Myosin Heavy Chain Myopathy (MYHM)
Myosin heavy chain myopathy (MYHM) is a disorder that impacts horses of Quarter Horse-related breeds, such as American Paint, Appaloosa, and their crosses, due to a specific genetic mutation in the MYH1 gene. MYHM presents as two overlapping clinical conditions: nonexertional rhabdomyolysis and immune-mediated myositis (IMM)
Immune mediated Myositis & MYH1 Myopathy (IMM and MYH1 Myopathy)
Quarter Horse and related breeds are susceptible to developing rapidly progressing form of muscle atrophy and severe damage to muscles at rest known as nonexertional rhabdomyolysis. The severe atrophy can be caused by an autoimmune muscle disease called immune-mediated myositis (IMM), which can result in the loss of 40% of muscle mass within 72 hours in Quarter Horse and related breeds. The disease is characterized by infiltration of inflammatory cells, in particular the lymphocytes, into the muscles and surrounding the muscle blood vessels, with preference to target the rump and the muscles alongside the vertebral column. IMM is characterized by stiffness, weakness and nonspecific malaise. The disease affects either horses who are 8 years and younger or 17 years and older. IMM maybe triggered by factors such as Streptococcus equi subsp. Equi infection, respiratory virus or vaccination with influenza, Equine Herpes Virus 4 or Streptococcus equi subsp. Equi.
Researchers at Michigan State University and University of California-Davis identified a mutation in the Myosin Heavy Chain 1 (MYH1) gene that is associated with increased susceptibility to develop IMM in Quarter Horses and related breeds which is characterized by significant muscle atrophy.
IMM and nonexertional rhabdomyolysis belong to the group of muscle diseases known as MYH1 myopathy (MYHM).
