Meet “George”: A Polysaccharide Storage Myopathy (PSSM) Horse

I’m proud to introduce you to George, a “Heinz-57” draft cross, and my current equine partner.  He is the result of a ½ Thoroughbred, ½ Percheron (dam) x ½ Hanoverian, ½ Paint horse (sire). I’ve had the privilege of knowing George since he was a weanling, and bought him as a yearling.  It wasn’t until I started him under-saddle as a three and a half year old that I started noticing behavioral changes (crankiness – not like George), non-specific muscle soreness, and a transient, almost undetectable gait abnormality, all of which happened to be associated with new hay delivery.  I won’t mention how much I’ve spent having him worked-up, imaged, adjusted, fitted and many more things to get to the bottom of what his body was trying to tell me.  We were coming up empty handed and frustrated.

It wasn’t until after I returned from an equine nutrition symposia that it occurred to me to have him tested for polysaccharide storage myopathy (PSSM). PSSM causes the horse’s muscle cells to store energy (glycogen) in excess, which can result in a variety of symptoms, the most severe of which is tying up after aerobic exercise.  Nearly all of the classic signs were there, short of a bad tying up episode.  Wouldn’t you know it, he came back positive for Type-1 PSSM a.k.a. EPSM, tying- up syndrome/exertional rhabdomyolysis/Monday morning disease, set fast or azoturia.  There is more than one version of PSSM (Type-1 is most common) and the diagnostic tests for each are unique.

Recent advances in equine genetics have made testing a blood, muscle, or hair follicle sample possible.  As it turns out, three of the four breeds that George represents have been identified as prone to carrying the genetic mutation responsible for PSSM.  Unlike some other recessive genetic diseases, PSSM is inherited as a dominant gene; in other words having just one copy of the mutated gene means the horse has the disease.  Horses lucky enough to inherit 2 copies of the gene can be more severely affected.   The good news is, with a little diligence, these horses can be managed and go on to have a good quality of life and successful athletic careers; both of which I want for George. 

Diagnostic information can be found at the University of Minnesota Neuromuscular Diagnostic lab website:

10 Replies to “Meet “George”: A Polysaccharide Storage Myopathy (PSSM) Horse”

  1. I have a talented 16 yr old Hanovarian with PSSM. I bout bought him in Canada as a 6 yr old dressage horse. No one thought to mention the PSSM, though they had to know. He tied up severely and it took a long time to go through all the vets and specialists. Finally I took him to the U of MN clinic where a vet had made a life study of PSSM. She had co-developed a feed called Releve, which is the only feed he can have. After another couple of years of experimenting I realized that he cannot have ANY supplements. Not even Ferrier’s Formula or Stongid-C. Since then I can keep him competitive and active most of the time, though he does flare with hay change. For that he has to switch over very gradually to the new hay.

    Good luck, and don’t give up on him!

  2. I have a weanling halter baby who was genetically tested and is n/P1 for PSSM. he is showing some signs of epiphysitis (slight swelling in the ankle joints). I was feeding strategy, grass hay, and some alfalfa. My vet would like me to decrease the protein–so, I have eliminated the alfalfa but I am not sure what feed would be appropriate for him. I need something with less protein but still good for a growing weanling–but not too much starch and sugar. Suggestions please???

    1. Hi Vicki,
      Thanks for the question. For growing horses that may be experiencing developmental challenges such as contracted tendons, physitis, etc. often veterinarians will advise to cut way back on dietary protein. Considering that protein content in tendons and ligaments is greater than 90% , and protein content in skeletal muscle is 73% protein, horse owners may want to think twice about doing this. Growing horses require adequate amounts of essential amino acids for proper growth, and joint development. Physitis is actually a sign of amino acid deficiency.

      Although I don’t know your horse’s daily intake of Strategy and hay or have the nutrient analysis, the non-structural carbohydrate content of your hay and grain combination may be exceeding the 15% calorie maximum from NSC for horses with PSSM. To complicate matters, the other nutrients needed for proper development may be slightly deficient in the total diet. An energy (calorie) surplus with a nutrient (amino acid, fatty acid, mineral, vitamin) deficiency may be contributing to the physitis. Horses with a combination of metabolic challenges, such as your weanling, would benefit from limiting calories from sugar and starch (non-structural carbohydrates) while ensuring that other essential nutrients (amino acids or quality protein, fatty acids, minerals, vitamins) are provided in the diet to support the high demands of growth and development.

      My first recommendation would be to work with a nutritionist have your hay tested to understand the nutrient content, specifically the NSC content. It is recommended that the hay not exceed 12% NSC, and that the total diet (hay + grain & supplements) not exceed 15% of calories coming from NSC, and at least 15% of the calories coming from fat. Nutrena and Progressive Nutrition will happily provide these services and consultation for you and your veterinarian. Nutrena and Progressive have several products balanced for growth and in the format of low NSC low calorie ration balancers that will provide concentrated nutrients, without the added calories coming from NSC that will be appropriate for managing the PSSM as well as helping to ameliorate the physitis and support optimal joint development. Understanding your forage/hay nutrient content will determine which concentrate will be best for your weanling. Please let us know if you and/or your veterinarian have any additional questions or concerns.
      Thank you ~ Emily L.

  3. I have had good progress feeding and keeping a PSSM horse in work and meeting his nutritional needs by supplementing using a product by Pro Manna called Cool Calories. It is a powder and is 99.9% fat.

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