is also less common in Warmbloods but may be found in Quarter Horses and QH related western breeds. Type 2 is rare in Morgans.
Symptoms of Equine Polysaccharide Storage Myopathy
Tying up, muscle spasms and elevated muscle enzymes will be seen in QH and light breeds. Drafts and Warmbloods may show only muscle tenderness or a reluctance to engage the hindquarters. They may exhibit back issues, show gait abnormalities, have muscle atrophy and weakness but often without elevated muscle enzymes.
Shivers is not caused by EPSM.
The mechanism of equine polysaccharide storage myopathy
Tying-up or weakness with EPSM is an energy crisis in the muscle cell. Both muscle contraction and relaxation require energy. Higher fat feeding can have beneficial effects by slowing glucose entry into glycogen. It appears there is actually a shortage of glucose to burn in the muscles because:
- The EPSM muscle preferentially produces glycogen rather than burning glucose.
- The abnormal glycogen found in type 2 EPSM is not easily broken down by the cell.
- The EPSM muscle has decreased mitochondria (cellular structures that burn fat and glucose with oxygen and have a high energy yield) and higher levels of lactate indicating inefficient use of glucose.
All drafts, EPSM or not, and Quarter Horses with EPSM, are very insulin sensitive. However, this does not cause the EPSM. It is a reflection of their high need for glucose because of how inefficiently they utilize it.
Treatment/Management of equine polysaccharide storage myopathy
Turnout, 24/7 if available and as much formal exercise as possible is important to controlling EPSM. Exercise triggers the AMPK enzyme which stops glycogen production and directs glucose into energy pathways.
A forage based, low carbohydrate diet with combined sugar and starch of 12% or less is used to avoid surges of glucose being presented to the muscle and stimulating glycogen storage.
High fat feeding (choose a high omega three source such as micronised linseed or a good medium chain tri-glyceride source such as copra) a minimum of 450 grams of fat for 450 kg of body weight (often much more), is a common treatment. Several months are required for maximum benefit. High fat feeding can train the muscle to use more fat to some extent. The major effect is likely through mechanisms that limit glucose uptake by muscle when there are high circulating levels of fat.
Acetyl-L-carnitine [ALCAR], a metabolite of the amino acid L-carnitine, is a naturally occurring compound in the muscle. Like exercise, ALCAR switches on the AMPK enzyme that directs glucose into energy rather than glycogen. The acetyl group can also be switched to produce acetyl-CoA, which favors burning of glucose/glycogen over fats. ALCAR typically produces obvious clinical benefit in less than a week, with improvements continuing as the horse becomes more fit and fine tuning of the diet improves muscular function. Dosage is 1 gram per 45 kgs of body weight. Please note however that if supplementing with ALCAR, high fat should not be fed.
Regular daily exercise is extremely important for managing horses with PSSM. Even 10 min of exercise has been shown to be extremely beneficial in reducing muscle damage with exercise. Once conditioned, some PSSM horses thrive with 4 days of exercise as long as they receive daily turn out. For riding horses with type 2 PSSM, a prolonged warm-up with adequate stretching is recommended. Rest periods that allow horses to relax and stretch their muscles between 2 – 5 min periods of collection under saddle may be of benefit.
University of Minnesota
Basic Nutritional Strategies to help Horses with Equine Polysaccharide Storage Myopathy
Several other basic nutrients impact muscle function, regardless of whether or not the horse has EPSM. Many horses with a diagnosis of “mild” type I EPSM by biopsy have responded to measures correcting intakes of these nutrients with no EPSM specific interventions. If the horse does have EPSM, these are issues that can interfere with a strong positive response to treatment. They are completely preventable, so no reason not to actively cover these bases.
- Salt [sodium chloride – basic table, rock or sea salt is fine and cheap] As little as 2% dehydration can have serious effects on muscle strength, power and endurance. Sodium is the major ion responsible for keeping water levels in the body at optimum. It is can be severely deficient in the equine diet due to low levels in forage, a forage analysis will confirm the amounts needing to be fed. If a forage analysis cannot be carried out then a rough guide is to feed an average size horse 20-30 grams of salt daily in cold weather, and 4 times this in warm weather which causes sweating.
- Thyroid dysfunction has a major impact on skeletal muscle energy generation. Typical equine diets are borderline to clearly deficient in iodine due to low levels in forage. Avoid problems related to deficiency by providing at least the bare minimum of 2 mg, on top of that contained in forage for the average size horse, by feeding a forage focused supplement. Additional iodine can be provided by feeding 2.5 – 5 mls of seaweed each day should a forage analysis show that levels are so low that extra are needed. Bear in mind that working horses require higher levels of iodine.
- Magnesium deficiency causes muscle irritability and inflammation. If a forage analysis is available, adjust the calcium:magnesium ratio to between 2:1 and 1.5:1. If not, 1 gram per 45 kgs of body weight is a good approximation for most hays.
- Research has documented increased oxidative stress in muscle from EPSM horses, making intake of vitamin E and selenium even more important than normal. The usual starting dose for EPSM horses of average size is 2 mg/day of selenium and 5000 IU of vitamin E oil. Higher intakes of vitamin E, up to 10,000 iu can be of benefit to some horses.
- Another important antioxidant is glutathione. L-glutamine supplementation can support these levels and also doubles as an alternate energy source in muscle. Dosage is 1 gram per 45 kgs of body weight. Higher levels, up to 2 grams per 45 kgs of body weight, can be used for horses with ongoing issues of muscle pain.
- When horses have issues with muscle atrophy or low energy levels for exercise, L-leucine supplementation can often help, this is one of the branch chain amino acids. This is the major amino acid in muscle tissue and has an anabolic effect. It can also serve as an alternate energy source. Give 2 grams per 45 kgs of body weight 30 minutes before and after exercise. When exercise is prolonged, repeat the dosing at hourly intervals.
Can you cure a Horse diagnosed with Equine Polysaccharide Storage Myopathy?
When the described diet and formal exercise routine were followed we found that all horses improved, and >75% of horses stopped tying-up. PSSM horses, however, will always be susceptible to this condition and if their exercise schedule is disrupted. If they become ill from other causes, they may again develop clinical signs again.
University of Minnesota
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