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Myofibrillar myopathy (MFM) is a newly identified muscle disorder that causes exercise intolerance in horses.
MFM is a genetic condition that describes abnormal build-up of desmin in muscle tissue. Desmin is a protein in muscle filaments.
In response to strenuous exercise, horses with MFM may experience pain, stiffness, lameness, poor stamina and intermittent gait abnormalities. [5]
This disorder has been identified mostly in Warmblood, Arabian horses and their crosses. Affected Warmbloods often refuse to collect under saddle while Arabians tend to have episodes of tying-up or extreme cramping. [7]
There is no cure for Myofibrillar Myopathy in horses, but tolerance to exercise can be improved with management strategies to address diet and conditioning. However, some horses may be retired from competition due to poor performance.
Myofibrillar Myopathy
Myofibrillar myopathy is an exertional muscle disorder that shares clinical signs with more common exercise disorders, such as polysaccharide storage myopathy type 1 or type 2 (PSSM1 or PSSM2) or recurrent exertional rhabdomyolysis (RER).
Unlike RER, Warmbloods with MFM do not have elevations in serum creatine kinase (CK) or aspartate transaminase (AST), but Arabians do. [6][8]
MFM can present differently in individual cases, so muscle biopsy is required for a definitive diagnosis.
Signs of MFM in Horses
Muscle disorders can dramatically impair the abilities of horses in various levels of work by causing pain and weakness in response to exercise. [6]
Clinical signs of MFM in horses can present during and after an exercise bout, resembling other muscle disorders such as PSSM or tying up. [6]
The signs of MFM include: [7][8][9]
- Poor performance
- Muscle pain and stiffness
- Tying-up or rhabdomyolysis
- Reluctance to move forward
- Vague hindlimb lameness
- Tremors
- Extreme sweating
- Movement abnormalities
- Pigmenturia or dark urine
- Elevated muscle enzymes in Arabians
It has been suggested that the clinical presentation of MFM can differ between breeds. [7] For example, Arabians are more likely to experience tying up than Warmbloods. The actual cause of these differences is unknown but may be related to breed genetics.
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Cause of MFM
The genetic cause of MFM remains under investigation and likely involves a complex interaction between several genes, instead of one specific gene. [9][12]
MFM-affected horses have a disorderly alignment of broken myofibrils (contractile proteins) in their muscle cells. These damaged myofibrils cause a clumpy build-up of the cytoskeleton protein desmin near the breakage sites in fast-twitch muscles. [6][7]
The alignment of muscle proteins is essential for strength and effective muscle contractions. [6] When desmin cannot stabilize muscle filaments and maintain alignment, muscle contraction becomes less effective. This leads to exercise intolerance and early exhaustion.
MFM in Arabians
The Arabian is a successful endurance breed because of metabolic adaptations that allow the horse to efficiently break down fatty acids, providing energy for long-distance exercise.
However, this adaptation can also lead to oxidative stress during aerobic exercise, in which antioxidant defences in the body are overwhelmed. [1][7] This imbalance can affect several physiological processes, including muscle function.
Arabians with MFM seem to have a deficiency of cysteine antioxidants in the body, which are responsible for combatting inflammation and oxidative stress. [10]
This can lead to chronic muscle breakdown during or following exercise.
MFM in Warmbloods
The onset of exercise intolerance in MFM-affected warmblood horses usually occurs between 6-8 years of age. [6] Even high-performing horses can lose stamina and the ability to collect in several gaits.
Many warmbloods lose the ability to sustain a normal canter for more than a few strides along with sloppy and stiff canter transitions. They may demonstrate hindlimb lameness with no apparent cause.
Exercise intolerance in warmbloods with MFM may be explained by abnormal adjustment of muscle fibre signalling pathways to exercise demands. Warmbloods with MFM may also show slow-onset muscle atrophy. [7]
Other breeds
In rare cases, MFM has been identified in Thoroughbreds, Quarter Horses and Paso Finos through muscle biopsy. [7] However, little is known about the causes, symptoms, management or prognosis of MFM in these breeds.
Diagnosis
A detailed examination including biopsy and immunohistochemical staining is required for diagnosis.
There are commercially available genetic tests for MFM that report the presence of the variants (P2, P3, P4). However, when comparing control horses to those with features of MFM based on muscle biopsy analysis, the presence of these variances does not differ. [11] Therefore, the current genetic tests should be considered inaccurate for an MFM diagnosis.
Because MFM is relatively rare, your veterinarian will first examine your horse to rule out other conditions and causes of exercise intolerance.
Physical Examination
A physical examination must be conducted to rule out lameness and neurological conditions that could be causing MFM symptoms.
If a horse shows alterations in their gait or symptoms of hoof pain, your veterinarian will ask about your horse’s medical history and conduct a hands-on assessment to examine and palpate muscles, joints and tendons.
A horse with MFM may have lameness in the hindlimb without apparent cause, so isolating the specific location of pain through physical examination is often difficult. [7]
Muscle Biopsy
MFM can be diagnosed with a muscle biopsy in which tissue is removed from affected muscles to be analyzed microscopically. These biopsies are sensitive and can yield false negative or positive results. [7] Therefore, other conditions must be ruled out first.
Muscle tissue can be removed for sampling safely, quickly and painlessly through a percutaneous needle biopsy. [3] The gluteus medius muscle or semimembranosus tendinosis muscle are commonly sampled for diagnosis.
The muscle fibres are examined to determine if Type 2A or 2B (fast-twitch) muscle fibres contain desmin build-up. [6][8]
If the desmin aggregates are found in mature muscle cells, the horse can be diagnosed with MFM. [6] However, a false positive diagnosis may occur if the sample is taken from a horse with actively regenerating muscle fibres.
If a biopsy is needed for diagnosis, it should be conducted following the physical examination and before other treatment plans are commenced. Treatment can decrease the build-up of desmin, potentially leading to a false negative result for MFM. [6]
Immunohistochemical Stains
Immunohistochemical staining for desmin is currently the most accurate way to diagnose myofibrillar myopathy. Immunohistochemistry uses antibodies to detect the presence of specific protein markers in biological tissue.
Stains can identify desmin aggregates in tissue samples obtained during a muscle biopsy. Desmin build-up is identified by abnormal red clumping in scattered muscle fibres. [8]
This method of diagnosis is not reliable for horses under eight years of age, horses that are out of work or horses under specific management plans for MFM. Desmin build-up is difficult to identify in young horses, as significant accumulation is needed for diagnosis. [6]
Given the limitations of current diagnostic techniques, more sensitive markers to identify MFM are needed.
Differential Diagnosis
It is recommended that horses undergo neurological and lameness examinations before considering an MFM diagnosis. Other conditions that cause poor performance are more common.
The following conditions can mimic the clinical signs of MFM and should be ruled out before diagnosis:
- Lameness
- Ill-fitting tack
- Neurological disorders
- Suspensory injuries
- Sacroiliac disorders
- Inflammation of the vertebrae
- Stomach ulcers
MFM vs. PSSM2
Horses with MFM are commonly misdiagnosed with PSSM2, which is a more common and better-understood equine muscle disorder. Neither disorder has a known cause. [7]
PSSM2 is a collective term for several muscle disorders that are included under the same diagnosis and characterized by similar clinical signs. PSSM2 occurs when there is an abnormal accumulation of glycogen in muscle cells that is not caused by the genetic defect that causes PSSM1. [4]
Glycogen stains from horses with MFM reveal similar accumulation between breakages in myofibrils, which can indicate PSSM. This can lead to an incorrect diagnosis.
It has been suggested that PSSM2 is an early-stage MFM. If muscle biopsies show findings that are consistent with both PSSM2 and MFM, the diagnosis of MFM is used to characterize the horse.
Management of MFM
Horses with MFM require alterations to their diet, care and training routine to manage clinical signs of the disorder.
Discuss treatment options with your veterinarian and equine nutritionist to develop an individualized management plan for your horse.
Feeding Plan
All horses require a balanced diet with adequate protein, vitamins and minerals. [6]
Choose forage that matches your horse’s nutritional needs and feed a vitamin and mineral supplement to ensure there are no deficiencies in your horse’s diet.
Horses with myofibrillar myopathy may benefit from supplementation to support protein synthesis and muscle function.
High-fat, low-starch and sugar diets have been suggested for horses with muscle disorders such as PSSM. However, there is no evidence suggesting this diet is effective for horses with MFM. In contrast, due to impaired antioxidant function identified in Warmbloods, a high-fat diet may be detrimental as burning fat for energy generates more oxidants than burning carbohydrates.
Concentrates should contain 12-14% crude protein (CP), a moderate level (20-30%) of NSCs and fat content of 4-8%. This may vary for easy-keepers or horses that do not require extra concentrates added to the diet. [7]
Supplementation
High-quality amino acids should be supplemented in the diet to promote protein synthesis for muscle repair and improved muscle mass in affected horses.
Lysine, threonine and methionine are considered limiting amino acids in the equine diet. These are most commonly deficient leading to impaired protein synthesis. For all horses, adequately supplying these is required for optimally supporting protein synthesis.
Mad Barn’s Three Amigos supplement provides lysine, methionine and threonine, which support muscle function and repair.
Defects in cysteine metabolism have been noted in horses with MFM. [10] This can impact protein synthesis and antioxidant capacity in the body. Therefore, cysteine supplementation in the form of N-acetyl cysteine is often recommended for horses with MFM to increase antioxidant production in the body.
Whey-based protein is high in cysteine as well as the amino acids leucine and lysine which are important for protein synthesis.
Alternatively, cysteine can be supplemented directly. In one study, supplementation with 10 grams of N-acetyl cysteine (NAC) and 1.6g coenzyme Q10 increased antioxidants in the skeletal muscles of healthy Thoroughbreds. [2]
Acetyl-L-carnitine supplementation may also be of use in reducing oxidative stress and stimulating production of new mitochondria. [13]
Further research is required to determine whether these supplements benefit horses with MFM.
Exercise
Training programs for horses with MFM vary depending on the severity of exercise intolerance. Your horse may require more rest between exercise bouts or lower-intensity exercise.
Many horses benefit from a balance of work and rest to promote muscle recovery and tissue repair following exercise. A schedule of three work days followed by two rest days is believed to reduce stiffness and support post-exercise recovery. [7]
Avoid periods of complete inactivity for endurance and performance horses. Provide a consistent training routine to maintain conditioning and prevent MFM episodes.
When first beginning treatment, the total ride time should only be 15 – 20 minutes. As your horse adjusts to treatment, total ride time can be increased to 30 – 45 minutes.
Rest periods throughout the exercise should allow the horse to stretch their muscles.
Warm-Up
Before exercising your MFM horse, ensure they are properly warmed up. Lungeline work in a long, low frame for 5-15 minutes before riding is recommended to promote relaxation and lifting of the neck and back. [6][7]
Warming up at the canter is recommended once a treatment plan has been established to strengthen the muscles.
Stretching of the topline and engagement of the abdominal muscles is important during warm-up. To engage the abdominal and hind-end muscles, hill work and poles can be introduced gradually.
Prognosis
The prognosis for horses with MFM varies greatly. Many endurance and performance horses return to full-time work under appropriate management plans.
Affected horses may not be able to compete in the same level of sport, but may excel at a lower-level sport.
In some cases, horses with MFM must be retired from work due to exercise intolerance. However, low-intensity exercise, such as trail-riding or recreational riding, may be tolerable.
Frequently Asked Questions
Here are some frequently asked questions about myofibrillar myopathy in horses:
Myofibrillar myopathy is an exercise-related muscle disorder linked to abnormal desmin build-up inside muscle fibres. This disruption affects muscle contraction and can reduce stamina, coordination, and willingness to work. Horses with MFM may develop stiffness, muscle pain, poor performance, or gait abnormalities during or after exercise, especially following more demanding training sessions.
Warmbloods and Arabians are most commonly associated with myofibrillar myopathy, although cases have also been identified in Thoroughbreds, Quarter Horses, and Paso Finos. Warmbloods often lose strength and collection under saddle, while Arabians are more likely to experience tying-up episodes or severe cramping linked to exercise intolerance and muscle stress.
Signs of myofibrillar myopathy can include poor performance, muscle stiffness, reluctance to move forward, vague hindlimb lameness, trembling, heavy sweating, and tying-up episodes. Some horses also develop abnormal movement patterns or dark urine after exercise. Clinical signs may appear during work or several hours later and can vary between breeds and individual horses.
Myofibrillar myopathy can cause tying-up episodes, particularly in Arabian horses. Affected horses may develop painful muscle cramping, stiffness, sweating, and reluctance to move after exercise. Arabians with MFM are more likely to show elevated muscle enzymes during these episodes, while Warmbloods with the condition often show exercise intolerance without major enzyme increases in bloodwork.
Myofibrillar myopathy differs from PSSM2 because MFM involves structural damage and desmin accumulation within muscle fibres rather than abnormal glycogen storage alone. Both conditions can produce similar signs, including stiffness and poor performance, which makes diagnosis difficult. Muscle biopsy findings may overlap, and some horses previously labelled with PSSM2 may later receive an MFM diagnosis.
Current genetic tests do not reliably diagnose myofibrillar myopathy in horses. Available tests report variants linked to MFM, but studies have not found meaningful differences between affected horses and healthy controls. Diagnosis currently depends on clinical evaluation, muscle biopsy findings, and immunohistochemical staining to identify abnormal desmin accumulation within muscle tissue.
Diagnosis of myofibrillar myopathy usually involves a physical examination, muscle biopsy, and specialized staining techniques that identify desmin aggregates in muscle fibres. Veterinarians also rule out more common causes of poor performance, including lameness, neurological disease, ulcers, and tack problems. Immunohistochemical staining is currently considered the most accurate diagnostic method for confirming MFM.
Myofibrillar myopathy can cause vague hindlimb lameness and movement abnormalities that are difficult to localize during examination. Some horses appear stiff behind, resist transitions, or struggle to maintain a balanced canter without obvious joint or tendon injury. These subtle signs often resemble other performance problems, which can delay diagnosis and complicate treatment planning.
Diet plans for horses with myofibrillar myopathy focus on balanced nutrition, adequate protein intake, and support for muscle repair and antioxidant function. Concentrates often contain 12 - 14% crude protein with moderate starch, sugar, and fat levels. Amino acids such as lysine, methionine, and threonine are commonly added to support muscle protein synthesis and recovery.
N-acetyl cysteine is commonly used in horses with myofibrillar myopathy to support antioxidant production and reduce oxidative stress linked to muscle damage. Horses with MFM may have impaired cysteine metabolism, especially Arabians. Research in healthy Thoroughbreds found that supplementation with 10 grams of NAC and 1.6 grams of coenzyme Q10 increased antioxidant levels within skeletal muscle tissue.
Consistent exercise with planned recovery periods is often recommended for horses with myofibrillar myopathy. Many horses tolerate shorter, moderate-intensity sessions better than infrequent strenuous work. Initial rides may last 15 - 20 minutes before gradually increasing to 30 - 45 minutes as conditioning improves. Long periods of inactivity can worsen fitness loss and reduce exercise tolerance.
Warm-up routines are an important part of managing myofibrillar myopathy because gradual muscle engagement may reduce stiffness and improve movement quality during exercise. Lunging in a long, low frame for 5 - 15 minutes before riding is commonly recommended. Stretching exercises, hill work, and pole work may also support topline strength and abdominal muscle engagement over time.
Many horses with myofibrillar myopathy return to regular exercise with careful management of diet, conditioning, and recovery time. Some performance horses continue competing successfully, although they may perform better at a lower intensity than before diagnosis. Horses with more severe exercise intolerance may require retirement from high-level sport but still tolerate light recreational riding or trail work.
Summary
Myofibrillar myopathy (MFM) is a muscle disorder in which the supportive proteins inside muscle cells become disorganized and clump together instead of maintaining normal structure. These changes weaken how a horse's muscle fibers function under stress, reducing their ability to contract smoothly and recover after exercise.
- MFM disrupts the internal framework that keeps muscle fibers aligned and functioning during movement
- Horses can show stiffness, soreness, gait changes, sweating, tying-up episodes, or reluctance to move forward
- Because signs resemble other muscle diseases, confirmation requires a muscle biopsy examined for desmin clumping
- Management focuses on diet, targeted supplementation, conditioning, and workload adjustments to support muscle function
References
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- Williams, Z.J. et al. Integrated proteomic and transcriptomic profiling identifies aberrant gene and protein expression in the sarcomere, mitochondrial complex I, and the extracellular matrix in Warmblood horses with myofibrillar myopathy. BMC Genomics. 2021. View Summary
- Pesce, V. et al. Acetyl-L-carnitine supplementation to old rats partially reverts the age-related mitochondrial decay of soleus muscle by activating peroxisome proliferator-activated receptor gamma coactivator-1alpha-dependent mitochondrial biogenesis. Rejuvenation Res. 2010.
