Myofibrillar Myopathy in Horses: Dietary Help Possible

Researchers have unveiled yet another facet in the multidimensional muscle disease known as exertional myopathy or colloquially as tying-up. While the new disease, named myofibrillar myopathy (MFM), shares certain characteristics with the more familiar recurrent exertional rhabdomyolysis and polysaccharide storage myopathy, namely exercise intolerance, the underlying cause is novel.

Myofibrillar myopathy occurs when the orderly alignment of myofibrils becomes disrupted. Myofibrils are the microscopic contractile units within a muscle fiber composed primarily of two proteins: actin and myosin. During contraction, these two proteins, which lie in staggered formation side-by-side, slide along one another to shorten the overall length of a muscle fiber. Another protein, called desmin, helps stabilize the actin filaments to help maintain their side-by-side alignment with myosin so that muscle contraction can occur.

In muscle biopsies from horses with MFM, the alignment between the actin and myosin strands becomes disrupted and can actually break. Desmin accumulates at these disruption sites. Telltale signs of the disease exist microscopically, underscoring the need for muscle biopsy for accurate diagnosis.

“Dietary recommendations for horses with MFM are different than other myopathies. Instead of focusing on a diet that limits glycogen storage, the diet should be one that is more focused on increasing muscle protein turnover and building muscle strength,” explained Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research. “Unlike horses diagnosed with polysaccharide storage myopathy, those with MFM tolerate diets with moderate starch and sugar levels with up to 6% fat if needed to maintain good body condition.”

In addition to a well-balanced base diet, the provision of key amino acids is necessary for horses diagnosed with MFM. Kentucky Equine Research worked to create a nutritional solution in concert with Stephanie Valberg, D.V.M., Ph.D., Dipl. ACVIM, ACVSMR, Director of the Equine Neuromuscular Diagnostic Laboratory and Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University, Department of Large Animal Clinical Sciences.

“We’ve found these horses don’t seem to have an appropriate amount of cysteine-based antioxidants,” said Valberg, “so we designed a supplement called MFM Pellet that supplies specific amino acids that stimulate muscle repair and build strength. The supplement includes n-acetyl cysteine, which forms cysteine, an amino acid that helps build antioxidants not found in appropriate concentrations in MFM-affected horses.”

In addition to MFM Pellet, Valberg recommends coenzyme Q10, which assists in maintaining cellular integrity by protecting muscle cells from oxidative damage that occurs during exercise.

Valberg has supplemented affected horses with a combination of MFM Pellet and Nano-Q10. Read about one Warmblood gelding that benefited from this diet.

MFM Pellet is currently only available to purchase in the United States. For more information on feeding the horse with MFM, share a few details here and a KER nutrition advisor will be in touch.

Williams, Z.J., D. Velez-Irizarry, J.L. Petersen, J. Ochala, C.J. Finno, and S.J. Valberg. 2020. Candidate gene expression and coding sequence variants in Warmblood horses with myofibrillar myopathy. Equine Veterinary Journal:13286.