11/18/2025
๐งฌ ๐๐ฒ๐จ๐ฌ๐ข๐ง ๐๐๐๐ฏ๐ฒ ๐๐ก๐๐ข๐ง ๐๐ฒ๐จ๐ฉ๐๐ญ๐ก๐ฒ (๐๐๐๐): ๐๐ง๐๐๐ซ๐ฌ๐ญ๐๐ง๐๐ข๐ง๐ ๐๐ก๐ข๐ฌ ๐๐๐ง๐๐ญ๐ข๐ ๐๐ฎ๐ฌ๐๐ฅ๐ ๐๐ข๐ฌ๐จ๐ซ๐๐๐ซ ๐ด
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:
๐น ๐. ๐๐ฆ๐ฆ๐ฎ๐ง๐-๐๐๐๐ข๐๐ญ๐๐ ๐๐ฒ๐จ๐ฌ๐ข๐ญ๐ข๐ฌ (๐๐๐):
An autoimmune condition that causes the body to attack its own skeletal muscle fibers, specifically type 2X fibers.
๐ ๐๐ฅ๐ข๐ง๐ข๐๐๐ฅ ๐๐ข๐ ๐ง๐ฌ:
โข Rapid, symmetrical muscle atrophy over the topline and croup
โข Depression, stiffness, reduced appetite
โข Often follows respiratory illness, exposure to Streptococcus equi, or recent vaccination
๐ ๐๐ข๐๐ ๐ง๐จ๐ฌ๐ข๐ฌ:
โข Elevated muscle enzymes (CK, AST)
โข Genetic testing via hair for MYH1 mutation
โข Muscle biopsy, if performed early, shows lymphocytic infiltration and muscle fiber regeneration
๐ ๐๐ซ๐๐๐ญ๐ฆ๐๐ง๐ญ & ๐๐๐ง๐๐ ๐๐ฆ๐๐ง๐ญ:
โข Rapid response with corticosteroids (dexamethasone followed by prednisolone)
โข Supportive care with high-protein diets, alfalfa, and amino acid supplements
โข Adjust vaccine protocols if necessary
โข Monitor closely after infections or immune stimulation
๐น ๐. ๐๐จ๐ง๐๐ฑ๐๐ซ๐ญ๐ข๐จ๐ง๐๐ฅ ๐๐ก๐๐๐๐จ๐ฆ๐ฒ๐จ๐ฅ๐ฒ๐ฌ๐ข๐ฌ:
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.
๐ ๐๐ฅ๐ข๐ง๐ข๐๐๐ฅ ๐๐ข๐ ๐ง๐ฌ:
โข Painful, swollen back and haunch muscles
โข Reluctance to move, difficulty rising after laying down
โข Brown urine (from myoglobinuria)
โข Often accompanied by fever or nasal discharge
๐ ๐๐ข๐๐ ๐ง๐จ๐ฌ๐ข๐ฌ:
โข Very high CK (often >50,000 U/L) and AST levels
โข Confirmed S. equi infection may be present
โข MYH1 genetic test confirms susceptibility
๐ ๐๐ซ๐๐๐ญ๐ฆ๐๐ง๐ญ:
โข Dantrolene to reduce muscle damage
โข Corticosteroids in select cases
โข Antibiotics and guttural pouch lavage if S. equi is involved
๐งฌ ๐๐๐ง๐๐ญ๐ข๐๐ฌ ๐จ๐ ๐๐๐๐:
โข N/N: No copies of the mutation; no increased disease risk
โข N/My: One copy - at risk for both IMM and rhabdomyolysis; may pass gene to 50% of offspring.
โข My/My: Two copies - higher risk of severe or recurrent disease; 100% of offspring will inherit mutation.
๐งช ๐๐ก๐ฒ ๐๐๐ฌ๐ญ๐ข๐ง๐ ๐๐๐ญ๐ญ๐๐ซ๐ฌ:
โข MYHM is inherited in an autosomal codominant manner with variable penetrance which means not all carriers show clinical signs.
โข Genetic testing helps with diagnosis, risk assessment, breeding decisions, and vaccination planning.
โข 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!
๐ธ: UC Davis
