11/18/2025
🧬 𝐌𝐲𝐨𝐬𝐢𝐧 𝐇𝐞𝐚𝐯𝐲 𝐂𝐡𝐚𝐢𝐧 𝐌𝐲𝐨𝐩𝐚𝐭𝐡𝐲 (𝐌𝐘𝐇𝐌): 𝐔𝐧𝐝𝐞𝐫𝐬𝐭𝐚𝐧𝐝𝐢𝐧𝐠 𝐓𝐡𝐢𝐬 𝐆𝐞𝐧𝐞𝐭𝐢𝐜 𝐌𝐮𝐬𝐜𝐥𝐞 𝐃𝐢𝐬𝐨𝐫𝐝𝐞𝐫 🐴
MYHM is a genetically linked muscle disease seen in Quarter Horses, Paints, Appaloosas, and crosses. It is caused by a mutation in the MYH1 gene, which encodes a protein (myosin heavy chain 1) vital for fast-twitch (type 2X) muscle fibers. Horses with this mutation are at risk for two distinct disease presentations:
🔹 𝟏. 𝐈𝐦𝐦𝐮𝐧𝐞-𝐌𝐞𝐝𝐢𝐚𝐭𝐞𝐝 𝐌𝐲𝐨𝐬𝐢𝐭𝐢𝐬 (𝐈𝐌𝐌):
An autoimmune condition that causes the body to attack its own skeletal muscle fibers, specifically type 2X fibers.
📌 𝐂𝐥𝐢𝐧𝐢𝐜𝐚𝐥 𝐒𝐢𝐠𝐧𝐬:
• Rapid, symmetrical muscle atrophy over the topline and croup
• Depression, stiffness, reduced appetite
• Often follows respiratory illness, exposure to Streptococcus equi, or recent vaccination
📋 𝐃𝐢𝐚𝐠𝐧𝐨𝐬𝐢𝐬:
• Elevated muscle enzymes (CK, AST)
• Genetic testing via hair for MYH1 mutation
• Muscle biopsy, if performed early, shows lymphocytic infiltration and muscle fiber regeneration
💊 𝐓𝐫𝐞𝐚𝐭𝐦𝐞𝐧𝐭 & 𝐌𝐚𝐧𝐚𝐠𝐞𝐦𝐞𝐧𝐭:
• Rapid response with corticosteroids (dexamethasone followed by prednisolone)
• Supportive care with high-protein diets, alfalfa, and amino acid supplements
• Adjust vaccine protocols if necessary
• Monitor closely after infections or immune stimulation
🔹 𝟐. 𝐍𝐨𝐧𝐞𝐱𝐞𝐫𝐭𝐢𝐨𝐧𝐚𝐥 𝐑𝐡𝐚𝐛𝐝𝐨𝐦𝐲𝐨𝐥𝐲𝐬𝐢𝐬:
In addition to immune-mediated myositis, the MYH1 mutation can also lead to a second, distinct muscle disorder: nonexertional rhabdomyolysis. Unlike typical tying-up episodes related to exercise, this form occurs spontaneously, often causing significant muscle pain and stiffness. Though the underlying mechanisms are still being studied, this condition highlights another way the MYH1 mutation can impact muscle health in Quarter Horses and related breeds.
While nonexertional rhabdomyolysis (NER) and PSSM (Polysaccharide Storage Myopathy) can both cause muscle pain, stiffness, and elevated muscle enzymes, they are fundamentally different in cause, presentation, and management. PSSM, particularly type 1, is a glycogen storage disorder caused by a mutation in the GYS1 gene, leading to abnormal sugar storage in muscle cells and symptoms typically triggered by exercise or dietary imbalances. In contrast, NER associated with the MYH1 mutation occurs without exercise and is thought to result from an immune-mediated process or altered muscle fiber function. Horses with NER often experience sudden, severe muscle damage, sometimes in connection with infections or vaccination, and may have extremely high CK and AST levels.
While diet and turnout are key components of managing PSSM, treatment for NER may include corticosteroids or dantrolene, especially if infection is present. Genetic testing is available for both conditions and plays a crucial role in accurate diagnosis and long-term management.
📌 𝐂𝐥𝐢𝐧𝐢𝐜𝐚𝐥 𝐒𝐢𝐠𝐧𝐬:
• Painful, swollen back and haunch muscles
• Reluctance to move, difficulty rising after laying down
• Brown urine (from myoglobinuria)
• Often accompanied by fever or nasal discharge
📋 𝐃𝐢𝐚𝐠𝐧𝐨𝐬𝐢𝐬:
• Very high CK (often >50,000 U/L) and AST levels
• Confirmed S. equi infection may be present
• MYH1 genetic test confirms susceptibility
💊 𝐓𝐫𝐞𝐚𝐭𝐦𝐞𝐧𝐭:
• Dantrolene to reduce muscle damage
• Corticosteroids in select cases
• Antibiotics and guttural pouch lavage if S. equi is involved
🧬 𝐆𝐞𝐧𝐞𝐭𝐢𝐜𝐬 𝐨𝐟 𝐌𝐘𝐇𝐌:
• N/N: No copies of the mutation; no increased disease risk
• N/My: One copy - at risk for both IMM and rhabdomyolysis; may pass gene to 50% of offspring.
• My/My: Two copies - higher risk of severe or recurrent disease; 100% of offspring will inherit mutation.
🧪 𝐖𝐡𝐲 𝐓𝐞𝐬𝐭𝐢𝐧𝐠 𝐌𝐚𝐭𝐭𝐞𝐫𝐬:
• MYHM is inherited in an autosomal codominant manner with variable penetrance which means not all carriers show clinical signs.
• Genetic testing helps with diagnosis, risk assessment, breeding decisions, and vaccination planning.
• Horses can live full, productive lives with appropriate management if diagnosed early.
Understanding MYHM and its two clinical forms, immune-mediated myositis and nonexertional rhabdomyolysis, is key to managing affected horses and making informed breeding, vaccination, and treatment decisions! With a simple genetic test, owners can identify at-risk horses early and work with their veterinarian to minimize potential triggers and monitor for signs of disease. Early recognition and intervention can significantly improve outcomes and help maintain long-term health and performance!
📸: UC Davis
