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Polysaccharide Storage Myopathy in Warmblood HorsesBy Kentucky Equine Research Staff · January 12, 2017

Your Warmblood horse’s gait seems a little off; he feels stiff and unwilling to go forward under saddle. An initial lameness examination does not find the cause. He’s never tied-up before, but could it be polysaccharide storage myopathy (PSSM)? Susannah Lewis, D.V.M., Ph.D., of Hagyard Equine Medical Institute in Lexington, Kentucky, and Stephanie Valberg, D.V.M., Ph.D., of Michigan State University presented research* regarding the clinical presentation of PSSM in Warmbloods at the 2016 American Association of Equine Practitioners Annual Convention.

Type 1 vs. Type 2 PSSM

There are two types of PSSM currently recognized by the veterinary community. Type 1 PSSM is brought about by a genetic mutation of the glycogen synthase 1 (GYS1) gene, which causes increased glycogen storage in the muscle. The main clinical sign of Type 1 PSSM is exertional rhabdomyolysis, commonly known as “tying-up.” The cause or causes of Type 2 PSSM is not yet known, and the diagnosis is based on tissue appearance from a muscle biopsy. Type 2 PSSM horses have an abnormal appearance of muscle glycogen under the microscope, but there is no mutation of the GYS1 gene.

Are Warmbloods Different?

Lewis and colleagues wanted to determine if Warmbloods had a unique presentation of Type 1 or Type 2 PSSM compared to other breeds, and if muscle glycogen concentrations were different between Type 1 and Type 2 PSSM. A database containing 3,615 muscle biopsy samples were analyzed, 581 of which were from Warmbloods and 3,034 from other breeds. With respect to Type 1 PSSM, the GYS1 mutation was rare in Warmbloods (8% of Warmbloods with PSSM had the Type 1 mutation). Warmblood crosses were disproportionately represented in the Type 1 group. There was no evidence that Type 1 PSSM presented differently in Warmbloods vs. non-Warmbloods. The most common clinical sign in all horses with Type 1 PSSM was tying-up.

In contrast, about 92% of Warmbloods diagnosed with PSSM had the Type 2 form. The main clinical signs were abnormal gait and muscle pain. Unlike non-Warmbloods with Type 2 PSSM, warmbloods were much less likely to have tied-up. Muscle glycogen concentrations in horses with Type 2 PSSM were not elevated like they were with Type 1 PSSM.

Further investigation showed no difference in breed, age, or gender of Warmbloods with and without Type 2 PSSM. Warmbloods were more likely to show an “abnormal gait,” often a mild, poorly localized hind limb lameness, or a stiffness and reluctance to move forward.

In Sum

Warmbloods are rarely diagnosed with Type 1 PSSM, but when they are, the clinical signs of tying-up are similar to non-Warmblood horses. Warmbloods with Type 2 PSSM are less likely to tie-up and more likely to have an abnormal gait, whereas other breeds with Type 2 PSSM are more prone to tying-up. Currently, Type 1 and Type 2 PSSM horses are treated and managed in a similar way. Since muscle glycogen concentrations differ between Type 1 and Type 2 PSSM, future research is needed to determine if different management practices are warranted based on the etiology of the condition.

Learn how to best nourish horses diagnosed with PSSM. If you have a specific question about your horse’s diet, including how to appropriately feed a PSSM horse, check in with a Kentucky Equine Research nutritional consultant at any time. Go now!

*Lewis, S.S., S.J. Valberg, and A.M. Nicholson. 2016. Warmblood horses with polysaccharide storage myopathy: Clinical characteristics and muscle glycogen concentrations. Proceedings of the 2016 American Association of Equine Practitioners. 62:511-512