The latest research on muscle disorders in horses

Researchers have made great strides in understanding the various forms of tying up and how best to help susceptible horses.
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A classic case of tying up is pretty dramatic. The affected horse will hardly move as the large muscles of his hindquarters cramp into a solid, rock-hard mass. He is in obvious pain, shaking and sweating profusely. You’ll know something is wrong right away and have clear indications that the problem involves his muscles.

More often, however, equine muscle disorders produce vague and shifting signs that are easy to overlook or misinterpret. A horse may simply seem slightly stiff after a day off, for example, or unaccountably lose suppleness. Or he may suddenly be resistant to work. As a result, many horses with chronic muscle disease go through life without accurate diagnosis or treatment.

One person hoping to change that is Stephanie Valberg, DVM, PhD. Director of the Equine Neuromuscular Diagnostic Laboratory at Michigan State University, Valberg has spent her career identifying the various muscle disorders that affect horses and helping owners and veterinarians recognize and manage them.

That’s no small task. Even equine muscle problems that arise from different causes often look similar and can be difficult to categorize.

“‘Tying up’ is an umbrella term now, just like the word ‘colic’ is a generalized description of a variety of conditions,” explains Valberg. “Tying up can be a one-off event caused by a confluence of factors plus bad luck. When a horse repeatedly shows signs of tying up, however, that is classified as chronic exertional rhabdomyolysis (ER) and it’s much more likely there’s an ongoing physiological, and even genetic, cause.”

Working with a team of talented scientists, Valberg has developed better and more precise methods of diagnosis for equine myopathies. Her work has also led to the development of management protocols that can keep affected horses healthy and active, even at the highest levels of competition.

The breakthrough: PSSM type 1

One of Valberg’s first milestones came in 1992 when she identified polysaccharide storage myopathy (PSSM), a condition characterized by an inability of a horse’s body to properly store and utilize glycogen for energy during periods of exercise. Her initial work focused on abnormal findings in muscle biopsies taken from a subset of horses prone to chronic ER.

For your bookshelf: Physical Therapy for Horses: A Visual Course in Massage, Stretching, Rehabilitation, Anatomy and Biomechanics

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Originally, PSSM was diagnosed only through muscle biopsy. Pathologists would stain the samples to identify the type and levels of accumulated glycogen. “In normal horses, sugar is stored in skeletal muscle in the form of a highly branched polymer called glycogen, which is sensitive to digestion by the salivary and pancreatic enzyme amylase,” says Valberg. Normal horses can readily break down glycogen to fuel muscles as it is needed, but Valberg recognized that horses with PSSM store sugar as an abnormal amylase-resistant polysaccharide that cannot be easily utilized. By 2008, Valberg and her team discovered there was a genetic mutation in a gene known as GYS1 in horses with amylase-resistant polysaccharide. Now a genetic test can identify a horse with this form of PSSM before he shows clinical signs. Valberg called this form PSSM type 1 (PSSM1).

While the exact physiological mechanism at work in PSSM1 is complex, the upshot is that the accumulation of excess and/or abnormal glycogen disturbs the normal patterns of energy metabolism during exercise, leaving the horse with an energy deficit in the muscle cells that leads to mild to moderate cramping, resistance to work and sometimes full-blown tying up. These signs are most often seen in horses when they return to work after a lay-up period, about 10 to 20 minutes into a ride.

Signs of PSSM are often seen after a horse returns to work following a lay up period.

Signs of PSSM in horses are most often seen in horses when they return to work after a lay-up period, 

“This genetic defect with PSSM1 originated thousands of years ago and was incorporated into light horse breeds at the time,” says Valberg. “Our best theory is that when some heavier European draft horses, particularly Belgian drafts, were crossed with lighter breeds to create a horse that could carry men in armor breeders unknowingly included horses with PSSM1. At that time, before the era of improved pastures and concentrates, the ability to store an extra amount of energy in your muscles was a good thing. These horses were working eight-hour days, every day, and would utilize much of that glycogen.” In the modern horse, though, that mutation becomes problematic.

“In the modern breeds, you see PSSM1 in the Quarter Horses, Paints, Appaloosas and, to a lesser extent, the Morgan, Tennessee Walking Horse and Rocky Mountain Spotted Saddle horses,” says Valberg. “You’ll also see it in some horses with draft blood, like the Belgian, Percheron and Haflingers. We see it in warmbloods at a lower frequency and don’t see it at all in Thoroughbreds, Arabians or Standardbreds.”

The effects of PSSM1 vary by breed. “In Quarter Horses, the primary complaint is tying up,” says Valberg. “That’s the dramatic, very apparent cramping of the large muscle groups and muscle cell damage. In some draft horses, PSSM1 can be asymptomatic.”

Same effect, different causes: PSSM type 2 and MFM

While working to develop the genetic test for PSSM, Valberg made a surprising discovery. “We had literally hundreds of biopsy samples from horses with clinical signs of PSSM and abnormal glycogen accumulations in their muscle cells,” she says. “Once we were able to test for the mutation, we went back and tested all those samples. Only 72 percent of Quarter Horses diagnosed with PSSM by muscle biopsy had the genetic mutation. And in warmbloods, we found the mutation in only 8 percent of horses diagnosed with PSSM by muscle biopsy. That told us that all cases of PSSM didn’t have the same underlying cause and it might not even be the same disease process.” Valberg began classifying these horses ---whose biopsies showed abnormal glycogen accumulations but who did not carry the genetic defect---as having PSSM type 2 (PSSM2).

“The primary difference, from a practical standpoint, between PSSM type 1 and type 2 is that one can be tested for genetically and the other cannot,” she says. “You might be able to find genetic variants between the horses, but there isn’t a scientifically verified test for a mutation that is clearly linked to the classic muscle biopsy findings associated with PSSM type 2 horses.”

Valberg estimates that approximately 20 percent of Quarter Horses and 80 percent of warmbloods with PSSM have type 2. That is not to say that 20 percent of all Quarter Horses have type 2 PSSM, she says, rather it means that 20 percent of those horses with muscle biopsies showing abnormal glycogen staining have PSSM2. Unlike type 1, type 2 has been found in other light breeds, such as Arabians, Standardbreds and Thoroughbreds.

PSSM type 2 isn’t fully understood, but a key difference from PSSM type 1 might be the amount of glycogen stored in cells. Horses with type 1 have higher levels of glycogen (stored glucose) in their muscle cells, in addition to abnormal amylase-resistant clumping. There appear to be breed differences with PSSM2: Recent findings show that Quarter Horses with PSSM2 have high muscle glycogen, but warmbloods and Arabians with type 2 do not appear to store excessive amounts of glucose.

“This is where our research is currently focused. The glycogen looks abnormal under the microscope in both types of PSSM, but our breed-specific findings suggest that this means there are different causes in different breeds. We are measuring the amount of glycogen biochemically, and that information may eventually lead us to a better understanding of the difference between forms of PSSM.”

To further explore these differences Valberg developed new stains to evaluate muscle with PSSM2. She found that Arabians and warmbloods previously diagnosed with PSSM2 actually have clumping of glycogen that forms in breaks in the muscle’s contractile proteins. “We have found a protein that is abnormal called desmin. Desmin’s job is to create an orderly alignment of the contractile proteins in the muscle cells,” says Valberg. “With a new stain for desmin, muscle biopsies can show abnormal desmin clumps, and this has led us to believe that a subset of Arabians and warmbloods previously diagnosed with PSSM2 don’t have an underlying glycogen storage disease like PSSM1. Ultimately, we suspect, weakness or degeneration occurs in the contractile proteins, and this could be responsible for the clinical signs of muscle pain and reluctance to want to go forward or collect that we see in certain PSSM2 horses.”

For your bookshelf: Physical Therapy for Horses: A Visual Course in Massage, Stretching, Rehabilitation, Anatomy and Biomechanics 

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Termed myofibrillar myopathy (MFM), this condition is defined by clumping of desmin in specific areas of the muscle cells. (Valberg notes that this condition is different than MFM in people, and relying on human literature for information is unwise.) Evaluating a biopsy to detect MFM can be tricky because the horse’s age and the handling of the sample can produce false positive or negative results. Valberg suspects there is a genetic component to MFM, but no scientifically validated genetic test for the condition is available yet. “We find genetic variants exist between MFM and healthy horses, but there has not been one specific variant that is present in all MFM horses and absent in controls in our research,” Valberg says.

Challenges of diagnosis

Valberg stresses that not every sore, resistant horse has a muscle disorder or myopathy. In fact, most don’t. “There are a million things that can make a horse muscle sore,” she says. “It’s not always, or even usually, muscle disease.”

The diagnostic process begins with an investigation of the more common causes of soreness and reluctance to exercise, Valberg explains. “You have to look at saddle fit, possible training issues, whether or not it’s a mechanical lameness or sacroiliac disease. These are all diagnostics that your veterinarian can do. The problem is this can be expensive, so many people are reluctant to go through that whole process. But it’s the only way to find those much more common causes.”

Your veterinarian will start by taking a blood sample to measure muscle enzymes. “If those are elevated, it’s clear evidence of exertional rhabdomyolysis. Something is going on to damage the muscles, says Valberg. “From there, we subdivide: Is this the first time this has happened? If so, this might be a one-time dietary or exercise issue. We also look at the horse’s diet. Is he getting too much grain, not enough vitamin E or selenium? If we find a problem, we can fix it. Then the problem isn’t going to recur.”

If the horse continues to have episodes of tying up or soreness and reluctance to work, however, then it’s time to consider that there may be something intrinsically wrong with how the muscles function.

“The least invasive testing method is now genetic analysis, and there are two tests that are scientifically validated by peer-reviewed research,” says Valberg. “One is for PSSM type 1, but if you have an Arabian or Standardbred, there’s no point in going that route because they just don’t have it. If you have a warmblood, the likelihood of PSSM type 1 is small, but it’s a place to start and if the genetic test comes back positive, you’ve found your problem without having to take a biopsy.” In Quarter Horses there is another form of exertional rhabdomyolysis, called malignant hyperthermia, that can be diagnosed by a genetic test. Horses with this form of tying up often have an increased body temperature when they have muscle cramping and they can develop severe muscle damage.

Click here to read more about genetic tests in horses. 

If the genetic test comes back negative, Valberg then considers the horse’s clinical signs, breed and even temperament to determine what is the most likely cause of the ongoing problem. “One disorder, that we call recurrent rhabdomyolysis (RER) is found mainly in Thoroughbreds and Standardbreds who are very successful when they aren’t tying up, but also tend to be very reactive and with a nervous temperament. It occurs when they are being held back at a gallop or jigging. If the horse is tying up after a situation like that, he likely has RER.”

For a horse who frequently ties up but doesn’t fit the profile for RER and tests negative for PSSM1, the next step is a muscle biopsy. “When we look at a tissue sample under the microscope, we are looking at the size and shape of the cells, evidence of degeneration and regeneration, the amount of glycogen stored in them as well as clumping of glycogen and desmin,” says Valberg. “From all of that, we will diagnose a subset of these horses with PSSM type 2 or myofibrillar myopathy.”

Valberg stresses that, absent a genetic test, diagnosing PSSM2 remains a bit subjective: “You can’t base the diagnosis on the muscle biopsy alone. The biopsy is not specific enough to use as a tool for prepurchase exams or making breeding decisions. Muscle biopsy is most accurate when it is used in horses that have a high suspicion of disease based on their clinical signs.”

Two-step management

As tricky as myopathy diagnostics can be, the treatments are remarkably straightforward. In fact, for PSSM1 and PSSM2 in Quarter Horses where the problem is too much glycogen, the management recommendations are identical.

The first step is a change in diet. Feedstuffs that are high in sugar and starch, such as corn, wheat, oats, barley and molasses, appear to exacerbate PSSM1 and PSSM2 in Quarter Horses. “Getting these horses onto a low-sugar/low-starch diet is key,” says Valberg. “That might mean restricting grazing if the pastures are high in sugar. We replace the energy of starch and sugar with fat in the diet and this improves the horses’ exercise tolerance.”

A horse who coughs after eating could have a dental problem or structural issue in his esophagus. Or, it could be the result fo dusty feed.

Feeds that are high in sugar and starch can exacerbate PSSM1 and PSSM2

The second management step is faithful daily exercise, which enhances glucose utilization and improves energy metabolism in skeletal muscles. The horse doesn’t need to work hard, Valberg stresses, but he does need some organized exercise every single day. “Turnout in a pasture isn’t enough because some horses just stand around,” she says. “You need to get on these horses and ride them or hand-walk them or longe them. Even if it’s only a bit of trotting for a few minutes. That can make all the difference.”

The combination of diet and exercise works well in Quarter Horses---up to 90 percent have no more episodes of tying up---but Valberg says warmbloods may not respond to the same degree: “The fact that [treatments] don’t work as well in some breeds as others speaks to the different underlying factors that we don’t fully understand. But those recommendations were the best we had at that point for all horses, and if you’ve ruled out everything else, it’s worth trying.”

Dietary changes of low starch and sugar don’t seem to be as crucial in horses with MFM, but Valberg still recommends a diet that is moderate in sugars and starches: “Many feed companies have now developed supplements of high-quality amino acids and we now recommend these supplements for warmbloods and Arabian horses with PSSM2 or MFM.”

Valberg hopes, through research, to determine if this type of diet optimizes turnover of structural proteins in the muscle and builds muscle strength. “We are also working on supplements to enhance energy generation and antioxidant function,” she says.

Meanwhile, exercise recommendations for horses with MFM are a bit more specific than for those with either type of PSSM. Valberg has found that MFM horses seem to respond best to work that initially encourages them to stretch their topline, in a “long and low” frame for about five to 15 minutes each day before being asked to do more collected work. And bringing an MFM horse back from a lay off needs to be done gradually, with a least two weeks of walking, followed by at least two weeks of trotting before the horse canters. Intervals of walk and trot are recommended.

Research into myopathies continues. “With each new thing we learn, we realize how much we still don’t know,” Valberg says, “but we’ve come such a long way already.” Even though there are much more common causes of mild to moderate muscle soreness in horses, being able to accurately identify and successfully manage the relative few who have these conditions can mean the world of difference to those horses and their owners.

Disclosure statement: Stephanie Valberg, DVM, PhD, and her colleagues license the PSSM1 test and receive royalties. This conflict is managed by Michigan State University, East Lansing, Michigan.

CASE STUDY: KEEPING A STAR FOXHUNTER BUSY

Emma James bought Maggie Mae as a 4-year-old in 2006 specifically for foxhunting. The Paint with Quarter Horse racing bloodlines proved to be a star in the hunt field—brave and forward, but well-mannered.

“She was great and easy to manage in the field,” says James. “I had hunted her for three seasons when the horse I was using for three-day eventing passed away.”

Thinking the mare would excel at a new sport, James started preparing Maggie for training-level events. “She was super fit already and only got fitter,” says James. “I had to feed her more and more to keep weight on her. And that’s when she started to have funky episodes.”

These “funky episodes” occurred only during training sessions. “She’d get tight in her back while being ridden and not want to go forward,” says James. “She’d be cantering and then just slow to a walk. I’d put my leg on but she’d just stop. For a while, I thought she was being resistant. It happened most often when we were doing arena work, so I thought maybe she was being barn sour. It was also the first year I’d asked her to do dressage work, so I thought maybe she really didn’t like it. Why would she suddenly develop an attitude after being so great for years? It didn’t make sense.”

When the episodes increased from once every few weeks to two or three times a week, James started looking for a physical cause. “We did lameness exams, checking her feet and back,” she says. “We did find she was sore in her back, but that could have been from the episodes she was having. She was getting massages and adjustments. I was stretching her legs out be- fore I tacked her up. Just when we’d think we had it figured out, she’d have another episode.”

The timing of one of these episodes, however, turned out to be lucky. “We were foxhunting,” says James. “We’d just pulled up at a check and Maggie started to feel tense and tight. She was taking small steps. It wasn’t the same as what I’d felt in the arena. It was worse.” As fate would have it, Stephanie Valberg, DVM, PhD, one of the world’s leading authorities on equine muscle disorders, was nearby, mounted on her own horse.

“[Valberg] immediately said it looked like Maggie was tying up,” says James. “We took Maggie home and called Stephanie the next day, and that’s when things started to come together.”

Maggie was trailered to the University of Minnesota for a full diagnostic workup, including a muscle biopsy. The results arrived a few days later, showing the mare had PSSM type 1. “Stephanie sent us the results, along with a bunch of information about the condition so we could read up on it.”

Under Valberg’s guidance, James took Maggie off concentrated feed, giving the mare only a mineral balancer pellet. She also put her on a dry lot, with no access to pasture. “We were going into fall at that point and the grass was very rich,” she says. “In the summer, the horses spend half the day on dry lot and half on pasture so they don’t overgraze the fields, but that fall the pasture was really thick and the horses hadn’t eaten it down, so they were turned out more. I hadn’t realized that in the fall the grass was still so full of sugar.”

The second part of Maggie’s new regimen was regular exercise. “We had bloodwork done that indicated a high level of muscle damage. We knew Maggie needed exercise, but we proceeded carefully because she was still recovering from the damage she had sustained. We started out hand-walking just three minutes a day. It wasn’t much but it was enough to burn off the excess glycogen.”

The hand-walking routine continued through Christmas and then Maggie was started under saddle. That spring, however, Maggie had a setback.“I was walking her with the hounds, just to get her fit,” says James, “when a few hounds broke off to chase a deer. I took off after them without thinking. She had only been doing walk and trot work until that point, and that one little sprint set her off. She was stiff the next day and didn’t want to go forward, and I knew enough by then to understand what had happened. I lost most of that hunt season because I was so nervous about it happening again.”

James kept Maggie in regular, light work throughout the remainder of the year and gave foxhunting another try the following summer. “It took a while to figure out how to keep weight on her. We ended up putting her on a 25 percent rice bran meal and even then it was a struggle. We couldn’t put her on pasture, or give her beet pulp or sweet feed, but she had to work all the time to keep the glycogen from building up. Hunting was a good balance; she worked hard a few days a week, then the rest of the week she walked with the hounds. That wasn’t making her drop a bunch of weight, but it let her burn off the glycogen.”

The dietary changes combined with regular, moderate exercise seemed to be working. “I hunted her the whole season in the field without a single episode. I think we hit the right balance of diet and exercise.”

Maggie, James learned, needed at least 15 minutes of hand-walking or a short trotting hack every single day, even if she spent time turned out. Once James started college, however, she couldn’t maintain that schedule. “After my sophomore year, we gave Maggie to a friend in Aiken, South Carolina,” she says. “That friend whipped in for Aiken hounds and two other private hunts in the area with Maggie.”

The change of location made a world of difference to the mare. “As soon as she went to Aiken, she stopped having episodes,” says James. “The soil there is very sandy and the grass and hay are a lot less rich. Diet is a huge part of managing her and the forages in South Carolina are just naturally better for her.” Maggie’s talent didn’t go unnoticed in her new home, and she has since been passed on to another avid foxhunter who enjoys her athleticism and sensibility in the field.

James says that dealing with PSSM can be stressful but she wouldn’t consider the condition a deal breaker when evaluating a new horse. “I wouldn’t shy away from buying a great horse just because he had [PSSM],” she says. “It does take a bit of work to manage, but for the right horse, it’s worth it.”

CASE STUDY: MANAGING AN ELITE ATHLETE

Pippy was 8 years old and a rising star in the eventing world when Mary Ann Ghadban purchased the Holsteiner mare and imported her from Germany in 2012. Ghadban had high hopes for her. “We were told that she had tied up before but that it was manageable,” says Ghadban. “We didn’t know anything else about her condition at the time.”

Two years after she came to the United States, Pippy—whose show name is “Under Suspection”—was successfully completing three-star events with a professional rider. She was brave and talented and seemingly unstoppable. Then, she tied up.

“It was at an event after the cross-country phase,” says Ghadban. “It didn’t look like much from the ground, but the rider knew something was wrong and the veterinarian confirmed. We pulled her from that event, obviously, and had bloodwork done that showed muscle damage. We thought it was just ‘normal’ tying up, so we changed her diet and knew we’d have to be better at monitoring her.”

Not long afterward, though, at an event in The Netherlands, Pippy tied up severely after the initial soundness jog, before the actual competition even began. “We immediately took her to the hospital,” says Ghadban. “We knew we were dealing with something serious and wanted to get the best treatment.”

The Dutch veterinarian told Ghabdan that Pippy probably had PSSM, but she’d need an in-depth diagnostic workup to confirm it. The best place to do that, the veterinarian continued, was in Minnesota, where a researcher named Stephanie Valberg, DVM, PhD, ran an equine muscle disorder lab. (Valberg’s lab has since moved to Michigan State University.)

After two weeks of recuperation in Europe, Pippy returned home to Virginia, where her veterinarian worked with Valberg to investigate the source of the mare’s muscle problem. A genetic test for PSSM type 1 came back negative, which wasn’t surprising given Pippy’s warmblood breeding. A muscle biopsy was the next logical step, and the results of that, combined with the mare’s history, led Valberg to make a diagnosis of PSSM type 2.

“Pippy underwent a year of rehab and we changed the way we managed her,” says Ghadban. “We had to make big changes to her diet. She’s on a low-starch diet we devised with Joe Pagan [PhD], an equine nutritionist in Kentucky. She gets a special grain and supplement and she can’t be on any grass at all here at home in Virginia; there’s just too much sugar in it. When we travel down to Florida, she’s fine on pasture. I wish there was a way to grow those grasses up here.” When she can’t graze, Pippy is fed only bagged hay, which has known levels of sugar and starch.

Pippy’s exercise regimen was also overhauled. “She always has to be moving. The very worst thing that she can do is stand still. She has a dry lot attached to her stall that she can walk in and out of. We take her for a walk, either under saddle or in hand, once or twice a day on days when she’s not in training. She simply cannot have a day off.”

While diet and exercise are the cornerstone of Pippy’s management, Ghadban has also discovered other ways to help the mare maintain her muscle health. “We’ve found that it’s important to keep her well hydrated, particularly when she has to travel long distances in the trailer or prior to starting a cross-country course.” Sometimes this means having a veterinarian administer intravenous fluids.

“We also monitor her muscle enzymes before and after every gallop session and before and after every competition. We are constantly monitoring her so we know how she’s doing. I can be a little obsessive about it,” says Ghadban, who adds that if Pippy weren’t an elite level athlete, the PSSM type 2 might not be a significant issue for her. “I’m not sure it would affect her that much, but when you take these horses and ask them to work at the top level of an extreme sport, any physical problem can be significant, and I think it’s your responsibility to do everything necessary to keep them healthy.”

Ghadban’s awareness of PSSM has helped other horses in her care. “I have a Percheron/Thoroughbred cross who is my foxhunter. A few years ago, she started acting up. She was grumpy and just not herself. Now that I’m in tune to these things, I asked my veterinarian to check her muscle enzymes. They were elevated, indicating something physical was going on. We now muzzle her and have her on the same diet as Pippy. She’s been fine since.”

The instinct to rest hardworking horses with this problem can be difficult to overcome, says Ghadban: “I think that’s the biggest problem with these horses. People think, ‘Oh, I hunted for three hours yesterday so I need to give him the day off today.’ That’s wrong. They need to be kept moving, even if it’s just a 20-minute casual stroll around the farm. Then you can turn them out.”

The efforts are paying off, says Ghadban. With all of these changes, Pippy hasn’t tied up again, even while competing at the pinnacle of her sport. “Once we had Dr. Valberg and Dr. Pagan on board and started following their plan, it’s all worked out. We just traveled 14 hours in a trailer each way to a three-star event and she came in second. I do feel a bit guilty that we didn’t know enough before and we didn’t control her situation better. She’s the most wonderful horse on Earth.”

CASE STUDY: FAMILY TIES

Syrocco Rime wasn’t diagnosed with a myofibrillar myopathy (MFM) until long after she retired from elite endurance competition.

“I had bred Rime myself and started riding her when she was 4,” says Meg Sleeper, VMD, a veterinarian who specializes in cardiology. “I remember her tying up at two competitions. The first time was very mild, so mild that none of the race veterinarians knew it was happening and I wasn’t exactly sure myself what was going on. But we pulled blood after the race and found elevated muscle enzymes, so it was clear it was some type of tying up. It happened a second time at the 2005 Pan Am Championships. By then, I knew she was predisposed to tying up so I recognized it and withdrew her right away.”

Still, Rime’s episodes didn’t look like classic cases of tying up, says Sleeper. “The muscles over her croup would get a little tight, but her gait wasn’t really affected,” she says. “If you weren’t sitting on her and didn’t pay close attention, you could miss it entirely.”

Through research and a bit of trial and error, Sleeper learned how to manage Rime’s tying up with a low-starch diet and exercise. The mare was able to compete for several years without any further episodes, but eventually she developed uveitis and was retired due to blindness at age 10. By then, Sleeper was bringing her full sister, Syrocco Reveille, up through the endurance ranks. It soon became clear, however, that Rime’s younger sibling also had a predisposition to tying up.

“Rev never tied up during competitions,” says Sleeper. “But she did once or twice during conditioning. By then I was aware of the problem and knew I had to manage her.”

Although she knew her horses were prone to tying up, Sleeper didn’t have a more precise diagnosis until she met Stephanie Valberg, DVM, PhD, who heads a laboratory dedicated to researching the various types of equine muscle myopathies. Having identified causes of tying up in stock breeds, Thoroughbreds and warmbloods, Valberg had turned her attention to Arabians. “I had heard that Dr. Valberg was looking for Arabians with a history of tying up,” says Sleeper. “She had already approached the endurance riding community and was referred to me by a fellow rider who knew I had been dealing with these issues.”

After several phone calls to discuss Rime and Rev’s history, Valberg traveled to New Jersey and did bloodwork and biopsies on Sleeper’s horses.

Sleeper said she wasn’t surprised when the biopsies from Rime and Rev and one other horse revealed they had PSSM. “After talking with [Valberg], that is what I’d expected.” At that time, Valberg knew Arabians had a variation of PSSM in which the glycogen in muscle cells clumps abnormally, but the condition was still being investigated and didn’t yet have its own name.

The next year, however, Valberg published a paper describing MFM. “For me the only thing that changed is it finally has a name,” says Sleeper. “We figured out the management long ago.”

Exercise is key in keeping MFM horses healthy, says Sleeper. Her horses live outside 24 hours a day and come inside only to eat. Although she’s blind, Rime stays active enough in the herd to stay healthy, but the rest of the horses have some formal exercise every day.

“To manage this, they need to be kept in work every single day,” says Sleeper. “Even when they have a day ‘off’ they usually go on the eurociser [horse walker]. Regular exercise keeps the glycogen moving out of the cells. If the muscles are full of sugar and you ask a lot of the horse, that’s when you can have trouble. You don’t have much room for error. To be successful with horses who have this, you’ve got to stay on top of things and not get lazy or cut corners.”

While the genetics of MFM aren’t understood, Sleeper says she is certain it’s inherited, given the patterns she has seen in her own related horses. “That brings up the question, ‘Should I breed a horse who has this?’ From my background as a veterinary cardiologist, I know that you have to be careful when trying to eradicate inherited diseases because in the process you can breed out good genetic material. I also strongly believe that MFM is found in the highest class of athlete, so do we want to remove them from the gene pool entirely? If the condition wasn’t manageable that would be different, but this is manageable and these horses can still compete successfully and contribute to the breed.”  

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