The Horse

FEB 2019

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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TheHorse.com | The Horse February 2019 35 exam for horses that tie up involves: ■ Getting a detailed history; ■ Conducting a thorough physical exam; ■ Collecting a blood sample and per- forming a complete blood count and chemistry profile; ■ Collecting urine to examine for frac- tional electrolyte exertion; ■ If there's no evidence of muscle damage on all previous tests, conducting an exercise test and evaluating pre- and post-exercise CK; and ■ Performing a muscle biopsy. 4. The UC Davis Neuromuscular Diagnostic Laboratory Scientists at the University of California, Davis, (UC Davis) Neuromuscular Diagnostic Laboratory in- troduced the concept of using specialized freezing techniques to preserve muscle biopsy samples, rather than fixing them in formalin. This allowed researchers to see structures better during analysis. They also found that using different stains on samples can reveal different diseases. Valberg described a case in which she and colleagues used this new muscle biopsy assessment technique to identify a muscle disorder in a 3-year-old Quarter Horse mare with a history of chronic stiff- ness, firm muscles, exercise intolerance, and persistently high serum CK levels. Valberg determined that the mare had abnormal glycogen (stored sugar) building up in her muscles, something not seen in Thoroughbreds and Standard- breds. A biochemistry evaluation revealed twice as much glycogen as normal. She and colleagues diagnosed the mare with polysaccharide storage myopathy (PSSM), now found in roughly 10% of Quarter Horses and 36% of Drafts. They eventually determined that PSSM horses had too much glycogen, but not enough energy, in their muscles. They worked to find a diet to slow glycogen synthesis and encourage glycogen metabolism. They found that a diet low in starch and sugars and high in fat (to provide ex- tra energy), along with regular exercise to burn the glycogen, helped relieve PSSM horses' signs and let them return to work. Valberg said the Quarter Horse mare responded well to this regimen. 5. The Equine Genome Advances in equine genome mapping allowed Valberg and colleagues to identify the gene responsible for PSSM in Quarter Horses. They eventually landed on the glycogen synthase gene GSY1, confirming that a GSY1 mutation altered glycogen synthase activity and that mutations were present in 6% of a random Quarter Horse popula- tion and 80% of PSSM horses. Scientists sequenced the complete equine genome in 2009, allowing Valberg and others to more easily analyze genes of interest and identify those associated with specific traits and disorders. MFM in Arabians Valberg also described Supplementing With Vitamin E Vitamin E is a potent antioxidant primarily found in green pasture grass that plays a role in muscle atrophy (wasting) and neurodegenera- tion in horses. As land becomes increasingly limited and more horses are housed on less acreage, vitamin E deficiency becomes a real prob- lem. Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, described conditions linked to vitamin E deficiencies and how to manage them. Veterinarians see three associated neurologic conditions, she said. Neuroaxonal Dystrophy and Equine Degenerative Myeloencepha- lopathy (NAD/EDM) This neuromuscular disorder typically appears in horses 6 months to 3 years of age. Clinical signs of ataxia (incoordi- nation) and proprioceptive deficits (awareness of where one's limbs are) are similar to those of horses with wobbler syndrome (spinal cord compression) and persist through adulthood. "We believe it results from a genetic predisposition and vitamin E deficiency in utero and the early six months of life," said Valberg, explaining that the equine nervous system depends on adequate vitamin E to develop normally. While owners can supplement susceptible horses (i.e., broodmares and foals living on farms that have had cases) with vitamin E to try to prevent development of this disease or reduce its severity, once horses develop clinical signs, supplementation has no effect, Valberg said. It typically crops up on farms that have experienced a decrease in pasture quality and green grass, she added. Vitamin E Deficient Myopathy This muscle disorder occurs in adult horses ages 7 to 10. Signs include an inability to lock the stifles, weak- ness, trembling, a low head position, difficulty lying down, weight loss, and muscle atrophy. In acute cases, she said, the most obvious sign is trembling, while in chronic cases owners typically notice reduced muscle mass in the hindquarters and some trembling. Vets can test for this by taking a biopsy of the sacrocaudalis dorsalis muscle (located above the tailhead on either side of the spine) and looking for abnormal mitochondrial staining. Fortunately, she said, it's reversible with supplementation. Equine Motor Neuron Disease (EMND) This neurodegenerative disorder affects the spinal cord where the nerves come out to control muscle contraction. Affected horses are typically older (>10), said Val- berg, and have been vitamin E-deficient for a long time. They typically display similar signs as horses with vitamin E deficiencies. They might also have a distinct pigmented pattern to their retinas. "Once horses reach this stage, they might stabilize with supplemen- tation but might not return to performance," she said. Supplementing With Vitamin E "The impact of vitamin E deficien- cies causing subtle but significant muscle atrophy and a decline in performance are underrecognized," Valberg said. "It should be on every- one's radar because it's easy to diagnose with blood samples for vita- min E and can be readily treated with liquid vitamin E supplements." What supplement to provide varies by case. For healthy horses in at-risk areas, Valberg suggests using 1,000-2,000 IU/day of the oral powdered natural form. If your horse already suffers from EMND or vitamin E deficient myopathy, she recommends using 5,000 IU/day of the natural liquid form until all clinical signs are gone and, then, transitioning to powder over a series of weeks once the horse returns to normal. Expect it to take several months for signs to disappear. Because responses to vitamin E supplementation vary, Valberg urges vets and/or nutritionists to measure vitamin E levels before and four weeks after supplementation, adjusting the dose accordingly. She added that while vitamin E supplementation won't resolve other neurodegenerative diseases such as shivers, owners should know that a deficiency might exacerbate them. "I think it's important to maintain horses at normal vitamin E levels," Valberg said. "I'm a big fan of measuring vitamin E in horses and supplementing as needed. As we have less and less pasture, we'll see more of these cases."—Alexandra Beckstett

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