Polysaccharide Storage Myopathy (PSSM) is a genetic disorder affecting many breeds of horses. In essence, this disease results in abnormal sugar (glycogen) regulation in the horse’s muscle. Horses carrying this genetic mutation may experience mild to severe symptoms resulting in pain, poor performance and overall apathy. Fortunately, for horses carrying this disease, they can be carefully managed with a well-planned diet coupled with a strict exercise program to live a full and happy life.

What is PSSM Type-1?

There are currently two known types of PSSM: Type-1 and Type-2. PSSM Type-1 has been identified as a genetic mutation in a gene responsible for glycogen storage in the muscles of horses, glycogen synthase 1 (GYS1). This is a dominant trait, thus only one parent (sire or dam) needs to be a carrier to pass it on to their offspring.

Glycogen is made up of many branches of glucose (sugar), which is primarily stored in muscle and the liver. In PSSM Type-1 horses, due to the mutation in the GYS1 gene, muscle cells continually produce glycogen. This results in an abnormal accumulation of glycogen in the muscle cells. The origination of PSSM Type-2 has yet to be identified, as these horses do not carry the genetic mutation of GYS1.

How does PSSM affect horses?

Many horses that carry the GYS1 mutation may not even show clinical signs of the disease and are considered asymptotic. However, a shift in diet or training may bring about reluctance to train or exercise. Also, due to muscle soreness, stiffness, and pain, PSSM Type-1 horses will have a general lethargic disposition or reluctance for any movement.

Dr. Stephanie Valberg from Michigan State University was the first to report on PSSM in horses. She estimates in PSSM Type-1 horses, muscle cells can store up to 1.8 to 4x above normal levels of glycogen. With these high levels of glycogen in the muscles, horses that are exercised can experience “tying up,” as well as suffer from more severe signs of PSSM with shaking, shivers, lameness, excessive sweating, and overall lethargy.

Which Horses Are Affected by PSSM Type-1?

The origination of PSSM Type-1 in horses is thought to date back many hundreds of years with the breeding of warhorses. Today, there are approximately 20 horse breeds that can carry the GYS1 gene mutation that causes PSSM Type-1. These are primarily those breeds with an American Quarter Horse (QH) lineage, or draft horses and warmbloods from continental Europe. Horse breeds originating from the United Kingdom, like the Shire or Clydesdale, have lower incidence of PSSM.

Briefly, up to 10% of all Quarter Horses are PSSM Type-1 horses. Within the Quarter Horse breed, the highest incidence of PSSM Type-1 are those bred as halter horses (~28%). The top horse breeds thought to be carriers of the GYS1 genetic mutation for PSSM Type-1 and their percent affected are:

  • Comtois (80%)
  • Trekkpard (74%)
  • Breton (63%)
  • Percheron (62%)
  • Belgian (39%)
  • American Quarter Horse (10%)
  • American Paint Horse (8%)
  • Appaloosa (7%)

While the prevalence has yet to be established, other horse breeds thought to be afflicted with PSSM Type-1 include Morgan, Hanoverian, Irish Sport Horse, Gerdlander, Tennessee Walking Horses, as well as other breeds crossed with Warmbloods.

How Can We Manage PSSM Type-1 In Our Horses?

The positive news about PSSM Type-1 horses is that they can be managed to live long, healthy, and productive lives. Even after bouts of clinical signs of PSSM Type-1, horses with a careful reintroduction of exercise and alterations in diet can resume their regular activities.

With advancements in genetics and genetic testing, horses can now be tested to see if they are carriers of the GYS1 mutation that causes PSSM Type-1. Many services now offer this genetic test for reasonable prices, with tests on average about $40 USD. We highly suggest you consult with your veterinarian if you are considering a genetic test for your horse for PSSM Type-1.

It is critical that the diet of horses with PSSM Type-1 be carefully controlled. Diets higher in non-structural carbohydrates (NSC) can lead to clinical symptoms of PSSM Type-1. Thus, owners of these horses need to fed low NSC diets and the energy sources of the diet should be replaced with fat. To gain a greater understanding of NSC in horse diets, the following articles are helpful:

The other critical management strategy for PSSM Type-1 horses is that exercise must be consistent. Horses that have been given long periods of rest and carry the GYS1 mutation are at the highest risk of experiencing tying up and other symptoms associated with PSSM Type-1. This is also true for horses that are starting their initial training. Thus, any horses that are identified as a PSSM Type-1 must receive 10 minutes of daily exercise, at minimum. Once a horse is in training and the horse is fit, a day of rest with daily turnout of free-choice exercise can be substituted two to three days a week. A combination of diet and exercise has led to about 90% of PSSM Type-1 horses improving and episodes of tying up are quite rare.

Finally, if a PSSM Type-1 horse has suffered a bout of tying up or expresses other clinical symptoms, you should consult with your veterinarian immediately. It is not recommended to completely stop exercise, but rather slowly reintroduce them to work. However, a recovery plan is best discussed with them.

If you have any worries or concerns about what you are feeding your horse or have identified your horse as having PSSM Type-1 and need help, please contact us for a free consultation.   
 
References: 
Presence of the glycogen synthase 1 (GSY1) mutation causing type 1 polysaccharide storage myopathy in continental European draught breeds. Baird et al Vet Rec. 2010 Nov 13;167(20):781-4.

Glycogen Synthase 1 (GYS1) Mutation in Diverse Breeds with Polysaccharide Storage Myopathy.
McCue et al J Vet Intern Med. 2008 Sep-Oct;22(5):1228-33.
 
Exertional rhabdomyolysis and polysaccharide storage myopathy in Quarter Horses. American Assoc. Valberg SJ.  Equine Pract. 1995;228-230.

Chris Mortensen, Ph.D.