Muscle Maladies

Learn about PSSM and RER, two genetic muscle diseases behind the condition known as ‘tying up.’ We’ll tell you what they are, how they happen, and what you can do to minimize your horse’s risks.
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Has your horse ever “tied up”—not with a rope attached to his halter, but with his large muscles so painfully locked that he stops dead in his tracks, sweats profusely, and refuses to move? If so, you know just how frightening an episode can be. Even the veterinary treatment can be scary to witness, as it often involves insertion of a long tube through the horse’s nostril, down to his stomach, in order to pump critical fluids directly into his system.

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The tying-up phenomenon isn’t news. Horsemen have been plagued by it since horse-and-buggy days. What is news is that researchers have found a genetic component associated with tying up, and continue to work hard to learn more, with new information becoming available regularly.

In this article, I’m going to tell you what they’ve learned. I’ll explain about the different underlying muscle disorders that can cause a horse to have repeated tie-up episodes, including polysaccharide storage myopathy (PSSM) and recurrent exertional rhabdomyolysis (RER). I’ll tell you what they are, how to tell if your horse is affected, and what you can do about it if he is. With this information in hand, you can minimize the chances that you’ll ever see another tie-up episode—and if you do, you’ll know how to protect your horse against potentially damaging long-term effects.

MALADY: PSSM
What it is:
Polysaccharide storage myopathy, a heritable disease, is a result of abnormal storage of polysaccharide (sugars) within the muscles. It’s been suggested that there may be two types of this disease.

The first, most stringent diagnosis can be made when there’s an increase in an abnormal form of polysaccharide that’s resistant to breakdown by the enzyme called amylase (amylase-resistant polysaccharide), in addition to excessive amounts of amylase-sensitive glycogen—a normal sugar that can be found in muscles of normal horses. The muscles of horses with PSSM are more sensitive to insulin—the hormone released to help with sugar metabolism—meaning more sugar gets transported from the bloodstream into the muscles after a high-carbohydrate meal, and more will be stored as glycogen.

In an exciting new breakthrough, a specific gene that affects the enzyme responsible for glycogen synthesis has been identified in horses diagnosed with Type 1 PSSM. A genetic test for the gene’s presence is available.

A second type of PSSM has been described only where there’s excessive accumulation of amylase-sensitive glycogen, without the abnormal amylase-resistant polysaccharide playing a role. Type 2 PSSM is also believed to be inherited, although a specific gene has yet to be identified.

Who’s at risk? Type 1 PSSM is inherited as a dominant gene. This means that any horse carrying the gene is likely to show clinical signs. Type 1 PSSM is seen most commonly in Quarter Horses, and breeds that employ Quarter Horses as outcrosses, as well as draft horses and warmbloods. If your horse is one of these breeds and has experienced an episode of tying-up, he could be at risk. Other signs of the disease include general muscle stiffness and soreness, difficulty backing up, and visible muscle atrophy. Type 2 PSSM is also most common in the breeds named above, although it can be seen in a wide variety of other breeds.

How is it diagnosed? With the identification of its causative gene, Type 1 PSSM is easy to diagnose with a genetic test. A blood or hair sample submitted to the testing laboratory will confirm a diagnosis in 80 percent of affected horses. If the genetic test comes up negative, a muscle biopsy is the next step to confirming a diagnosis, and can be useful for determining whether your horse has Type 2 PSSM or other muscle abnormalities.

Although muscle biopsy may sound scary, it’s really a fairly simple procedure. With your horse sedated, your vet will make an incision just next to your horse’s tail. He or she needs to harvest a sample of muscle that’s about the size of your little finger, which requires an incision several inches in length. 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 treated with special stains that look for abnormal storage of glycogen as well as amylase-resistant polysaccharide within the cells.

For genetic testing: For detailed information on genetic testing, or to submit samples, contact the University of Minnesota Equine Center, Veterinary Diagnostic Laboratory, (800) 605-8787.

MALADY: RER
What it is:
If your horse has tied up several times but doesn’t fit the picture of PSSM, it’s possible he’s affected with a different underlying muscle disorder known as recurrent exertional rhabdomyolysis (RER). This muscle disease is due to an abnormality in calcium regulation within your horse’s cells. Unlike PSSM, it has nothing to do with abnormal polysaccharide storage. Typically, horses with RER will tie up when they become overly excited.

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Who’s at risk? RER is also believed to be inherited as a dominant trait in Thoroughbreds, Standardbreds, and Arabians, although as of yet, no gene has been identified. If inheritance is indeed through a dominant gene, then crossbred offspring of the affected breeds (Half Arabians, for instance) can be expected to be affected as well. Horses with a more excitable temperament appear to be more at risk, and fillies are more commonly affected than colts or geldings. Younger horses with the condition are more likely to tie up than their more mature, possibly calmer, barnmates. If you have a high-strung 2-year-old filly of an above-named breed, with history of tying up, it might be wise to consider RER.

How is it diagnosed? Although researchers are working hard, no genetic test is yet available. Muscle biopsies taken from horses with RER show a subtle abnormality where the nuclei of the muscle cells are more centrally located than in normal horses.
Research request: If you have a family of related horses with RER, Dr. Molly McCue at the University of Minnesota would be interested in hearing from you in her ongoing efforts to identify the genetic link. Contact her at mccu0173@umn.edu.

MANAGEMENT TIPS
Whether your horse is diagnosed with PSSM, RER, or has simply tied up a time or two, there are things you can do to help prevent future tie-up episodes and keep him performing at his best.

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Diet: By eliminating carbohydrates and adding fat, you’ll limit the amount of sugar available to be converted into glycogen in the muscles. A low-starch/high-fat diet is critical for horses diagnosed with PSSM, and can be beneficial for horses with RER as well. Avoid high-starch feeds including corn, wheat, oats, barley, and molasses, and feed 15 to 20 percent of the daily calories as fat.

The most straightforward way to accomplish this goal is to feed a low-starch concentrate ration, with 2 to 3 cups of corn oil added daily. Unfortunately, that’s not always as easy as it seems. 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 feedroom maintenance. For a list of optional sources of fat to add to your horse’s diet, refer to the sidebar at left.

Exercise: Exercise suppresses uptake of sugars and enhances sugar metabolism, which is beneficial for any horse that may tie up. As an added benefit, PSSM horses are able to use excess muscle glycogen as fuel—meaning increased exercise will result in decreased accumulation. In general, regular daily exercise without a day off is recommended for any horse diagnosed with a muscle disease. Full-time pasture turn out is also beneficial. At minimum, an affected horse’s stall should have an attached paddock that allows him to move freely throughout the day. Studies have shown that 50 percent of horses diagnosed with PSSM will improve with diet changes alone, while 90 percent show improvement with diet changes and exercise.

Supplements: Blood levels of selenium and vitamin E are also important in horses that tie up. Simple blood tests can be run to check for vitamin E and selenium levels, and both can be supplemented easily if levels are low. Because of the narrow toxicity range of selenium, 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.

Medications: Other than treatment necessary during an actual tie-up episode, there are no medications recommended for routine management of PSSM. However, if your horse fits in the RER category of muscle disease, there are several things that can be helpful. Fillies may improve when maintained on progesterone, and light sedation administered prior to an exercise session may reduce the chances for a tie-up episode. In addition, dantrolene, a medication that helps regulate calcium within the cells, can be administered prior to exercise to reduce the risk of a tie-up episode.

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