This mysterious muscle disorder has been in the news. Learn what it is, what it does, and how it can affect your horse’s performance.

Have you noticed all of the high-fat diets on the feed store shelves these days? And why is it that half the horses in your barn have a jug of corn oil by their stalls? Are you missing something? Could be. The benefits of feeding fat to horses are well established—and range from a beautiful, shiny coat to better endurance because of more efficient energy storage in the muscles. Owners of performance horses have been routinely adding fat to their horse’s rations for many years.

Recently, a specific muscle disorder called polysaccharide storage myopathy (PSM) has been identified that’s brought even more attention to the benefits of feeding fat. This disease, a result of abnormal storage of sugars within the muscles, is becoming more commonly recognized as an underlying cause for a wide range of vague symptoms that can affect your horse’s performance—including stiff, sore muscles, intermittent lameness and repeated episodes of tying up. The good news? It’s easy to diagnose, and usually easy to manage—simply adding fat to the daily ration can result in dramatic improvement in the majority of affected horses.

In this article, I’m going to explain what PSM is all about. I’ll tell you what it is, will describe the wide range of different symptoms it can cause and explain how your vet can easily make a diagnosis. If your horse is diagnosed with PSM, I’ll help you formulate a diet plan to help control the signs. Finally, I’ll introduce you to a couple of PSM horses and tell you their stories of success.

What is PSM?
PSM is a disorder associated with the abnormal storage of sugars in the muscles. A muscle sample from a horse with this condition will have increased glycogen along with an increase in an abnormal form of polysaccharide (a type of sugar) that is resistant to breakdown by the enzyme called amylase. Abnormal muscle fibers are found in horses where these increased sugars are detected.

The disease was first recognized and is most commonly seen in Draft Horses—a 2005 study reported by Dr. Beth Valentine of Oregon State University revealed an astounding incidence of 86% in these breeds. However, we now know that other breeds can also be affected, including, Quarter Horses Warmbloods, Morgans, Arabians, Thoroughbreds, Welsh Ponies—and probably many others. The disease is believed to be genetic, although a precise mechanism for inheritance hasn’t been identified.

What will you see? The typical PSM horse is stiff, sore and resistant. He may seem anxious and spooky and will often appear weak through his hindquarters—with visible muscle atrophy. He can have episodes of hind limb lameness that prove difficult to diagnose. If the disease goes unrecognized more serious scenarios can result, including:

Tying-up: Your horse experiences a sudden episode of muscle cramping that primarily affects the large muscles of his hindquarters. He stops in the middle of a work session and refuses to move. His heart-rate is elevated, he’s breathing hard and sweat covers his body due to the pain he is experiencing. You call your vet who administers a sedative to help relax his muscles, control pain and relieve his anxiety. He also gives your horse fluids either through a nasogastric tube (a tube that passes through his nose into his stomach) or intravenously (directly into his bloodstream) to help flush out the muscle breakdown products that have accumulated in his system. In severe cases large amounts of the protein called myoglobin will be released from dying muscle cells. This substance will turn your horse’s urine coffee-colored. More importantly, it can be damaging to his kidneys, putting him at risk for kidney failure.

A blood sample taken after an episode of tying-up will have increased levels of muscle enzymes, confirming the diagnosis. If your horse ties up repeatedly or shows other typical symptoms, your vet might suggest PSM as an underlying cause.

Stringhalt: Your horse suddenly begins hyper-flexing his hind legs when he moves. In some cases this movement is so dramatic that he kicks his own belly with every step he takes, making it difficult for him to get around at all. A horse with stringhalt is often unable to back up and, in severe cases, will be unable to move because of the uncontrollable action of his hind legs.

Here in the Northwest, stringhalt has also been associated with a plant toxicity—ingestion of the multi-stemmed dandelions that thrive in our pastures is thought to cause the symptoms. This can lead to further complications if it occurs in a horse with PSM. It’s usually recommended that you remove your horse from the pasture if he develops a stringhalt-type gait, to minimize his exposure to the dandelion. However, symptoms in a horse with PSM typically become even more severe if exercise is restricted, meaning his abnormal gait will become even more pronounced when he’s locked up. This, in turn, can lead to a vicious cycle where his gait is so severely affected that he’s not stable enough to move back into a pasture situation.

Collapse: Your horse’s weakness becomes so severe that he collapses and is unable to stand. Although rare, this scenario has been reported most commonly in severely affected draft horses. If a horse with PSM becomes so weak that he’s unable to stand, his chances for recovery are slim.

Making a diagnosis: If your vet suspects PSM, it’s easy to confirm a diagnosis with a simple muscle biopsy. With your horse sedated, your vet will make an incision just next to your horse’s tail. He needs to harvest a sample of muscle that’s about the size of your little finger, which requires an incision several inches in length. However, the scars are barely visible once they’ve healed and complications with the incision site are rare. Most horses can continue their normal activities immediately after the procedure.

The muscle sample will be cut into tiny sections and stained with special stains that look for abnormal storage of glycogen, as well as amylase resistant polysaccharide within the cells. If these abnormal substances are found, a diagnosis of PSM can be confirmed.

Your vet may also suggest testing selenium levels in your horse’s blood, as low selenium levels are often associated with muscle abnormalities. Many areas in the Northwest are selenium deficient—so making sure your horse receives adequate selenium supplementation is critical.

What should you do? Although PSM can’t be treated (the abnormalities within the muscles will remain), it can be managed with a simple change in diet. By eliminating carbohydrates and adding fat, the amount of sugar available to be converted into glycogen in the muscles is limited. The goal is to provide approximately 20 percent of your horse’s daily calories from fat—which can be the most challenging aspect of getting PSM under control. Eliminating all cereal grains (i.e. corn, oats and barley) and feeding corn oil is the simplest way to accomplish this goal. Unfortunately, your horse may turn up his nose in disgust when offered this food selection—and you may find the greasy goo a frustrating challenge when it comes to feed room maintenance. For tips on how to adjust your horses ration to a low starch, high fat formulation refer to the case studies.

If blood selenium levels are low, selenium should also be supplemented. However, because of the narrow toxicity range of this mineral, levels should be rechecked following diet adjustments and reduced if needed. In some horses, selenium levels will increase following diet changes, even without additional supplementation.
Regular exercise is an extremely important part of management for a PSM affected horse. Full-time pasture turn out is ideal. At minimum, his stall should have an attached paddock that allows him to move freely throughout the day. When possible, daily lunging for 15 to 30 minutes is recommended. And if your horse is rideable, he should be worked consistently.

What to expect: It takes a full four months for your horse’s muscle fibers to fat adapt—so don’t be frustrated if you don’t see a difference right away. However, many owners report a significant change in their horse’s behavior and performance within a few weeks of starting the diet. If your horse has severe symptoms, such as stringhalt or weakness to the point of collapse it may take even longer for his improvement to be really noticeable. Sadly, very severe cases may never completely return to normal. It has been reported that some horses will improve initially, then experience a set-back at the two or three month mark. If your horse is diagnosed with PSM and improves when you first begin the diet, don’t stop feeding fat even if he seems to get worse again for a period of time. PSM horses should be maintained on a high fat diet for life.

One thing is certain—PSM is a common condition that can be completely overlooked if you don’t put it on your list of possibilities. If you think your horse is showing signs of this disease, a simple muscle biopsy may be all it takes to put him on the road to recovery.

Feeding Fat

Accomplishing the goal of 20% fat in the horse’s ration isn’t always as easy as it seems. Some horses just won’t eat oil, and finding a low starch/high fat diet isn’t always easy. When formulating a diet, it should contain approximately 1 pounds of daily fat for an average 1,000 pound horse. The following strategies will give you some alternatives for adding fat to your horse’s ration (recommendations based on 1,000 pound body weight);

Strategy 1: Oil

Formula: Two cups of oil is approximately one pound of fat. Feed 2 cups of corn, vegetable or canola oil.

How to feed it: Pour over alfalfa pellets, alfalfa cubes or chopped hay. Begin gradually, with cup of oil a day for a week, increasing by cup per week until the two cup daily goal is met.

Tips: If your horse is a picky eater and won’t eat oil on alfalfa pellets or cubes, try a more palatable senior diet or low starch complete feed such as Purina Strategy or Nutrena Complete. Although these diets do contain small amounts of starch, the carbohydrate content isn’t high enough to cause significant problems. To minimize the mess, buy a large pump for your oil containers. Calculate the number of pumps per cup, and add oil by the pump.

Strategy 2: Rice bran

Formula: Powdered and pelleted forms of this high fat option are available and can be mixed with the same pellet or complete feed choices suggested for oil. Natural Glo and Nutrena Empower are common rice bran products. Some of these feeds contain Vitamin E, selenium and other fatty acids in addition to the rice bran—all substances that can be beneficial for PSM horses.

How to feed it: Rice bran products vary, although they typically contain 20% fat—meaning your horse needs between five pounds per day to meet his requirement. Begin with one pound per day, and add one pound per week until you reach the full amount.

Tips: Because rice bran is low in protein and calcium, you should include alfalfa hay in your horse’s ration if you use this as your fat source. Choose only stabilized rice bran products to avoid chemical preservatives and ensure a longer shelf life. If your horse won’t eat five pounds of rice bran daily, you can feed rice bran in combination with oil or a commercial high fat diet to achieve the desired amount.

Strategy 3: Commercial High Fat diets

Formula: Commercially formulated high fat diets are becoming more widely available, with variable formulas. Examples include Kent Feeds Omegatin (20% fat) and Buckeye Ultimate Finish (25% fat). To calculate how much to feed, simply multiply the pounds of feed you’ll give per day by the percentage of fat. (For example, 5 pounds of Kent Feeds Omegatin would be 5 x 0.2 =1.0 pound of fat, enough to meet your horse’s daily requirement).

How to feed it: As with all feed changes, begin with a approximately the total amount you plan to feed, and increase by increments weekly until you meet the full amount.

Tips: These feeds may be the easiest feeding option, and the most palatable. They’re also most expensive. To cut cost, feed less in combination with oil, rice bran or powdered fat to meet the full fat requirement.

Strategy 4: Powdered Fat

Formula: Powdered fat products such as Cool Calories by Performance Horse Nutrition or Performance Pak 100 by Milk Specialties are simply 100% fat products formulated into powder form for easy feeding and less mess.

How to feed it: Simply feed one pound per day, mixed with a low starch pellet such as those listed for feeding oil. Begin gradually, with pound the first week, increasing by pound at weekly intervals until you are feeding one pound daily.

Tips: Your horse may not be willing to eat a full pound of a powdered fat product every day. Like oil and rice bran, powdered fat can be mixed with a more palatable high fat feed.

CASE STUDIES

Horse #1: Imprint, a 10-year-old Quarter Horse/Warmblood cross mare.

Her Story: Imprint was purchased as a three year-old. During her prepurchase exam, mild atrophy of her left hindquarters was noted by the veterinarian—an issue her owners felt they could live with. She progressed well in her training for several years, although that left hind always seemed a little weak—and Impy presented her owners with some temperament challenges as she was often tense and spooky. Over several years, her weak left hind became a lame hind leg, but a specific diagnosis proved impossible to make—even with extensive diagnostics including nerve blocks, radiographs, ultrasound and nuclear scintigraphy. After several years of frustration, she was retired to pasture.

She seemed happy in the field, until one day when she developed a gait abnormality known as stringhalt, which involves extreme flexion of the hind legs. Each step she took caused her to hit her own belly with her hind feet. Because she was in a pasture with exposure to plants believed to be associated with the development of stringhalt, she was immediately brought into a stall. Instead of the “spontaneous recovery” expected with the plant associated stringhalt, her condition worsened, until she was unable to move out of the stall at all. With each step, her hind legs would flex so violently that her entire hindquarters would rise several feet in the air. Euthanasia seemed imminent.

Her veterinarian had heard reports from Dr. Beth Valentine at Oregon State University of PSM causing this type of extreme gait abnormality. At Dr. Valentine’s recommendation, a biopsy was performed which confirmed the PSM diagnosis. Imprint was started on a high fat diet, and forced to leave her stall each day. First, she was just walked down the barn aisle. Eventually she worked up to a daily lunging session. As her gait started to improve over a period of six or more months, she was eventually put back out into the pasture. Slowly but surely she continued to recover.

The Outcome: 18 months after the diagnosis of PSM was made, Imprint is trotting soundly across the pasture. She occasionally takes a few stringhalt-like steps when she’s spooked, but otherwise appears normal. Her owner is considering putting her back to work under saddle

Horse #2: Chesapeake, a nine year-old warmblood gelding

His Story: Chesapeake was purchased as a two year-old dressage prospect. The vet noted mild muscle atrophy through the left hindquarters during his prepurchase examination, but it did not appear to be associated with any significant gait irregularity. He started training, and progressed well for several years—until he tied up. This initial episode resolved with sedatives and fluid therapy. However, his owner was concerned with taking any possible steps to reduce the chances he’d tie up again. Although PSM hadn’t been well described at that time, there had been some information about the benefits of feeding fat to combat muscle problems. As a result, Chesapeake was started on a high fat diet and did well with his training for another several years.

Then he moved barns. Feeding oil was inconvenient and messy. After all, he’d been doing really well—did he really need that high fat diet? Four months later, he tied up again. This time, PSM had been well described and a muscle biopsy was recommended. You guessed it—his muscles showed abnormal storage of glycogen and polysaccharide diagnostic for the disease.

The Outcome: Chesapeake was started back on a high fat diet and hasn’t looked back. He’s moving better than ever and hasn’t tied up again.