Knowde Enhanced TDS
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The understanding of muscle disease in horses has broadened considerably over the last few decades. Researchers continue to discover nuances in muscle dysfunction, and one disease continues to gain recognition within the Warmblood and endurance communities: myofibrillar myopathy (MFM).
Like those diagnosed with polysaccharide storage myopathy (PSSM), horses with MFM demonstrate exercise intolerance and occasional episodes of tying-up. Microscopic examination of muscle tissue reveals, however, that the underlying cause of MFM is much different than it is for PSSM.
Sarcomeres are the basic contractile units of muscle fiber. Each sarcomere is composed of two main protein filaments, actin and myosin, which are the structures responsible for muscular contraction. These structures lie side by side in a staggered formation, while another protein called desmin stabilizes the filaments to maintain their alignment. In horses with MFM, the alignment between the actin and myosin strands fails, causing tears and gaps in the filaments. Muscle biopsies of affected horses show abnormal desmin deposits at these disruption sites.
While researchers remain unsure why these changes in myofibrils occur, MFM has been identified and studied most extensively in two groups of horses, Warmbloods and Arabians. Crossbred horses of Arabian and Warmblood breeding as well as horses of unrelated breeds have also been affected. Warmbloods typically show exercise intolerance by the time they are six to eight years of age. This is characterized by an unwillingness to go forward, inability to achieve or maintain collection, reduced capacity for routine work, and difficulty with upward transitions to canter. Muscle pain, stiffness, and vague hindlimb lameness are also signs of MFM in Warmbloods.
According to Valberg, the “basis for MFM in Warmblood horses appears to be related to the individual effects of diet, exercise, and training on gene and protein responses to exercise with downstream effects on muscle mass, the alignment of contractile proteins, mitochondrial function, and oxidative stress.”
Arabians diagnosed with MFM are typically engaged in endurance riding. Horses have irregular elevations in serum creatine kinase activity after endurance rides or during exercise that follows a week or more of rest. Elevated creatine kinase is a sign of muscle damage. Other signs of tying-up, such as sweating, pain, and reluctance to move, are hit-or-miss and oftentimes less severe than those signs exhibited by horses with other forms of exertional rhabdomyolysis. Discolored urine has been noted in Arabian horses with only slight muscle stiffness.
The basis for MFM in Arabian horses appears to be related to a need for increased cysteine synthesis, decreased cysteine-based antioxidants, and oxidative stress. With this in mind, these horses would have an increased requirement for cysteine following exercise.
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- MFM Pellet Feeding Directions
- Consult your veterinarian to see if this product is appropriate for your horse.
- Top-dress on feed. Divide between two meals. Feed 120 g per meal twice daily
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- Nutritional Management
Researchers continue to delve into the intricacies of this recently discovered myopathy, including how best to nourish affected horses. Rations for horses diagnosed with MFM should focus on providing high-quality protein and specific amino acids, which may support the regeneration of sarcomeres.
Horses with MFM should consume 1.5%–2% of body weight per day of hay; use good-quality grass or mixed hays (aim for a range of 55%–65% NDF, 10%–12% CP, 10%–17% NSC). If concentrates are required to maintain body condition, look for products with moderate levels of NSC (20-30% NSC) and lower fat (4-6%), as fat increases oxidative stress. Use MFM Pellet to increase the amount of key amino acids and high-quality protein to protect the muscle from oxidative stress and rebuild damaged muscle fibers.