Other Names
MFM; myofibrillar myopathy; non-exertional rhabdomyolysis; atypical myopathy (partially); fibre-type disproportion myopathy.
What is Equine Myofibrillar Myopathy?
Equine Myofibrillar Myopathy (MFM) is a recently characterised muscle disorder in horses in which the structural proteins within muscle fibres — the myofibrils — degenerate abnormally. Unlike other forms of equine muscle disease, MFM is associated with alterations in the contractile proteins (desmin and other myofibrillar components) rather than energy metabolism dysfunction. This makes it a distinct entity from Recurrent Exertional Rhabdomyolysis (RER) and Polysaccharide Storage Myopathy (PSSM).
The condition was formally described in horses relatively recently, and understanding of its prevalence, genetic basis, and optimal management continues to evolve. MFM predominantly affects sport horses — particularly warmbloods and Arabians engaged in disciplines requiring sustained, controlled effort such as dressage, eventing, and endurance. It is most commonly diagnosed in horses that show poor performance, back stiffness, or reluctance to work without an obvious orthopaedic cause.
Because MFM shares clinical features with other myopathies and orthopaedic conditions, it is frequently underdiagnosed or misdiagnosed. Advances in muscle biopsy techniques and specialised staining have improved diagnostic accuracy considerably in recent years.
Causes
The precise cause of MFM in horses is not yet fully established. Research suggests a genetic predisposition is likely, given that certain bloodlines and breeds are over-represented, but the specific gene or genes involved have not been definitively characterised in all affected animals.
The fundamental abnormality appears to be a disruption to the organisation and stability of myofibrillar proteins within muscle fibres. Desmin, a structural protein that anchors the contractile apparatus of muscle cells, is frequently found to be abnormally aggregated or absent in affected horses. This leads to fibre degeneration under the sustained muscle contractions demanded by athletic work.
Nutritional factors, including vitamin E deficiency and selenium imbalance, may contribute to or exacerbate the condition, though they are unlikely to be the sole cause in genetically predisposed horses.
Symptoms and Signs
Clinical presentation in horses with MFM tends to be subtle and gradual rather than acute. Common signs include:
- Poor performance — the horse works below its known capability without a clear cause
- Back pain and stiffness, particularly in the epaxial muscles along the topline
- Resistance to collection, lateral work, or back engagement in ridden work
- Muscle wastage, particularly over the hindquarters and topline
- Reluctance to be groomed or touched over the back and hindquarters
- Gait abnormalities — shortened stride, reduced engagement, or inconsistent hind limb impulsion
- Occasional post-exercise stiffness or low-grade tying-up episodes
Serum creatine kinase (CK) and aspartate aminotransferase (AST) levels may be mildly elevated or normal at rest, though they can be markedly elevated following exercise. This inconsistency makes biochemical testing alone unreliable for diagnosis.
Diagnosis
Definitive diagnosis of MFM requires muscle biopsy with specialised immunohistochemical staining. The semimembranosus or gluteal muscles are commonly sampled. Histopathological examination reveals areas of myofibrillar disorganisation, protein aggregation (particularly of desmin), and fibre degeneration with subsequent regeneration.
Before biopsy, your vet will perform a thorough physical and lameness examination, exercise-induced CK and AST measurements, and exclusion of orthopaedic causes. Imaging such as nuclear scintigraphy (bone scanning) or ultrasonography may be used to rule out concurrent musculoskeletal problems, which commonly occur alongside MFM in sport horses.
Genetic testing is available for some myopathies (such as PSSM type 2, with which MFM overlaps in current diagnostic classification), and your vet or specialist equine internal medicine clinician can advise on the most appropriate diagnostic pathway for your horse.
Treatment
Management of MFM focuses on reducing muscle damage, supporting muscle repair, and modifying training to match the horse’s capacity. Key components include:
- Vitamin E supplementation — natural vitamin E (d-alpha-tocopherol) at 5,000–10,000 IU per day is frequently recommended, as it supports muscle membrane integrity and acts as an antioxidant
- Omega-3 fatty acid supplementation via linseed or fish oil to reduce inflammation
- Dietary modification — reducing high-starch, high-sugar feeds and replacing with fat and fibre-based energy sources
- Modified training — reducing training intensity and duration while the horse’s condition is assessed and nutritional support is established
- Regular, consistent exercise rather than irregular intense work
- Physiotherapy and massage to manage secondary muscle tightness
Some horses improve significantly with dietary and exercise management; others remain affected and may require long-term modification of their workload and competitive aspirations.
Prevention
Prevention in genetically predisposed horses centres on optimal nutrition and training management. Ensuring adequate vitamin E and selenium in the diet, avoiding sudden increases in workload intensity, and providing daily turnout with free movement all support muscle health. Horses with a family history of myopathy or recurring poor performance should be evaluated by a vet before being entered into demanding training programmes.
Prognosis
The prognosis for MFM is variable. Some horses respond well to nutritional management and modified training and can return to full competition. Others show only partial improvement and may need to be transitioned to lower-intensity work. Horses with severe, extensive muscle pathology on biopsy carry a more guarded prognosis for return to high-level sport. Early diagnosis and proactive management offer the best outcomes.
When to Call the Vet
Seek veterinary advice if your sport horse shows unexplained deterioration in performance, recurring back stiffness, muscle wastage over the topline, or resistance to work that does not improve with normal training and farriery management. These signs warrant full investigation rather than dismissal as training or behavioural issues.
Frequently Asked Questions
Is MFM the same as tying up?
They are related but distinct conditions. Tying up (exertional rhabdomyolysis) involves acute muscle fibre breakdown during or after exercise, often with marked pain and elevated CK levels. MFM is a structural abnormality of the myofibrils themselves and tends to present more chronically as poor performance and back pain. Some horses with MFM also experience tying-up episodes, but not all do.
Can MFM be managed with diet alone?
Diet, particularly vitamin E supplementation and low-starch feeding, is a cornerstone of management and many horses improve considerably with dietary changes. However, training modification and ongoing monitoring are equally important. Diet alone is unlikely to be sufficient without addressing workload appropriately.
Is MFM hereditary?
A genetic predisposition is strongly suspected, particularly given the breed over-representation in warmbloods and Arabians. However, the specific genetic markers for equine MFM have not been fully characterised, and genetic testing currently available covers overlapping but not identical conditions.
Which breeds are most commonly affected?
Warmbloods and Arabians are most frequently represented in MFM case reports, though the condition can occur in any sport horse breed. The prevalence in these breeds likely reflects both a possible genetic predisposition and the type of sustained, collected work these horses are typically asked to perform.
See also: Equine Shivers | Rhabdomyolysis