Uncovering a Case of Equine Polysaccharide Storage Myopathy

A kind mare turns into a grouch, sending her owner on a long journey to eliminate a list of causes for her sour behavior.

After I lost a wonderful trail horse to a tragic accident, I was depressed and did not even want to start looking for another one. But my loyal friends, knowing how much I need a horse in my life, dragged me all over Michigan. Yet my response to every horse for sale was always, “No, thank you. Not quite right.” This went on for the entire summer of 2009.

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Finally, while we were riding one day in our favorite local park, my friend and I met a couple with some very nice Quarter Horses. Casually I asked, “Do you know of anyone who might have a nice trail horse like yours for sale?” The man said he had a few, and he lived only 45 minutes away. On a very cold, windy November day, my friend and I made the trip.

I said that I really wanted a quiet horse so my granddaughter could ride too, and the man brought in from the pasture a 5-year-old Quarter Horse mare named Jodie. Her owner mentioned that she had not been ridden for a few months, so I was a bit wary. But he saddled her up, led her out into his 50-acre hay field and just hopped on. She walked right off and then he loped her around the field. Just then, a little old pony who roamed freely on the farm came cantering in and ran in circles around the mare. Jodie was unfazed and just kept loping along as nice as could be.

My friend and I took turns riding the mare, who seemed unconcerned about everything—just what I had been looking for. I purchased her the next day.

A sudden change
Jodie settled into her new home quickly. She was turned out 24-7 in a pasture with a run-in shed, keeping company with two ponies that belonged to my granddaughters. She also made an easy transition to our meal plan—grass hay to supplement the fall pasture, plus a token amount of complete feed.

The weather was beautiful, so I started riding Jodie right away at the state park. From our first outing, she was perfect. She walked out happily and freely and cantered beautifully down the trails. I rode as much as
possible that fall and through the winter and spring.

But the next summer, Jodie’s personality began to change. When I tightened the cinch, she’d pin her ears and try to bite me. I suspected the problem was back pain—she has a broad back and I’d been having a terrible time finding a saddle that fit her well. It seemed I was shipping saddles back and forth from tack shops almost every other week.

Despite my efforts to find a better saddle, Jodie got more and more miserable. In late October, she began stopping at the bottom of hills and would go up only with urging. Then one day she did not want to continue her canter. When I asked her to move on, she actually bucked and pinned her ears—this
from a horse who could have won the Miss Congeniality award just a few months earlier.

I knew this had to be more than just an attitude problem, so I called my veterinarian, Robert Renton, DVM, of Howell Animal Hospital. As he examined Jodie, it took him only a few minutes to discover the
nature of the problem: When he pushed gently on her side, she pinned her ears and tried to kick him. He carefully continued the exam and listened closely as I described the changes in the mare’s behavior.

Jodie was clearly in pain, Renton told me when he’d finished. That conclusion was easy. The difficult part would be figuring out what was causing it. My mind reeled as the veterinarian outlined what seemed like an endless list of potential causes, ranging from poor saddle fit to hock pain, ulcers and ovarian tumors.

A few of the possibilities didn’t seem likely. Gastric ulcers, for example, can cause overall grumpiness, but Jodie’s outdoor, hay-fed, low-stress lifestyle didn’t fit the profile for that condition very well. Renton also discussed a type of muscle disorder, but most horses affected with that have asymmetrical hindquarters and are on high-grain diets, neither of which was true for Jodie.

Renton suggested that I continue looking for a better-fitting saddle while we worked down the list of more probable causes of Jodie’s pain, beginning with the most likely one—her hocks. Renton took a series of radiographs of both Jodie’s hocks. Everything looked healthy and normal, but just to be sure, we started her on a course of phenylbutazone, which would reduce any pain and inflammation, to see if she would improve over the next few weeks. She didn’t.

To investigate the next possibility, Renton returned to do a full reproductive exam, including an ultrasound of Jodie’s ovaries and blood work to look for abnormal hormone levels. A growth on her ovary would be painful, and it would also secrete hormones that could explain her aggression. Once again, though, everything was normal.

And Jodie’s pain continued unabated. Things seemed to be at their worst around Christmas of that year. We had unusually nice weather just before the holiday and I slipped onto Jodie bareback to take a walk around the pasture. But the moment I sat on her, she pinned her ears, braced her back and sent me a very clear message that she hurt. I knew then it wasn’t the saddle, and I was starting to worry that my sweet mare was forever changed.

I wasn’t ready to give up, however. The next rule-out Renton suggested was equine protozoal myeloencephalitis (EPM), a progressive neurological disease caused by a parasitic infection of the central nervous system. Jodie didn’t seem particularly uncoordinated, as horses with EPM typically are, but she had started to drag her hind toes as she walked, which can be an early sign of the disease. A blood test for antibodies to the protozoa came back positive, which meant she had been exposed to the organism, as about half of U.S. horses are, but that doesn’t necessarily mean that they have the disease. Only about 1 percent of the horses who encounter the protozoa, which are consumed with contaminated feed or water, ever develop signs of illness.

To explore this possibility, we could have had a spinal tap performed. This procedure involves drawing a sample of cerebrospinal fluid from the spinal canal to look for the presence of the protozoa, but the test is invasive and can still yield false positives. What’s more, it would require taking her to a university hospital. Or, Renton suggested, we could just start Jodie on the medications for EPM and look for improvement. I opted to skip the spinal tap and begin the treatment. I had high hopes it would help. But it didn’t.

Gastric ulcers were next on our list. These painful lesions in the lining of the stomach can cause a number of subtle signs, including vague pain and touchiness, especially when being saddled. They are more likely to occur in horses kept in stalls, fed a high-grain diet and subjected to a stressful lifestyle, including travel and competition. But although Jodie lived outdoors and ate mostly hay, she did show some signs of digestive discomfort. Every morning when I went out to feed, she had moist ruffled hair on both her sides where she had bitten at herself.

Gastric ulcers can be diagnosed with an endoscope, a long tube with a camera inserted down the esophagus and into the stomach, but many veterinarians will simply administer the medication to treat ulcers, GastroGard, and watch to see if the signs of discomfort dissipate. We put Jodie on GastroGard, and after a month she did stop biting her sides, but she remained grumpy, and her body still seemed sore. In fact, if I just pointed to her sides, she would pin her ears.

By May of 2011, Renton and I had exhausted the obvious diagnoses that could be made on the farm. He suggested I take her to Michigan State University’s large animal clinic for a more complete exam. Two internists, a back specialist and a physical therapist saw her there, and together they performed a series of exams so thorough it took nearly two days to complete.

The only obvious problem they found was mild gastric ulcers, although the physical therapist also noted that Jodie’s back was oddly dipped. She wasn’t swaybacked, but it seemed as if her back muscles were weak and possibly strained by her extra weight. We were sent home with instructions to continue the ulcer treatment, begin a routine of exercises, such as belly lifts and carrot stretches, designed to limber and strengthen her back muscles, and have Jodie lose 100 pounds.

I faithfully did Jodie’s back exercises daily but saw little improvement. When I massaged or manipulated her back, she would crankily bite the cross ties. I was beginning to think that I would never be able to ride my poor mare again, but even more distressing was the thought that she’d always be uncomfortable and unhappy.

Finally, an answer
Over the Fourth of July weekend, I found myself searching the Internet for any situation that sounded even?remotely similar to Jodie’s. As I clicked from one false lead to the next, I grew increasingly despondent.

Then, suddenly, I was reading a description of a horse who sounded remarkably like Jodie. This horse had suddenly turned cranky with no obvious physical reason. The owner, like me, hadn’t wanted to give up and eventually got a diagnosis: polysaccharide storage myopathy (PSSM).

This was one of the muscle disorders Renton and I had discussed briefly a year ago, but we’d decided that Jodie did not fit the profile. Well, neither had this horse I was reading about.

I immediately called Renton to tell him what I’d read, and he agreed it was worth a shot to have Jodie tested, even if the results came back negative. Currently, researchers have described two forms of this disease—PSSM Type 1, which has been linked to two specific genetic mutations that can be tested for, and PSSM Type 2, which is diagnosed by a muscle biopsy. Both tests are available only through the University of Minnesota, where much of the research on this disease has been done.

Which testing approach is recommended—the genetic test or the muscle biopsy—depends on the breed of horse. For Quarter Horses, the Minnesota researchers suggest doing the genetic test first, and by the end of the afternoon, I had received instructions via email on how to pluck mane hair and send it to the lab. Three days later I had an answer: Jodie had PSSM.

In the meantime, I’d been doing plenty of reading on PSSM, starting with the University of Minnesota’s website. I learned that many Quarter Horses, as well as more than 20 breeds, carry a genetic mutation that causes this disease (PSSM 1). Other light breeds, including Arabians and Thoroughbreds as well as some Quarter Horses, may also develop PSSM without carrying the mutated gene (PSSM 2).

Both forms of the disease affect the way muscle cells absorb, store and use the sugars they burn for energy. In horses with PSSM, the muscle tissue pulls a sugar (glucose) out of the bloodstream faster than in normal horses, according to the Minnesota researchers. The genetic mutation for PSSM, GYS1, results in overproduction of the storage form of the sugar, called glycogen, in the muscle cell. As a result, abnormal amounts of glycogen, as well as another abnormal sugar (polysaccharide), tend to accumulate in the cells. One reason for this, according to their research, is that the abnormal GYS enzyme that produces glycogen is very sensitive to insulin, a hormone released when a horse eats carbohydrates, which stimulates the muscles to take up more sugars and make more glycogen.

Horses who have inherited either one or two copies of the gene mutation may develop episodes of tying up (exertional rhabdomyolysis): painful, severe muscle stiffness and cramping in the hindquarters, usually after exercise. The muscles will feel very hard to the touch, and affected horses will resist moving their hind feet. In severe cases, the urine may be dark, brick-red or coffee-colored—a sign that muscle cells have been damaged and released proteins into the bloodstream. These proteins can damage the kidneys.

Fortunately, however, some horses like Jodie may have milder muscle pain, which is why I’d never seen signs that obvious. The downside, of course, was that her muscle stiffness and soreness were so subtle that it took
us nearly a year to figure out what the problem was.

But there was good news, too. Even in severe cases of PSSM, episodes of tying up can be prevented with careful attention to the horse’s diet and exercise. Because meals rich in starches and sugars cause insulin spikes, diets based on grass hays, with no added grains, are recommended for these horses.

Jodie got barely any grain at all, so the source of the sugars in her diet could only be our lush pasture and hay. I immediately put a muzzle on Jodie to keep her from eating any grass at all, switched her to a low-nutrient grass hay and began feeding a low-sugar, high-fat concentrate specially formulated for PSSM horses. Within two weeks she was moving much more comfortably and no longer snapped at anyone who tried to touch her.

But she still wasn’t entirely comfortable. After consulting with Renton and the University of Minnesota, I decided to discontinue the grain mixture because, frankly, Jodie didn’t need the extra calories. Within another week, her demeanor improved even more.

Regular exercise and as much turnout as possible are also recommended for horses with PSSM. This helps the muscle cells to continuously burn off the stores of glycogen they accumulate.

Jodie was already on 24-hour turn-out, but I continued doing the back exercises, and I noticed that she was beginning to lift her back and stretch more comfortably when I asked. I switched from using carrots, which are high in sugar, to celery as a lure to move her head and neck. Eventually, her topline filled in as her muscle tone and strength improved.

Once Jodie was moving comfortably, the next phase of the plan was to begin working her under saddle again. I’ll admit I was a bit anxious when I got back on her, but because she was no longer in pain, she was her old, sweet self. We hit the trails again, starting at a walk, then added short stretches of trotting. But to me it all felt as if we were flying. I cried tears of happiness the day we finally cantered and hopped over a small log.

Jodie’s PSSM wasn’t apparent when I purchased her because of the sparse fields at her previous home. Only as the sugar levels increased in our pasture over the spring and into summer did the problem arise. This past winter, the weather wasn’t cold enough to convince me the grass was dormant until mid-December, so I left Jodie’s muzzle on until then. I’ll put it back on the moment the days start to warm up again this spring.

I’ve also made one other management change. Knowing just how sensitive Jodie is to sugars and starches, I now have each batch of hay I buy tested for both. This is a lot of work, but everyone who knows Jodie knows it’s worth it. In fact, my wonderful husband even made me a hay corer for Christmas, allowing me to easily pull hay samples from the center of the bales. It was the perfect gift.




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