How to Prevent Polysaccharide Storage Myopathy (PSSM) in Your Horse

My horse was just diagnosed with Equine Polysaccharide Storage Myopathy, he is now on Poulin Grain Carb Safe. Can you explain more about this, and how to keep the total carbohydrate level at 12% or below. Thank you. SK, Vermont

Dear SK,

I’m so glad someone finally asked a question about Polysaccharide Storage Myopathy (PSSM or EPSM, depending on whose research you read). Because my own horse has this, I’ve done quite a bit of research on it, and might be able to shed some light on this relatively new disorder in horses for you.

So that everyone knows what we’re talking about, let’s start from the beginning! PSSM is a form of “tying up,” or, exertional rhabdomyolysis, which means “muscle breakdown with exercise.” Some classic examples of this are the working draft horse that gets Sunday off then develops “Monday morning sickness,” the thoroughbred who “ties up” after a race or three-day event, and the Arabian who develops this painful cramping of muscles during an endurance competition. While PSSM is related to these other forms of “tying up,” it is a unique, inherited condition in quarter horses, draft horses and warmbloods that has to do with how sugar (glucose) is taken up and stored in the muscles for energy.

Here’s the current theory: First, PSSM horses are very efficient at pulling glucose out of the bloodstream and putting it into muscles because of heightened insulin sensitivity. Second, because of a mutated gene, the enzyme that transforms glucose into glycogen (the storage form of glucose) is faulty and instead transforms glucose into a different polysaccharide, one that is abnormal and unusable. Therefore managing a PSSM horse involves 1) limiting the amount of sugars and starches he eats, 2) providing fat for energy instead, and 3) keeping the horse’s muscles moving so abnormal polysaccharides don’t build up.

As you’re finding out, performing these three tasks well is not easy! Current recommendations for the maximum amount of sugars and starches range from 10 to 20% of the total daily calories. To achieve this, you’ll probably need to have your hay analyzed (www.DairyOne.com and www.Equi-Analytical.com are good choices), then stick with that one hay source, if possible. If not, you may want to purchase hay cubes in bags, which have a more predictable composition. You’ll also have to be careful allowing your horse access to pasture, as there are certain times when the sugars and starches in grass are very high. Visit www.safergrass.org to learn when grazing is safe and when it’s not. I’m glad to see you’ve pulled your horse off grain (sweet feed, corn, oats, etc.) and are using a low-starch alternative. That’s important, because these horses still need a complete and balanced diet—not just forage—especially if they are being asked to perform. For people who can’t find a low-starch grain in their area, I recommend feeding a ration/forage balancer or multi-vitamin/mineral supplement.

You don’t mention if you’re providing your horse with extra fat, but this is the next step in managing a PSSM horse. Because you’ve taken away much of the sugar and starch they had been using for energy, you need to replace it with another source of energy: fat. Some commercial feeds have added fat, other companies make high-fat products to go along with their feeds, or you can use vegetable fat in a powder or oil to supply these calories. Note: additional Vitamin E (an anti-oxidant) should be fed to horses receiving high fat diets as the increased aerobic metabolism associated with such diets may result in oxidative stress (free radicals). Some experts recommend feeding up to 1 pound of fat a day, but I’ve found a half pound works just fine in my own horse. I feed him four ounces of Cool Calories AM and PM, with SmartE in the morning and MSM in the evening.

Finally, it’s important to provide lots of exercise to PSSM horses. The worst thing you can do is put them in stall! Twelve hours max is the rule. In addition to as much turnout as possible, these horses do best if worked (lunged, ridden, driven) every day. In fact, some experts recommend two shorter bouts of exercise per day! Take your time warming up and cooling down, and if your horse has an extended layoff for any reason, start back very very slowly with him, adding on just a few minutes of additional exercise a day until he’s back at the former level of work.

For those of you reading this who are concerned your horse might have PSSM, here is a list of the other, more subtle signs PSSM horses can have, besides full-blown episodes of “tying up,” which can be as mild as shortened strides or as severe as an inability to move:

• Gait abnormalities
• Mild colic (pawing, rolling, sweating, not eating)
• Muscle wasting or atrophy
• Decrease in level of performance
• Painful and firm back muscles
• Reluctance to collect and engage the hindquarters
• Poor rounding over fences
• Tucked up abdomen
• Difficulty backing
• Difficulty holding up limbs for the farrier
• Muscle trembling
• Muscle weakness
• A “shivers”-like gait

To learn more, visit the University of Minnesota College of Veterinary Medicine Neuromuscular Diagnostic Laboratory website: www.cvm.umn.edu/umec/lab/home.html.

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