Immune-mediated myositis in horses

Immune-mediated myositis (IMM), also known as "myosin-heavy chain myopathy", is a genetic skeletal muscle disease that manifests itself by sudden and recurrent loss of muscle mass, particularly in the dorsal muscles and thighs. The diagnosis is usually confirmed histologically - the typical finding is the presence of lymphocytes in the affected muscle fibres.

The disease is caused by a missense mutation c.959A>G in the MYH1 gene encoding myosin heavy chain 2X. The mode of inheritance of this mutation is autosomal, but the exact type of inheritance (dominant vs. recessive) has not been clearly confirmed yet. However, the presence of the mutation alone does not always indicate the onset of the disease - often an external factor, such as infection or vaccination, triggers the inflammatory reaction.

The mutation is most found in the Quarter Horse breed, but has also been reported in Belgian Cold Blood Horses and Welsh ponies.

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Reference:

Finno, C.J., Gianino, G., Perumbakkam, S., Williams, Z.J., Bordbari, M.H., Gardner, K.L., Burns, E., Peng, S., Durward-Akhurst, S.A., Valberg, S.J. : A missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses. Skelet Muscle 8:7, 2018.