PSSM – Polysaccharide Storage Myopathy

Polysaccharide Storage Myopathy (PSSM) is also called Equine Polysaccharide Storage Myopathy (EPSM). It is an inherited condition that occurs most commonly in Quarter Horses, Draft Horses, and Warmbloods. PSSM is not just limited to the breeds listed above, it can also show up in other breeds. The muscles of a horse with PSSM are unable to properly store glucose (sugar), so it is unavailable when needed for energy.

What are the signs?

There are two forms of PSSM. Horses with both forms of PSSM have signs typically associated with tying-up such as muscle stiffness, sweating, and reluctance to move. The signs are most often seen in horses when they are put into initial training or after a lay-up period. A correlation to when they receive little active turn-out. The episodes usually begin after very light exercise such as 10-20 minutes of walking and trotting. However, horses affected with PSSM can exhibit symptoms even without exercise.

PSSM Episodes

During an episode, horses seem lazy, have a shifting lameness, tense up their abdomen, and develop tremors in their flank area. When horses stop moving they may stretch out as if to urinate. They are painful, stiff, sweat profusely, and have firm hard muscles, particularly over their hindquarters. Some horses will try pawing and rolling immediately after exercise. Most horses with PSSM have a history of numerous episodes of muscle stiffness at the commencement of training. Mildly affected horses, however, may have only one or two episodes/year. Rarely, episodes of muscle pain and stiffness can be quite severe. This can result in the horses being unable to stand and being uncomfortable even when lying down. The urine in such horses can be coffee-colored. This is due to muscle proteins being released into the bloodstream and passed into the urine. When this happens it is a very serious situation, as it can damage the horse’s kidneys if they become dehydrated.

PSSM in Foals, Weanlings and Yearlings

Very young foals with PSSM occasionally show signs of severe muscle pain and weakness. This occurs more often if they have a concurrent infection such as pneumonia or diarrhea. Some weanlings and yearlings, particularly those with type 2 PSSM can develop muscle stiffness with daily activities and have extreme difficulty rising.
Indoor turnout for PSSM horses

What causes PSSM in horses?

The abnormal accumulation of the normal form of sugar stored in muscle (glycogen), as well as an abnormal form of sugar (polysaccharide) in muscle tissue, characterizes Polysaccharide storage myopathy (PSSM). Tying-up associates with polysaccharide accumulation in muscle effects in thousands of horses. The two forms of PSSM are both the result of the accumulation of muscle glycogen which is the storage form of glucose in muscles.

Types of PSSM

Type 1 PSSM is a result of a mutation in the GYS1 gene. The mutation causing PSSM is a point mutation on the GYS1 gene which codes for the skeletal muscle form of the glycogen synthase enzyme. The cause of Type 2 PSSM is not yet identified. Both types have an abnormal type of glycogen staining in muscle biopsies, and Type 1 is findable through genetic testing. Horses with Type 2 PSSM lack the mutation that is specific for Type 1 PSSM. At present, there is not a specific genetic test for Type 2 PSSM and we do not have conclusive evidence that it is inherited.

Carbohydrates that are high in starch, such as sweet feed, corn, wheat, oats, barley, and molasses, appear to exacerbate Type 1 and Type 2 PSSM. That is why horses suffering from PSSM should avoid them and intake of extra calories can come from fats. An important part of the management of PSSM horses is daily exercise. This enhances glucose utilization and improves energy metabolism in skeletal muscle. If only the diet changes, it has been found that approximately 50% of horses improve. When altering both diet and exercise, 90% of horses have had no or few episodes of tying-up.

What is safe to feed my horse?

Because of the oxidative stress associated with this disorder, as well as the free radicals generated from the high-fat diet often required by horses with PSSM, experts recommend supplementing with vitamin E for its antioxidant properties. Other antioxidants include vitamin C, Grape Seed Extract and Super Oxide Dismutase. Anecdotal evidence also suggests that the anti-inflammatory/antioxidant Dimethylglycine (DMG) may be beneficial for horses with muscle disorders like PSSM.

Beth Valentine, DVM, proposed the first successful therapy to halt and reverse the progression of PSSM, except in advanced cases. This is a diet change. Due to the fact that PSSM is a genetic condition, there can be no ‘cure’ in the symptomatic horse, but many affected horses can be restored to full health and work status.

The approach considers that the horse gains no nutrition from grains, other than the calories. Since the starches and sugars in grains cause specific digestive problems, the approach is to replace starch and sugar calories with fat-based calories. The result is the ‘added fats’ diet.

In basic outline one divides the daily ration into two feedings:

  • 1.2% to 2.1% body weight in good quality forage (hay)
  • 2 cups vegetable oil (corn, soy, canola) per 1,000 lb body weight (1-liter vegetable oil per 1,000 kg bodyweight)
  • 1-2 IU (International Units) vitamin E per pound (2 to 5 IU/kg)
  • Selenium supplement
  • Plenty of fresh water
  • Exercise to the horse’s comfort level
  • Usual salt and mineral supplements
  • Improvement of EPSM symptoms may take 4–6 months.

The Importance of Fat Calories

Adding the fat calories (vegetable oil) to the diet is as important as removing the starches and sugars from grain sources. Oils that provide balanced omega 3 to omega 6 fatty acids, like ExcelEQ ProElite can be a healthy alternative energy source that can aid in the stabilization of blood sugar. Depending on the severity of the condition, the horse may have comfort restrictions in exercising or working, but some exercise needs to happen to begin rebuilding damaged muscles. As the recovery proceeds, exercise may increase to best occupy the horse’s attention and maintain recovery.

Watch Out For Molasses

You can find molasses in many feeds and grains to control dust and to sweeten (sugar) the feed to entice the appetite. The sugars and starches in molasses, however, are a problem for EPSM-symptomatic horses. The horse’s appetite will often decline when molasses is withdrawn. This will pass as the horse relearns the taste of feed without molasses.

Do You Know Your Feeds Selenium Level?

Some regions are naturally selenium-sufficient, and one needs to take caution with additional supplementation. A local veterinarian can advise about local selenium levels.

Transport of Important Nutrients

Part of the consumption of the hay ration can be beet pulp shreds, alfalfa pellets, or a combination, to provide a carrier to pour the oil over for feeding.

Know Your Numbers

The specified level of vegetable oil, 2 c./1,000 lb bodyweight, targets a dietary level of 24% calories from fat sources. Several commercial feed companies now provide feeds with reduced sugars and starches and added fat sources. These feeds can reduce the amount of additional vegetable oil needed to meet the dietary goal ((2 c./1,000 lb)/day), or in some cases will meet the goal without additional fat calories. At least one company manufactures a dry fat supplement that can substitute liquid vegetable oil.

“I never thought my guy was going to be able to keep a healthy weight again and he was only 15 at the time. After 3 months, he is the healthiest I have ever seen him!” 

How to manage horses in hot weather

How ExcelEQ™ can benefit your PSSM horses

Even though most owners with PSSM horses don’t supplement with grains they generally are still receiving alfalfa hay which is higher in Omega-6 (inflammation causing fatty acid). Even though adding vegetable oil or corn oil to the horse’s diet is a great option we can not forget the fact that corn oil is 54% Omega-6 and most vegetable oils range from 65% to 75% in Omega-6 fatty acids. Again we run into the same problem of simply way too much Omega-6 than what nature intended your horse to have and not near enough Omega-3. Even horses with PSSM will suffer from inflammation in their body. ExcelEQ™ is not only high in Omega-3 with a balance of Omega-6, but it is also naturally high in vitamin E which is also highly recommended for your equine partner suffering from PSSM.

Many of our customers have exciting success stories of Excel helping their PSSM horse. If your horse suffers from PSSM be sure to give us a call to speak with one of our representatives about which product is right for you. As always buy with confidence knowing that if you do not notice a difference in your horse after 30 days of constant use we will be more than happy to offer a full money-back guarantee on your purchase price by simply returning the unused portion of your ExcelEQ™.


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