Myosin-Heavy Chain Myopathy (MYHM)

MYHM is a genetic muscle disease that can result in two distinct clinical disease presentations that both involve muscle loss or damage and are linked to the same genetic variant. A horse with MYHM is prone to presenting with one or both during their lifetime, while some horses with the mutation may never experience symptoms.

MYHM is a relatively newly discovered genetic disorder. This mutation makes horses susceptible to disease. Horses with the mutation exposed to environmental triggers will 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. This makes it important to have your horse tested, as management is key to preventing an episode.

Immune-Mediated Myositis (IMM) is one form of clinical disease caused by MYHM, this results in muscle atrophy that is suspected to be the result of a response to a vaccine or infectious agent such as strangles. 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 affected by IMM or nonexertional rhabdomyolysis can recover but may have more frequent episodes.

MYHM is a dominant mutation, which means your horse only needs one copy to be affected, though not all horses with the mutation will become affected. They must be exposed to a trigger to experience symptoms. Horses that are homozygous (My/My) are likely to experience more severe symptoms.

Approximately 7% of quarter horses have the MYHM variant. It is most commonly found in reining horses, cowhorses, and halter horses.


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