March 17, 2021 7 min read
Muscular disorders represent a common cause of disability and of poor athletic performance in horses. Among them, polysaccharide storage myopathy (PSSM) is widely recognized as a cause of recurrent episodes of muscle disease.This muscle disease occurs primarily inQuarter Horses, Paint Horses and Appaloosas. PSSM also occurs less frequently in breeds such as Drafts, Draft crossbreeds, andWarmbloods.
PSSM is a widely described cause ofexertional rhabdomyolysis (ER-muscle breakdown with exercise) in horses. The primary clinical sign of this disease ismuscle cramping or tying-up. Most commonly, muscle stiffness, skin twitching, profuse sweating,abnormal gait, firm painful muscles (particularly over their hindquarters), colic-like signs of pain, and reluctance to move are seen. The signs are most often seen in horses when they are put into initial training or after a spelling period when they receive little active exercise. Episodes usually begin after very light exercise, such as 10-20 minutes of walking and trotting. Horses with PSSM can even exhibit symptoms without exercise. During an episode, horses seem lazy, have a shifting lameness, tense up their abdomen, and develop tremors in their flank area. When horses stop moving they may stretch out as if to urinate. Some horses will try pawing and rolling immediately after exercise.
However, clinical signs may vary with different breeds and severity of the condition. Most horses with PSSM have a history of numerous episodes of muscle stiffness at the beginning of training; however, mildly affected horses may have only one or two episodes/year. Episodes of muscle pain and stiffness can be quite severe, resulting in a horse being unable to stand and being uncomfortable even when lying down.
PSSM is primarily aglycogen storage disorder. In normal tissue, insulin drives glucose from the blood into muscle and liver cells to be stored as glycogen and later used as energy. PSSM is characterized by the abnormal accumulation of the normal form of sugar stored in muscle (glycogen) as well as an abnormal form of sugar (amylase-resistant polysaccharide) in muscle tissue.Muscle glycogen concentrations in affected horses are between 1.5 to 4 times greater than in normal horses.An excessive accumulation of glycogen, glucose-6-phosphate, and abnormal polysaccharide inclusions in skeletal muscle is observed.Serum aspartate transaminase andcreatine kinase (CK) are often persistently elevated in association with clinical signs. The urine in such horses is often coffee-colored (myoglobinuria), due to muscle proteins being released into the bloodstream and passed into the urine. This is a serious situation, as it can damage the horse's kidneys if they become dehydrated.
Clinical symptoms and results of traditional complementary exams such as measurement of muscle enzymes and urinalysis are generally not very useful for the specific diagnosis of PSSM, while muscle biopsy is considered an essential tool for this purpose. The first description of PSSM, in 1992, wasbased on the hallmark feature ofamylase-resistant polysaccharide in skeletal muscle biopsies. Later,aggregates of amylase-sensitive glycogen and subsarcolemmal lakes of muscle glycogen were added as potential diagnostic features.
Research into the genetic basis of this disease shows that there are two kinds of PSSM disorders. PSSM1 is associated with a mutation in a specificglycogen synthase gene (GYS1). Horses affected with PSSM2 exhibit abnormal histologic muscle glycogen but lack theGYS1 mutation. The genetic basis of PSSM2 is currently unknown. Clinical signs are similar between PSSM1 and PSSM2 horses.
Diet and exercise play a very important role in improving this condition in horses. The goal is tostabilize blood glucose and insulin and provide a regular exercise regimen that enhances glycogen metabolism. Regardless of the severity of the score given for the amount of abnormal polysaccharide in muscle biopsy specimens, both exercise and dietary changes have been shown to result in an improvement in symptoms.
Because horses with PSSM have enhanced insulin sensitivity, a fat supplemented diet appears to produce a beneficial effect by providing energy without stimulating insulin secretion and glucose uptake into muscle cells.Significantly reducing starch and sugar, and increasing the number of calories supplied by dietary fat should help decrease uptake of glucose into the muscle and reduce glycogen stores. Therefore,recommendations include a diet that provides < 12% of digestible energy (DE) as non-structural carbohydrate (NSC), 15–25% of DE as fat and fermentable fibre, and regular daily exercise. This feeding regime produces a low glycaemic and insulinaemic response, while providing an alternate energy source in the form of plasma-free fatty acids and volatile fatty acids.
It is necessary to analyze the forage given to horses diagnosed with PSSM. Studies suggest that feeding hay with an NSC of 17%, or greater, that provides 31% of DE as NSC, could be a poor choice for PSSM horses, because of the glycaemic and insulinaemic response such hay induces. In contrast, hay with anNSC content of < 12% does not appear to produce the same high post-prandial glycaemic or insulinemic responses in PSSM horses. The importance of limiting the NSC content of hay may depend upon the total daily caloric requirements and the necessity of providing calories in the form of fat without producing excessive weight gain.
Since grass possibly can contain high amounts of sugar, when on pasture, horses that are not fed any concentrates can still develop symptoms. Well-maintained pastures should contain low-sugar grasses and few legumes (clover, alfalfa or lucerne). Appropriate hay is often mature and in most cases should be selected over more energy-dense immature hay, though all hay should be free of mold, dust, and foreign material. Also, as ingestion of high levels ofwater-soluble carbohydrates (WSC) has been implicated in insulin resistance and PSSM,such hays would be unsuitable for animals at risk of these conditions.Soaking hay in water is thus a common strategy used to remove WSCs and manage the nutrition of these diseased horses. Current hay soaking recommendations include soaking hay for 30 minutes in warm or 60 minutes in cold water for reducing the WSC content. This treatment should get rid of other culprits that trigger ER such as dust, mold (mycotoxins) and chemicals sprayed on the hay too.
Several studies show that feedingsweet feed or other grain-based concentrated feeds that contain a large amount of starch to PSSM horses exacerbates exercise intolerance and muscle pain in PSSM. Soy, wheat, and molasses appear to be specific triggers. Therefore, the typical recommendation is that all grain, sweet feed, and molasses should be eliminated from the diet and replaced with high quality forages such as alfalfa hay or grass-alfalfa hay mix diets. Grain hay such asoat hay should be avoided, as high-starch grain kernels can cling to the stems, increasing starch content in the diet. However, as additional calories are generally necessary due to the strict exercise regimen needed to control clinical signs; fat is added to the diet to provide an alternative energy source. Fat supplements of vegetable oil, powdered animal fat, and other alternative energy sources such as fermentable beet pulp can be used.Soyabean and canola oil have a higher omega-3:omega-6 fatty acid ratio compared to corn oil making them preferred additions to the PSSM horse’s diet. Although flaxseed oil and fish oil have a superior omega-3:omega-6 fatty acid ratio, these oils are highly prone to oxidation and easily turn rancid at room temperature. Cold-pressed coconut oil - with greater stability at storage conditions typically encountered in a barn - represents an excellent choice as an alternative energy source for horses experiencing symptoms of ‘typing up’.Copra Meal, Cool Stance® coconut meal, or Renew GoldTM are all feeds that could be considered for use in diets for horses needing to remain on a low NSC diet that will cause minimal disturbance to their post feeding glucose and insulin levels. It is important to remember that these feeds need to be stored properly in a cool, dry place to prevent oxidation. The addition of these fats to the diet could cause an upswing in the production of free radicals. To prevent cellular oxidation triggered by free radicals, PSSM horses should be fed avitamin E supplement.
Improving the feeding of affected horses is often not enough to achieve a decrease in severity and regular exercise is often recommended.Exercise therapy consists ofdaily turnout and as little stall rest as possible. The horse should be longed for 15 minutes a day and, if no increases in CK are evident, the workload can be gradually increased. When the horse can be worked for 30 minutes without difficulty, active riding can be initiated.
Another potent ingredient, a protein rich cocktail, bovine colostrum (BC) which is found in Immubiome Lean Muscle, can be included in the diet of horses diagnosed with PSSM.It is the first milk produced by cows after calving and contains numerous beneficial substancesthat provide a strong nutritional base for the new-born animal. It is a demonstrated nutritional supplement for musculoskeletal repair and growth, muscle recovery and improvement in muscle mass. As muscle breakdown following exercise is observed in horses with PSSM, the muscle recovery and mass gain supported by BC supplementation can be very beneficial. BC supplementation has also been shown to have a positive effect onblood sugar control. This should prove particularly useful for PSSM horses where the primary goal of maintenance therapy is to control blood glucose levels.
TheGanoderma lucidum (Reishi) mushroom which is found in Immubiome Lean Muscle, can also be a helpful addition to the PSSM horse’s diet. Reishi supplementation has been shown to aidblood sugar control. Effective recovery from muscle wear and tear has also been documented for the numerous bioactive compounds, such as polysaccharides, triterpenoids and proteins, that can be extracted from the fruiting bodies, cultured mycelia and spores of Reishi.
Several studies highlight important links between gut bacteria and skeletal muscle metabolism and glucose homeostasis in animals. (Also found in Immubiome Lean Muscle.) The short chain fatty acids (SCFA) - acetate, propionate, and butyrate - are an important energy source in the colon and are mainly produced by bacterial fermentation of undigested carbohydrates in the horse’s hindgut. Besides their local role as energy substrates within the colon, SCFA act as signalling molecules involved in systemic lipid metabolism and glucose/insulin regulation. Probioticscan positively influence dietary protein digestion and utilization in the gastrointestinal tract helping the equine athlete better assimilate protein from its diet togain lean muscle. Gut microbiota also play an important role in theproduction, storage, and expenditure of energy obtained from the diet. Restoring the gut bacterial community of horses, by supplementing their diet with probiotics, can help alleviate some of the symptoms of PSSM by supporting glucose homeostasis and building muscle mass and muscle strength.
Altering dietary components has become a key strategy for managing muscle disorders in horses. In the case of PSSM too,the management recommendations provided to horse owners are directed at reducing muscle glycogen synthesis, promoting oxidative metabolism and providing fat as an alternative fuel. In addition to this basic recommendation, supplementing the horse’s diet with more beneficial ingredients such as bovine colostrum, Reishi and probiotics can provide the necessary support towards positively influencing carbohydrate metabolism, establishing glucose homeostasis, and helping to rebuild muscle mass and strength.
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