Immune-mediated myositis, IMM

 

Gene: MYH1

Transmission: Autosomal dominant, variable expression

For an autosomal dominant genetic disease, an animal must have at least one copy of the mutation in question to be at risk of developing the disease. Animals with two copies of the mutation generally have more severe symptoms and an earlier onset of the disease than animals with just one copy of the mutation. One or both of the parents of an animal with the mutation has one or two copies of the mutation. Animals that have one or two copies of the mutation can pass the mutation on to future generations.

Mutation: Substitution, MYH1 gene; c.959, A>G, exon 9

Medical system: Muscular, immune

Breed: Quarter Horse

Age of onset of symptoms: Horses 8 years and younger or 17 years and older

Immune-mediated Myositis (IMM) is a genetic disorder that particularly affects the Quarter Horse. Horses with IMM have symptoms such as rapid muscle atrophy, weakness, and body stiffness. In less than 72 hours, affected horses can experience a loss of muscle mass of up to 40%. Genetics, the environment and exercise are all components of the disease. The onset of IMM can be due to exercise that is too intense, rhabdomyolysis (rapid muscle breakdown) not connected to exercise or following a recent infection. The presence of an infection stimulates the animal’s immune system and makes it more likely to develop IMM. Horses with IMM can usually be treated with corticosteroids. Fast treatment leads to a better prognosis in the animal. In severe cases, immune-mediated myositis may require urgent intervention.

 

References:

OMIA link: [2141-9796]

Faccin M, Landsgaard KA, Milliron SM, et al. (2023) Myosin heavy-chain myopathy in 2 American quarter horses. Vet Pathol :3009858231204253.  [pubmed/37818977]

Aleman M, Scalco, R, Malvick J, et al. (2022) Prevalence of genetic mutations in horses with muscle disease from a neuromuscular disease laboratory. J Equine Vet Sci 118:104129.  [pubmed/36150530]

Valberg SJ, Schultz AE, Finno, CJ, et al. (2022) Prevalence of clinical signs and factors impacting expression of myosin heavy chain myopathy in Quarter Horse-related breeds with the MYH1 E321G mutation. J Vet Intern Med 36:1152-1159. [pubmed/35426178]

Gianino GM, Valberg SJ, Perumbakkam S, et al. (2019) Prevalence of the E321G MYH1 variant for immune-mediated myositis and nonexertional rhabdomyolysis in performance subgroups of American Quarter Horses. J Vet Intern Med. (e-pub). [pubmed/30623495]

Finno CJ, Gianino G, Perumbakkam S, et al. (2018) A missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses. Skelet Muscle 8(1):7. [pubmed/29510741]

Valberg SJ, Henry ML, Perumbakkam S, et al. (2018) An E321G MYH1 mutation is strongly associated with nonexertional rhabdomyolysis in Quarter Horses. J Vet Intern Med. 32(5):1718-1725. [pubmed/30079499]

Lewis SS, Valberg SJ, Nielsen IL. (2007) Suspected immune-mediated myositis in horses. J Vet Intern Med 21(3): 495-503. [pubmed/17552457]

 

Contributed by: Marilie Brouillard and Fannie Pelletier, Class of 2027, Faculty of Veterinary Medicine, University of Montreal.