(Appaloosa, Quarter Horse, Paint Horse)plus
autosomal dominant with variable penetrance
Appaloosa, Paint Horse, Quarter Horse
HeredityAn autoimmune muscle disease called immune-mediated myositis (IMM) can cause severe muscle atrophy, which can result in the loss of 40% of muscle mass within 72 hours in Quarter Horse and related breeds. IMM is characterized by stiffness, weakness and nonspecific malaise. Affected horses are usually 8 years and younger or 17 years and older, with no sex predilection. Environmental factors combined with genetic susceptibility are important triggers for the development of muscle atrophy or severe rhabdomyolysis. The described variant causes an amino acid change detrimental to normal function of the myosin protein in muscle cells. This variant is associated with increased susceptibility to develop IMM characterized by significant muscle atrophy. Another clinical presentation of the MYH1 variant in young Quarter Horses is severe, sudden muscle damage not associated with exercise (nonexertional rhabdomyolysis). Horses with nonexertional rhabdomyolysis do not necessarily have muscle atrophy.