Part I: Misdiagnosed & Misunderstood – PSSM in Horses

I've heard it all ... "You've lost your place in the herd, she doesn't respect you. She looks fine. You need to ride her more. She's smart, but she's lazy. It's her hocks. It's her stifle. If I were you, I'd sell her and get a horse you can ride."

Story originally posted by: by Eleanor D. Van

Your Horse May Be Willing But Unable

I heard all the above and much more over the years of trying to find a diagnosis for my 1999 Quarter Horse mare, purchased in 2001. Doc Bar, Three Bars, Poco Bueno, and more great horses all lined up in her pedigree, and all the potential in the world, but I could barely get one good day of riding out of her. It took me ten years, but I recently figured out that she has been afflicted all this time with something called Polysaccharide Storage Myopathy or PSSM (referred to as EPSM in draft horse breeds).

I and my horse are not alone: the horse world is slowly learning that this is a fairly common disease in certain breeds that very often goes undiagnosed.

Unbeknownst to me, for the ten years I was searching for a name to attach to whatever my horse had, there was a research veterinarian, Dr. Beth Valentine, DVM, PhD, a veterinary pathologist at Oregon State University a few hours by car south of me who was working on EPSM. The multiple and various equine veterinarians and practitioners with whom I had consulted over the years had little understanding of this disease and thus never mentioned it as a possibility in regards to my horse. Granted, in my particular horse, there were usually no dramatic symptoms and no emergency tyeing up episodes, coupled with chronic bouts of diarrhea and on top of that, thin soled hooves.

Sage was a myriad of challenges and symptoms, some seemingly related and others not, but they all added up to one unhappy horse the majority of the time. A name for her disease as well as cure remained elusive for years. I couldn’t give up, though, as I was becoming more and more attached to her as we went along. Unfortunately, it would be a full decade before I would get in touch with Dr. Valentine of OSU.

Our lucky break came in Fall of 2010, after a friend’s horse was diagnosed with PSSM following a severe episode of tyeing up. My friend and her vet corresponded with Dr. Valentine. I reviewed a lot of information on the disease for my friend, and the two of us started talking about the symptoms looking a lot like the ones my horse Sage often exhibited. I was cautiously optimistic about trying the “cure” of high fat diet but quite frankly was hesitant to try one more thing that did not work. My current vets still were not on board with me, but then my horse took a turn for the worse over the winter. I spent hundreds of dollars and got no answers, so I contacted Dr. Valentine myself and decided to try the diet and exercise plan with her blessing.

There was almost immediate improvement, and three months into the diet my horse has lost almost all of the symptoms that she had for years.

This disease most likely covers a broad range of symptoms and severity. This means that there are probably many more horses out there like my own that are living undiagnosed or misdiagnosed — and thus, misunderstood.

When Sugar Sours

The primary PSSM/EPSM symptoms include tight muscles, sensitive skin, tight abdomen, back soreness, strange hind end gait, and tyeing up. In a nutshell, it is caused by an “abnormal accumulation of the normal form of sugar stored in muscle (glycogen) as well as an abnormal form of sugar (polyssacharide) in muscle tissue … due to an unregulated uptake of sugar (glucose) into their muscles and the synthesis of its storage form in the muscle called glycogen” (University of Minnesota Equine Center). Reducing insulin, and sugar in the diet, is an important part of the treatment plan.

According to Dr. Stephanie Valberg, DVM, PhD, of the University of Minnesota Equine Center, PSSM occurs in at least two forms. Type 1 PSSM is genetic in origin and is due to a mutation present in horses for over 1000 years (the same disorder described as “Monday Morning Disease” or azoturia in draft horses in the early 20th century). This mutation is present in over 20 breeds including: Quarter Horses, Paints, Appaloosas, numerous Continental European Draft breeds, some Warmblood breeds, Haflingers, Morgans, Mustangs, Rocky Mountain Horses and Tennessee Walking Horses. It can also rear its ugly and debilitating head in mixed breed horses. Type 2 PSSM represents horses diagnosed with PSSM by muscle biopsy that do not have the above genetic mutation. Type 2 PSSM is found primarily in Warmblood breeds such as Dutch Warmbloods, Hannoverian, Westfalian, Canadian Warmblood, Irish Sport Horse, Gerdlander, Hussien, and Rheinlander. It is also found in about 28% of Quarter Horses with PSSM as well as other light breeds of horses.

Dr. Valentine writes that EPSM “has been confirmed or suspected in virtually every draft horse breed, including Belgian, Percheron, Clydesdale, Shire, Haflinger, Norwegian Fjord, Irish Draught, Friesian, Gypsy Vanner, draft cross, and a draft mule. This newly-recognized disease, under research at the College of Veterinary Medicine at Oregon State University, has likely been around for hundreds of years.” (Rural Heritage site)

It is important to note that whether a horse has Type I or Type 2 PSSM, or EPSM, the horse will have a positive response to the same exercise and dietary recommendations.

Read Part II of this article here.


Eleanor D. Van Natta is a publicist and writer who lives outside of Portland, OR with her family. She has a degree in Zoology from the University of CA, Davis and is a lifelong lover of animals, especially of the equine kind. You can read more about her own horse’s struggle with PSSM on her website