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Immune-Mediated Myositis

Immune-Mediated Myositis (MY & IMM)

Gene or region: MYH1

Reference allele: G

Mutant allele: A

Affected Breeds: Quarter Horses

Research Confidence:                            Discovery*, Not yet validated in larger population, please exercise caution and ask your veterinarian for further information


What it does:


Immune-mediated myositis (IMM) is an autoimmune disease which, when triggered by an infection, or certain vaccinations (i.e. influenza) can cause severe muscle loss (atrophy) within a period of 3 days (up to 40% in 72 hours). Target areas of atrophy include the gluteal and vertebral regions (hind end and top line) as the characteristic inflammation attacks the muscle fibers and surrounding blood vessels.  Symptoms include general malaise, muscle stiffness and overall weakness, not associated with exercise, and usually affects horses older than 17 or younger than 8 years of age.


MYH1 or Myosin Heavy Chain 1 is a gene thought to have an altered amino acid which disrupts the function of the myosin cells in muscle tissue. The initial IMM study (cited below) indicates autosomal dominance, meaning heterozygous horses (IMM/n) may be at risk in addition to homozygous horses (IMM/IMM). As IMM is thought to have variable penetrance not all horses who carry copies of IMM will exhibit symptoms nor is there a particular increased affect for males versus females (both sexes appear to be equally affected) Environmental factors, infection exposure (such as Equine Herpes Type 4, Streptococcus equi, respiratory virus, etc.), which may act as triggers were also noted in the health histories of 39% of the IMM horses in the study.


* Additional studies with larger equine populations will continue to increase validation into the exact function of this genetic region and corresponding health of horses. As with any medical test, genetic test or identification of genetic sequences within your horse we recommend speaking with your veterinarian regarding any questions you may have for horse health or care. As additional horses are studied this information will be updated – your help and feedback in this ongoing study is much needed and appreciated!


-/-     No Immune Mediated Myositis (MY) variants detected. Normal susceptibility for IMM or nonexertional rhabdomyolysis

+/-    MY/n - One Immune Mediated Myositis (MY) variants detected. Moderate risk of IMM following infection, vaccination, or non-exertional rhabdomyolysis.


+/+   MY/MY – Two Immune Mediated Myositis (MY) variants detected. Increased risk of IMM following infection, vaccination, or non-exertional rhabdomyolysis.



Finno CJ, Gianino G, Perumbakkam S, Williams ZJ, Bordbari MH, Gardner KL, Burns E, Peng S, Durward-Akhurst SA, Valberg SJ. (2018). A missense mutation in MYH1 is associated with susceptibility to Immune-mediated myositis in Quarter Horses. Skelet Muscle 8(1):7. doi: 10.1186/s13395-018-0155-0.

Durward-Akhurst SA, Valberg SJ. (2018). Immune-Mediated Muscle Diseases of the Horse. Vet Pathol 55(1):68-75.

Lewis SS, Valberg SJ, Nielsen IL. (2007). Suspected immune-mediated myositis in horses. J Vet Intern Med 21:495-503.

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