Myofibrillar myopathy (MFM) is a newly identified muscle disorder that causes exercise intolerance in horses.

MFM is a genetic condition that results from the abnormal build-up of desmin in muscle tissue. Desmin is a protein that is important for muscle contraction.

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, horses with MFM do not have elevations in serum creatine kinase (CK) or aspartate transaminase (AST). [6]

MFM can present differently in individual cases, making diagnosing difficult without a muscle biopsy.

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 & stiffness
  • Tying-up or rhabdomyolysis
  • Reluctance to move forward
  • Vague hindlimb lameness
  • Tremors
  • Extreme sweating
  • Movement abnormalities
  • Pigmenturia or dark urine

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.

Mad About Horses
Join Dr. Chris Mortensen, PhD on an exciting adventure into the story of the horse and learn how we can make the world a better place for all equines.
Apple Podcasts Spotify