Myosin Heavy Chain Myopathy (MYHM)

Myosin-Heavy Chain Myopathy (MYHM) is a genetic muscle disease that can result in two distinct clinical disease presentations: Immune-Mediated Myositis (IMM) and Nonexertional Rhabdomyolysis. They both involve muscle loss or damage and are linked to a mutation in the MYH1gene. This gene is involved in the normal functioning of the myosin protein in muscle cells. A horse with MYHM is prone to presenting with one or both MYHM diseases during their lifetime.
The IMM variant of MYHM is known to occur in the Quarter Horse, American Paint and Appaloosa and is a co-dominant mutation also known as dominant with variable penetrance, which means your horse only needs one copy to be affected, though some horses with the mutation may never experience symptoms. They must be exposed to a trigger to experience symptoms. Horses that are homozygous for the mutation are likely to experience more severe symptoms.

Immune-Mediated Myositis (IMM) is one form of clinical disease caused by MYHM, this results in infiltration of inflammatory cells, particularly lymphocytes, into muscle fibers and surrounding blood vessels. It is suspected to be the result of a response to a trigger. These triggers can include respiratory infections (especially Streptococcus), muscle damage, immune stimulants, vaccination against strangles, and muscle-damaging vaccinations. The immune system misinterprets the muscle cells as foreign and rapidly attacks them. Horses initially experience stiffness, weakness, and a decreased appetite followed by the rapid loss of 40% of muscle mass within 72 hours.

The second presentation of MYHM is Nonexertional Rhabdomyolysis and often presents as stiffness, like "tying up", and possible swelling of muscles along the back and haunches without exercise. Nonexertional rhabdomyolysis causes pain, muscle cramping, muscle damage and may or may not result in muscle loss.

Horses with the mutation exposed to environmental triggers can develop symptoms of the disease. Not all environmental risk factors are currently known. Therefore, it is impossible to say if or how a horse with the MYHM mutation will be affected. Horses affected by IMM or Nonexertional Rhabdomyolysis can recover but may have more frequent episodes.

Pubmed ID: 29510741

Year published: 2018

Omia ID: 2141

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