Immune-Mediated Myositis: A Genetic Reason For Emaciation In Horses

This recently described syndrome, which affects mainly American Quarter Horses, can be confirmed using genetic tests, says Dr Mac.

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Immune-mediated myositis (IMM) occurs when the skeletal muscles are damaged because of an immune-related reaction to a disease or vaccine. Although previously diagnosed in dogs and humans, the causes and signs in horses were first described in detail in 2016. As it has been shown to be related to a specific gene, breeders can now have their stud animals tested.

The disease has been found mainly in American Quarter Horses, but also in English Thoroughbreds, Warmbloods and Arabians.

Horses with IMM, especially those with two copies of the gene MYH1, produce the protein 2Xmyosin. The immune system of an affected horse is stimulated to attack this protein in its own skeletal muscles after vaccination or infection with certain diseases, such as Streptococcus equi, S. zooepidemicus, and herpes 1. Certain vaccines, such as that used against equine influenza, have also been implicated.