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

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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.”