UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory

IMM and MYH1 Myopathy in Quarter Horse and Related Breeds


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.

Researchers at Michigan State University and University of California-Davis identified a mutation in the Myosin Heavy Chain 1 (MYH1) gene (Chr11:52,993,878T>C) that causes an amino acid change detrimental to normal function of the myosin protein in muscle cells. This mutation is associated with increased susceptibility to develop IMM in Quarter Horses and related breeds characterized by significant muscle atrophy. Another clinical presentation of the MYH1 mutation 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 mutation will develop IMM or nonexertional rhabdomyolysis. Horses with two copies (My/My) may be more severely affected. The frequency of the MYH1 mutation in the general Quarter Horse population is about 4%. About 7.5% of Quarter Horses have 1 copy of the mutation. The mutation frequency is higher in the reining (13.5%), working cow (8.5%) and halter (8%) categories, and not observed in barrel and racing categories.

The Veterinary Genetics Laboratory offers a test for IMM and MYH1 myopathy (MYHM). Results benefit clinicians by assisting with the diagnosis of IMM or nonexertional rhabdomyolysis cases suspected to be MYHM. The test assists breeders wanting to identify breeding stock that have 1 or 2 copies of the mutation in order to design appropriate breeding strategies that avoid producing at-risk offspring and help reduce the incidence of the disease in the breed.

Testing is recommended for Quarter Horses, Quarter Horse crosses, and related breeds with Quarter Horse influence.

Allow 2-6 business days for results.

Results reported as:

N/N No copies of the MYHM mutation. Horse does not have increased susceptibility for IMM or nonexertional rhabdomyolysis.
N/My 1 copy of the MYHM mutation is present. Horse may develop IMM following infection or vaccination, or nonexertional rhabdomyolysis. Horse can pass on the mutation to 50% of offspring.
My/My 2 copies of the MYHM mutation are present. Horse is at risk and may develop IMM following infection or vaccination, or nonexertional rhabdomyolysis. Horses will pass on the mutation to all offspring.


Finno CJ, Gianino G, Perumbakkam S, Williams ZJ, Bordbari MH, Gardner KL, Burns E, Peng S, Durward-Akhurst SA, Valberg SJ. (2018). A missense mutation in MYH1 is associated with susceptibility to Immune-mediated myositis in Quarter Horses. Skelet Muscle 8(1):7. doi: 10.1186/s13395-018-0155-0.

Lewis SS, Valberg SJ, Nielsen IL. (2007). Suspected immune-mediated myositis in horses. J Vet Intern Med 21:495-503.

Durward-Akhurst SA, Valberg SJ. (2018). Immune-Mediated Muscle Diseases of the Horse. Vet Pathol 55(1):68-75.

Veterinary Genetics Laboratory, Tel 530-752-2211, Email VGL