Myosin Heavy Chain Myopathy (MYHM)

Horses affected by the mutation are susceptible to sudden attacks of muscle wasting due to IMM, in which up to 40% of the horse’s topline and hindquarter musculature can be lost in as little as one week. IMM attacks are generally triggered by something that strongly influences the immune system. These triggers can include respiratory infections (especially Streptococcus), muscle damage, immune stimulants, vaccination against strangles, and muscle-damaging vaccinations. As anything that influences the immune system can trigger an attack in certain situations, it is not always possible to directly connect the clinical symptoms to any specific trigger.

Muscle mass lost to IMM atrophy can recover over a period of months. However, subsequent attacks are possible.

Horses carrying the MHYM mutation are also more susceptible to attacks of Non-Exertional Rhabdomyolysis, which is characterized by pain, muscle stiffness and an inability to rise.

IMM is a co-dominant disorder; carriers suffer a less severe form of the disease than fully affected horses.