Immune Mediated Myositis and MYH1 Myopathy is an incomplete dominant autoimmune disorder which causes muscular atrophy and stiffness in Quarter Horses. Horses with two copies of the mutation associated with IMM are more likely to develop symptoms than horses with a single copy, although environmental factors can play a role. IMM typically affects horses younger than eight years old and older than seventeen years old. IMM episodes typically last several days to several weeks and can be fatal if mismanaged.

An affected horse's immune system attacks the horse's skeletal muscles. This attack causes the muscular atrophy and stiffness seen in horses with IMM. Horses with IMM have a mutated MYH1 gene, which codes for a protein called 2X myosin. An affected horse's immune system is unable to tolerate the presence of this protein, leading to an attack on the muscles. Certain infections, such as a Streptococcus infection, and certain vaccines, like the influenza vaccine, are thought to potentially trigger symptoms of IMM. After an immune episode, muscle mass typically returns to the horse within a few months with proper care.

Symptoms include:

Muscular atrophy in the back and rear, Depression, Loss of appetite, Fever, Stiffness, Difficulty standing.