MYHM : Myosin-Heavy Chain Myopathy

As of 1 September 2023 Classified Stallion owners are required to have tested for MYHM and have their results recorded with AQHA-NZ.

Breeding mares will be required to be tested by 31 August 2025 and have their results recorded with AQHA-NZ.

AQHA-NZ have put together a quick fact sheet regarding MYHM – please click here to read information regarding this. Testing for MYHM is available in New Zealand at Massey University (MYHM/IMM Test). Massey are offering a discount to AQHA-NZ members until the end of August 2023. Contact the office for the code if you have not received the email with this information.

AQHA also have a fantastic resource available with full information regarding MYHM – click here to view.

Affects: approximately 7% of Quarterhorses. Most commonly found in reining horses, cowhorses and halterhorses


  • 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.