The Horse:Your Guide To Equine Health Care provides monthly equine health care information to horse owners, breeders, veterinarians, barn/farm managers, trainer/riding instructors, and others involved in the hands-on care of the horse.
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28 TheHorse.com THE HORSE March 2018 Of horses diagnosed with PSSM by muscle biopsy, says Valberg, about 28% of Quarter Horses have PSSM2, whereas 90% of Warmbloods and almost 100% of Arabians have PSSM2. Further, Quarter Horses with PSSM2 commonly experi- ence tying-up and have elevated circu- lating creatine kinase (CK, an enzyme indicating muscle damage). Warmbloods with PSSM2, on the other hand, predomi- nantly exhibit an abnormal gait; reluc- tance to collect; a mild, shifting lameness; and a less-consistent increase in CK. Points to Ponder When Diagnosing PSSM Genetic testing via blood or hair roots for PSSM1 is relatively inexpensive, painless, and very specifically identifies PSSM1. In horses with chronic tying-up that are among breeds known to carry the PSSM1 mutation, genetic testing is a good place to start, says Valberg. That said, many indicators of PSSM2— poor performance, subtle lameness, topline atrophy (loss of muscling over the back), difficulties changing leads and per- forming trot/canter transitions, changes in behavior toward work, and sensitivity to grooming—are common complaints that could be caused by a variety of conditions. "Owners are encouraged to work with their veterinarian to rule out more common causes for these clinical issues before considering muscle biopsies to evaluate horses for PSSM2," Valberg says. Only after a complete diagnostic workup has exhausted more common ex- planations for the presenting complaints (e.g., lameness, neurologic conditions) should owners pursue testing. To further reinforce this, muscle biopsy results are, like many other diagnostic tests in veterinary medicine, subjective. "This means that without a proper workup, horses can be falsely diagnosed with PSSM. This translates into institut- ing inappropriate or ineffective treat- ments that could prolong time off from work or even (lead to) early retirement," Valberg says. Managing the Disease Valberg says horses with PSSM1 im- prove markedly with a combination of a customized diet and routine exercise. "Diet alone will not fix these horses," she stresses. Implement a structured exercise pro- tocol, as well as turnout (using a grazing muzzle if pastures are lush). And as long as your horse is not actively tying up, do not give him time off from exercise. In terms of diet, the goal is to trick the muscle into burning that excess glycogen to fuel the muscle cells, instead of storing it, and to use fat as an alternate energy source. The "best" diet for a horse with PSSM1 depends on his current clinical condition, body condition score, and fitness level. In general, says Valberg, there are five key nutritional goals when it comes to managing horses with PSSM1: 1. Offer hay that's <12% nonstructural (water-soluble) carbohydrates, which contain high levels of sugar. Recall that the exuberant GYS enzyme (encoded from the GYS1 gene) converts excess sugar to glycogen that accumulates in muscle cells. 2. How much hay to offer depends on the horse's body condition. It is important to achieve and maintain an optimal body condition score (~5 or 6 out of 9) in affected animals. 3. If horses require additional energy (e.g., those still in moderate work), of- fer oil top-dressed on hay cubes or rice bran (naturally high in fat and low in sugar). Generally, 13% of calories fed to a horse daily in the form of fat provides TYING-UP: Where Do We Stand? TYPES OF EXERTIONAL MYOPATHIES BASED ON BREED ANY BREED Sporadic exertional rhabdomyolysis due to a horse training or competing beyond its athletic ability, causing muscle damage, electrolyte deficits Dehydration, electrolyte abnormalities, other THOROUGHBREDS, STANDARDBREDS Recurrent exertional rhabdomyolysis thought to be caused by abnormal calcium levels in muscle cells during contraction WARMBLOODS (Hanoverian, Dutch/Swedish/ Canadian/Danish Warmblood, Holsteiner, Oldenburg, Trakehner, Rheinlander, others) Primarily PSSM2; only a small proportion of these breeds appear to suffer from PSSM1 Myofibrillar myopathy, a disruption of the alignment of the contractile proteins called myofibrils in affected horses' muscle cells. DRAFT 69% of Percherons and 39% of Belgians in North America and more than 75% of European Belgian horses have the PSSM1 genetic mutation Clydesdales and Shires rarely have PSSM1 QUARTER HORSES AND RELATED BREEDS (Appaloosas, Paints) Primarily PSSM1, also PSSM2 Malignant hyperthermia caused by a genetic mutation in the ryanodine receptor 1 (RYR1) gene ARABIANS AND ARABIAN- CROSSES PSSM2 and myofibrillar myopathy MORGANS PSSM1 or PSSM2 DID YOU KNOW? Because the urine of horses with exertional rhabdomyolysis is dark, veterinarians in the 19th and early 20th centuries thought kidney disease (which also causes dark urine) was to blame. Researchers later discovered that the dark urine is due to the abnormal presence of the protein myoglobin in the urine due to muscle breakdown.