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Polysaccharide Storage Myopathy (PSSM), also called Equine Polysaccharide Storage Myopathy (EPSM or EPSSM), is likely an inherited condition, occurring most commonly in Quarter Horses, draft horses, and warmbloods, but also showing up in other breeds. The muscles of a horse with PSSM are unable to properly store glucose (sugar) so it is unavailable when needed for energy. Two forms have now been identified: Type 1, which is caused by a genetic mutation, and Type 2, the cause of which has yet to be determined. PSSM is related to “tying up”, but “tying up” can have many causes. For more information on the causes of “tying up” not related to PSSM, click here.
Both diet and the exercise/turnout program for a horse with PSSM must be adjusted in order to properly manage the condition.
Because PSSM causes horses to be more sensitive to insulin and therefore store more glucose in their muscles than normal, it is important to restrict the amount of sugars and starches in their diet. Nutritionists refer to the easily digestible sugar and starch portion of feeds and forages as non-structural carbohydrates (NSC). Since grain such as sweet feed is high in NSC, it should be removed from the diet and replaced with either a multi-vitamin/mineral supplement or a ration balancer. Also, hay that tests less than 10-12% NSC is more appropriate for PSSM horses, with soaking as one method to reduce sugars and starches in forage that tests higher. Access to fresh grass should be limited either with a grazing muzzle or by turning out onto a dry lot for exercise. Fat can gradually be added into the diet for those horses that require additional calories or energy.
Ample turnout is essential for horses with PSSM, as they should not be stalled for more than 12 hours at a time. Ideally these horses should be kept outside 24 hours per day. Another key to managing this condition is keeping the horse in a consistent exercise program – meaning controlled exercise such as hand walking, lunging, riding, or driving. When returning the horse to work after a bout of PSSM, start gradually. Always provide these horses with a long warm-up and cool-down, and offer frequent walk breaks.