Striving to provide the Gypsy and Drum Horse enthusiasts education pertaining to the health and ownership of the Gypsy Cob and the Drum Horse,
as well as, general horse industry standards, including vocabulary, genetic testing and care.
NEW: Equine Polysaccharide Storage Myopathy (EPSM or PSSM) has, recently, come under close scrutiny in the Gypsy breed and among breeders and enthusiasts, alike. With the advent of a relatively inexpensive and noninvasive test, it’s possible to determine if your horse is positive for the genetic mutation responsible for the condition, even if they are showing no symptoms. But, first, what is EPSM / PSSM, it is a genetic muscle disease found in many breeds of horses and is a mutation thought to be present for hundreds of years. The mutation causes an inability to properly metabolize carbohydrates, causing glycogen to build up in muscle tissue. There are 2 forms of PSSM, type 1 and type 2. They are similar in symptoms but only type 1 has the GYS1 gene mutation, making it a known genetic and heritable disease. For the purpose of this article, we are examining only type 1 (PSSM1).
On my path to better understanding this mutation and the affect, within our breed, I spoke in length to several vets, with varied horse backgrounds and interests. The key points, in each discussion, included;
In Quarter Horses and their related breeds, this condition is referred to as PSSM but in draft breeds, it is more commonly referred to as EPSM (Equine Polysaccharide Storage Myopathy), identified and so named by Dr. Beth Valentine, who is a leading authority on muscle diseases (in many species of animals), author of “Draft Horses: An Owner’s Manual” and breeder of draft cross sport horses. Dr. Valentine was one of the authorities I contacted, while researching the mutation. In our correspondence, Dr. Valentine speculated that the affected population, among all draft horse breeds, combined, may be as high as 66%. Dr. Valentine also noted that we may have, inadvertently, selectively bred the types most likely to carry the mutation. These would be horses who are heavily muscled, easy keepers with placid dispositions.
Unlike a recessive genetic mutation, EPSM/PSSM is dominant. Meaning there are no EPSM/PSSM carriers, they are either affected (positive) or unaffected (negative). Affected horses can be either heterozygous positive (1 copy of the mutation) or homozygous positive (2 copies of the mutation). The difference between a recessive genetic characteristic and a dominant genetic characteristic is; a recessive gene being where a horse with 1 copy of the gene is considered a carrier but, otherwise, unaffected. However, 2 copies of that gene would cause the horse to be affected, rather than simply being a carrier. And a dominant gene being a horse with 1 copy of the gene is considered affected. A horse being EPSM / PSSM, affected (positive) does not, necessarily, mean symptomatic, but rather there is a physiological response to the mutation. An EPSM / PSSM positive horse may be completely non-symptomatic until such time that bio or environmental conditions force the expression of symptoms. This can happen at any age, though, if it is going to occur, usually happens before the age of 5 – 7 years, or it may never happen. But, it should be noted that a significant change in feeding or exercise regimen could trigger a response, even in older horses. Among those who do show symptoms, sustainable management can frequently be achieved through a feeding regimen developed by Dr. Valentine and referred to as the EPSM Diet. This consists of removing as much sugar and starch from the horse’s diet, as possible, and replacing it with a high fat diet. In its simplest form, the EPSM diet would replace grain based feeds with alfalfa pellets and vegetable oil, or other high fat supplement. The key is to feed enough fat, as it replaces the now restricted sugars as the means of producing energy. Exercise is also an important factor and Dr. Valentine recommends no more than 12 hours of stall time, per day, at the maximum. Dr. Valentine’s EPSM diet can be found at the end of this article.
Symptoms of EPSM/PSSM may include a diminished top line, with little muscle, an awkward gait, shifting lameness or stringhalt, dark colored urine, irritability, stiffness, reluctance to move out, bucking and, in the worst cases, tying up or recumbency. (It’s important to note, there are 2 types of tying up, sporadic and chronic. Within the chronic, there are 2 subsets, 1 of which is EPSM / PSSM.) Symptoms may range from extremely mild to life threatening.
If you have a horse that is on a quality feeding program and being exercised regularly but cannot seem to build muscle or is losing muscle, EPSM / PSSM is highly likely. Horses with a large front end that tends to taper towards the rear end, or who always look very “tucked up” are also likely candidates. Any prolonged, mystery lameness or odd movement, especially in the hind legs, is highly suspicious. Drawing hind legs up and holding them or drawing them up and stamping down, often switching legs, is a symptom. (With the feathered breeds, stamping the hind legs is usually seen when the legs itch. If you see this and think it might be EPSM / PSSM, rather than itchy legs, watch the expression on the face and take note if the horse also rubs his legs on any available surface, which would indicate itch, rather than EPSM / PSSM.) Anytime you see dark urine or a horse that stretches out, as if to urinate, but does not, recognize it as a symptom. Has your horse suddenly developed an attitude or a poor work ethic? Or does he seem to be struggling with movements that he once performed well? These could be EPSM / PSSM symptoms. Horses who are homozygous positive are more likely to be symptomatic and symptoms are likely to be more severe than horses who are heterozygous positive.
It’s interesting to note that many of the clinical signs differ between Draft / Draft related breeds and those of Quarter Horse / Quarter Horse related breeds. Draft types are more likely to exhibit muscle soreness, atrophy and weakness. And, it seemed, Quarter Horses, and their related breeds, manifested more severe symptoms. This is explained by looking at another muscle disease. Quarter Horses and Quarter Horse related breeds have been found to have a secondary gene mutation, the RYR1 mutation responsible for Malignant Hyperthermia (MH). The presence of the MH mutation exacerbates the affects of PSSM.
Due to the difference in clinical signs and the presence of the MH mutation in horses with Quarter Horse blood, it might be feasible to stop the use of the term PSSM, with Gypsy and Drum Horses, and adopt the term EPSM, as the other draft horse breeds have done.
Steps to take if your horse appears symptomatic or tests positive for EPSM / PSSM; Even if your horse is non-symptomatic, but tests positive, the very first things to do are, review and, if necessary, replace your feeding program with a low carbohydrate / high fat approach and also GET YOUR HORSE OUT OF THE STALL. Positive horses need a minimum of 12 hours per day of turn out time and more is better. But be careful in spring and fall, when grasses can have a higher sugar content. You may have to resort to a dry lot when grass is particularly lush or after a heavy frost. If your only option is a small lot, make sure to set it up so your horse has to walk the farthest distance possible to go from his feed, to his hay, to his water. And include 15 minutes/day of exercise; riding, driving or longing.
Even horses who show the worst symptoms can often return to a full and normal life, once the feeding and exercise programs are put into place. It may take several months before he is completely back to normal but with a bit of patience and diligence, a full recovery is possible. However, management is a lifetime commitment.
When speaking to the veterinarians about breeding EPSM positive horses, there was an across the board acceptance of breeding these horses, albeit carefully. Test your breeding animals so you can make informed decisions. Never breed 2 affected horses, together. Do not breed homozygous positive horses. And Dr. Valentine noted she wouldn’t necessarily breed away from symptomatic horses or bloodlines, as long as they responded well to the EPSM diet. But any horse, or bloodline, that did not respond well to the EPSM diet should never be bred.
Many breeders have been proactive in testing their stallions but, while researching EPSM / PSSM, it came as a surprise to learn mares are more often affected than stallions/geldings.
To review the EPSM diet, click here
Resources and references include: Dr. Beth Valentine, renowned author of Draft Horses: An Owner’s Manual, Dr. Frank Stokes, Dr. Chris Ernst, Purdue University, University of MN, Animal Genetics, Rural Heritage