Advances in genetic research have made diagnosis straightforward in many cases, and feeding trials have fine-tuned nutritional approaches so that affected horses can lead normal, useful lives.

Simply put, PSSM arises from two specific glitches in skeletal muscle metabolism. First, an abnormal accumulation of glycogen occurs in the muscle. As a polysaccharide, glycogen serves as the primary storage form of glucose in horses.

While skeletal muscle is a major reservoir of glycogen, too much causes problems, including PSSM. Second, the presence of an abnormal polysaccharide may also occur in some horses with the disorder.

PSSM can be divided into two distinct types. Type 1 involves a mutation in the glycogen synthase-1 gene, which causes abnormal increased glycogen synthesis in skeletal muscles. Forms of PSSM that are not associated with this gene mutation are distinguished as type 2. Researchers have not yet been able to identify the cause for different forms of the disease in spite of biochemical studies.

The effects of PSSM manifest during exercise, usually 10 to 30 minutes after onset, and mirror many of those observed in other forms of tying-up. Mild signs include unwillingness to work, reluctance to engage the hindquarters, shifting lameness, and stopping and stretching as if to urinate.

As pain increases, gaits frequently change, becoming stilted with a shortened stride, and muscles of the hindquarters become firm and sore. The most severe cases are characterized by profuse sweating, elevated respiration and heart rate, muscle twitching, refusal to move or only walk in a slow, wooden fashion, and off-colored, reddish-brown urine. Horses with extreme PSSM may be unable to stand.

Diagnosis

Though the clinical signs of the disease are difficult to miss, the gold standard for diagnosis of PSSM remains muscle biopsy, as microscopic examination of muscle tissues reveals the presence of muscle damage with excessive normal glycogen (considered grade 1) or muscle damage with abnormal polysaccharide (grade 2). Muscle biopsies are not, however, easily obtained and require veterinary expertise.

Advances in genetic testing have allowed for diagnosis of type 1 PSSM through hair roots or whole blood samples. The American Quarter Horse Association offers PSSM testing as part of its five-panel genetic test.

The Michigan State University Equine Neuromuscular Diagnostic Laboratory also performs genetic testing. Submission of samples is easy, and guidelines are provided on sample submission forms. Type 2 PSSM must still be diagnosed definitively through muscle biopsy.

Guidelines for Feeding Affected Horses

How best to feed a horse diagnosed with PSSM depends on the individual. “Metabolism and performance expectations are two important factors in choosing an appropriate diet. Most horses with PSSM are typically in at least moderate body condition, and many are easy keepers bordering on obesity, so few PSSM horses require calorie-rich diets,” said Kathleen Crandell, Ph.D., a nutritionist with Kentucky Equine Research (KER). Forage and feed choices for PSSM horses are centered on minimizing sugar and starch intake.

Forage requirements. Forage can be supplied as pasture, hay, or hay alternatives such as pellets or cubes. Well-maintained pastures should contain low-sugar grasses and few legumes (clover, alfalfa or lucerne).

Grazing areas should not be lush, and low-yield acreage—one step above a drylot—seems to be especially suitable for PSSM horses. If you’re unsure about pasture suitability, pasture grasses can be analyzed by a reputable laboratory to determine if nonstructural carbohydrate (NSC) levels are less than 12%, which seems appropriate for most affected horses.

In addition to the nutritional advantages of turnout, foraging allows for an increase in exercise, which is essential for these horses. For times when too much forage is available, a grazing muzzle can limit intake.

Hay and alternative hay sources such as pellets and cubes should be made from grasses and should also have NSC levels less than 12%. 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.

“Although many hay producers take pride in their harvests and will offer hay analysis results, they may not have results for NSC, sugars, or starches, so it is best to have hay tested for these values by a laboratory to ensure its appropriateness for PSSM horses,” advised Crandell. Pellets and cubes are often packaged by manufacturers, and NSC values for these products may be printed on the label or available through the company.

Concentrated energy.If a horse requires additional calories to maintain weight while exercising, a concentrated source of energy should be offered. Neither straight cereal grains such as plain oats nor textured or sweet feeds containing cereal grains should be fed.

“Specialized feeds for horses that are intolerant to large amounts of starch or sugar are the most appropriate,” said Crandell. RE•LEVE® Concentrate, a low-starch, high-fat feed formulated by KER, provides adequate energy to horses in the form of alternative energy sources, primarily fermentable beet pulp and vegetable oil.

RE•LEVE Concentrate is designed with greater nutrient density to ensure horses with low to moderate energy needs receive proper nutrients. Most horses do well on 3-9 lb (1.5-4 kg) of RE•LEVE Concentrate daily. If more than this is needed to maintain weight, choose RE•LEVE Original, which contains the same low-starch ingredients but allows more to be fed daily (6-20 lb, 2.5-9 kg), which boosts calorie intake. Australian horse owners should contact KER Australasia at 1800 772 198 for an alternative.

“Based on research conducted on RE•LEVE, the beneficial effect of a low-starch diet is believed to be the result of less glucose uptake into muscle cells and provision of more plasma free fatty acids for use in muscle fibers during aerobic exercise,” explained Stephanie Valberg, D.V.M., Ph.D., a researcher at the Michigan State University College of Veterinary Medicine. Valberg, her colleagues, and researchers at KER performed the studies involving RE•LEVE.

“Quarter Horses naturally store very little lipid in muscle fibers and delivery of free fatty acids may overcome the disruption of energy metabolism that appears to occur during aerobic exercise,” she said.

Balanced vitamins and minerals. Horses on all-forage diets require vitamin and mineral supplementation for optimal health. “A ration balancer will make up for any shortfalls in protein, vitamin, and mineral nutrition, and will not add significantly to the NSC content of the diet. Most ration balancers are designed to be fed at a rate of 1-2 lb (0.45-0.9 kg) per day,” said Crandell.

In lieu of a ration balancer, a well-fortified vitamin and mineral supplement, such as Micro-Max™, can be fed if even 1 lb (0.45 kg) of a balancer pellet is too many calories for the horse.

Supplemental oil.Some studies suggest that signs of muscle dysfunction can persist even when horses are fed a low-NSC diet, but clinical signs diminish when even a little vegetable oil is added to the diet. For certain horses, additional calories in the form of vegetable oil might be necessary. In light of the benefits proffered by omega-3 fatty acids, choose soybean or canola oil, as these are richer in omega-3s than corn oil. Between 100 and 500 ml (0.25-2 cups) of oil can be fed daily, though it should be added gradually over a period of a week or so.

Antioxidant intake. The addition of fat 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 a vitamin E supplement, the most potent of which is a natural, nanodispersed product called Nano•E®.

According to Crandell, vitamin E should be offered at an intake of 1-1.5 IU per ml of supplemental oil in addition to 2-3 IU of vitamin E per kg body weight for horses in light work. An 1,100-lb (500-kg) horse, for example, should receive 1,000-2,000 IU of vitamin E per day.

Other Management Strategies

Exercise. Aside from a thoughtful diet, an exercise program must be implemented for PSSM horses to show clinical improvement, as it enhances energy metabolism. Horses with PSSM should have a well-defined and strictly enforced exercise program that includes daily paddock turnout and near-daily structured exercise, even if it is low-intensity work such as trail riding.

Return to exercise following a bout of tying-up should be slow but within a few days after all pain and stiffness has dissipated. Prolonged rest after an episode appears to be counterproductive, predisposing PSSM horses to further episodes. Similarly, too-rapid resumption of exercise could incite another episode of tying-up, causing further muscle damage.

Reintroduction of exercise to PSSM horses needs to be more gradual than approaches used in other forms of tying-up. Valberg advises adherence to the following guidelines:

• Providing adequate time for adaptation to a new diet prior to commencing exercise;
• Recognizing that the duration of exercise is more important to restrict than the intensity of exercise;
• Ensuring the exercise is gradually introduced and consistently performed; and
• Minimizing any days without some form of exercise.

Obesity. Many horses diagnosed with PSSM are obese. “Owners can enhance fat metabolism in obese horses by riding them after a 5- to 8-hour fast as a means to elevate free fatty acids,” said Valberg. Under normal circumstances, horses should not be fasted for this length of time as it sets the stage for gastric ulcers. In these cases, utilize a digestive tract buffer such as RiteTrac™.

A Step Ahead: Making Adjustments Prophylactically

Due in large part to the simplicity of genetic testing, horse owners no longer have to wait for an episode of tying-up to occur before management strategies are implemented. Regardless of the intended use of a horse, its diet can be designed to keep starch and sugar levels low while supplying all of the necessary energy and nutrients for top performance.

In many instances, management strategies reduce the frequency of tying-up episodes, and sometimes clinical signs diminish entirely.

In the past, muscle disorders such as PSSM prevented thousands of horses from fulfilling their athletic potential. New ways to manage horses, including how best to nourish them, have provided those horses and future generations the chance to be productive citizens in the equine world.

Kentucky Equine Research (KER) is an international equine nutrition, research and consultation company serving both the horse producer and the feed industry. Its goal is to advance the industry's knowledge of equine nutrition and exercise physiology and apply this knowledge to produce healthier, more athletic horses. For more information, see www.ker.com or call 888-873-1988.