ImmuneNoMediated Myositis (IMM)

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.

Publications

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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.

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

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

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