Immune Mediated Myositis & MYH1 Myopathy (MYHM)

Quarter Horse and related breeds are susceptible to developing rapid onset of muscle atrophy and severe muscle damage at rest (nonexertional rhabdomyolysis). An autoimmune muscle disease called immune-mediated myositis (IMM) can cause this severe 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 infiltration of inflammatory cells, particularly lymphocytes, into muscle fibers and surrounding blood vessels, with preferential targeting of the gluteal (rump) and epaxial (along the vertebral column) muscles. 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. About 39% of IMM horses have a history of exposure to a triggering factor such as Streptococcus equi subsp. equi infection, respiratory virus or vaccination with influenza, Equine Herpes Virus 4 or Streptococcus equi subsp. equi.
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.
IMM and nonexertional rhabdomyolysis belong to the group of muscle diseases known as MYH1 myopathy (MYHM). The mode of inheritance for MYHM is autosomal dominant with variable penetrance, which means that both males and females are affected and not all horses that have 1 (N/My) or 2 copies (My/My) of the genetic variant will develop IMM or nonexertional rhabdomyolysis. Horses with two copies (My/My) may be more severely affected.