Immune Mediated Myopathy (IMM)

Description

Immune Mediated Myositis (IMM) is an inherited autoimmune muscle disorder in Quarter Horses and related breeds that causes rapid muscle atrophy and stiffness. It is associated with a missense variant (E321G) in the MYH1 gene, which codes for the type 2X myosin heavy chain found in skeletal muscle. The variant is inherited with incomplete dominance: horses with one copy are susceptible, and horses with two copies are more likely to develop signs and to have recurring episodes. Environmental factors also play a role. IMM most often affects horses aged 8 years or younger or 17 years or older.

In affected horses, the immune system attacks muscle fibers containing the altered myosin, causing inflammation and loss of muscle mass, most noticeably along the back and hindquarters. Episodes may be triggered by infections such as Streptococcus equi or by vaccination, including influenza vaccines, although in many cases no trigger is identified. Other signs can include stiffness, difficulty standing, depression, loss of appetite, and fever. Episodes typically last from several days to several weeks and can be fatal if not properly managed.

The same variant is also strongly associated with non-exertional rhabdomyolysis, in which young horses suddenly develop severe muscle breakdown with stiffness, pain, and an inability to rise, with or without muscle atrophy. Because the variant causes both conditions, they are grouped together as MYH1 myopathy, also called myosin heavy chain myopathy (MYHM). The variant is present in about 6.8% of Quarter Horses and is more common in some performance lines, particularly reining and working cow horses.

IMM cannot be cured, but it can be managed. Corticosteroids are the primary treatment for an active episode and can take effect within about 72 hours, and early treatment improves the chance of recovery. Recovering horses benefit from a diet with high-quality protein to help rebuild muscle, which usually returns within a few months with proper care. If a vaccine is suspected as the trigger, identifying it is important for managing future vaccinations. Your veterinarian should be informed if a horse tests positive for the variant.

Because the variant is inherited, testing before breeding helps owners manage risk. A horse with two copies will pass the variant to all of its offspring, and a horse with one copy will pass it to about half.

Possible results

Breeds Affected

American Indian, Appaloosa, Appaloosa Cross, Appaloosa/Friesian, Appaloosa/Quarter Horse, Appaloosa/Thoroughbred, Appendix, AraAppaloosa, Arabian/Paint, Arabian/Quarter Horse, Australian Stock Horse, Azteca, Baroque Pinto, Canadian, Cleveland Bay, Clydesdale/Paint Cross, Colonial Spanish Horse, Crossbred, Curly Horse, Grade Horse, Grade Pony, Miniature Appaloosa, Mixed Breed (Horse), Morgan Horse, Origine Constatée, Paint Cross, Paint Horse, Paint Horse/Thoroughbred, Paint/Warmblood, Ponies of America, Pura Raza Espanola, Quarter Cross, Quarter Horse, Quarter Pony, Quarter/Paint Horse, Quarter/Stock Horse, Quarter/Thoroughbred, Quarter/Warmblood, Spanish Barb, Spanish Mustang, Stock Horse, Stonewall Sporthorse, Sugarbush Harlequin Draft, Thoroughbred

Reference

A missense mutation in MYH1 is associated with susceptibility to immune-mediated myositis in Quarter Horses.

[PMID: 29510741]

An E321G MYH1 mutation is strongly associated with nonexertional rhabdomyolysis in Quarter Horses.

[PMID: 30079499]