A syndrome called MYH-1 myopathy has been recognized in Quarter Horses and Quarter Horse-related breeds. While the frequency in the Quarter Horse population as a whole is only 4-7%, current studies reveal that 22% of the top reining Quarter Horse stallions are heterozygous; 16% of halter Quarter Horses are heterozygous, as are 17% of working cow horses. No barrel or racing Quarter Horses in this study were identified with the mutation.
The syndrome leads to rapid onset of muscle atrophy, especially along the epaxial and gluteal muscles. Up to two-thirds of muscle mass is lost within just days. An affected horse also experiences stiffness, weakness and general malaise. About half of the time, an inciting cause is identified, such as infection with Streptococcus equi, S. zooepidemicus, Anaplasmosis, Corynebacteria pseudotuberculosis, or subsequent to immunization with an equine influenza, equine herpesvirus-4, or strangles vaccine.
At the 2018 AAEP Convention in San Francisco, Stephanie Valberg, DVM, PhD, DACVIM, DACVSMR, spoke about this form of immune-mediated myositis (IMM). The syndrome is called MYH-1 myopathy or MYHM. [Further information is available in: Gionino, G.M.; Valberg, S.J., et al. Prevalence of the E321G MYH1 variant for immune-mediated myositis and non-exertional rhabdomyolysis in performance subgroups of American Quarter Horses. Journal of Veterinary Internal Medicine Nov 2018.]
MYH-1 myopathy seems to occur in young horses (less than 8 years old) and older horses (more than 17 years old), in particular. White blood cell counts, total protein and albumin are within normal limits unless the horse suffers from a concurrent infection. CK and AST increase and muscle biopsy reveals lymphocytic infiltration. CK elevations are not proportional to the amount of atrophy, and CK and AST eventually return to normal, although muscle atrophy persists.
Valberg explained that more than 50% of Quarter Horse skeletal muscle is composed of fast twitch fibers, with the inflammation mostly occurring in fast twitch IIx myofibers. Lymphocytes destroy these fibers and over time, affected muscle fibers become smaller.
This genetic mutation is not found in other breeds. In one study farm of Quarter Horses, 40% of the horses had the mutation. Heterozygous horses don’t tend to develop severe changes when muscle atrophy occurs, and there is good prognosis with return to full muscle mass when treated with prednisolone. Homozygous horses are most severely affected and have high recurrence rates and/or persistently reduced muscle mass.
The mutation interferes with normal myosin function in muscle. The MYH1 (myosin heavy chain 1) gene has a similar amino acid sequence to S. equi M protein, so exposure to S. equi might be sufficient to trigger MYHM. In a study of 165 Quarter Horses less than five years old that experienced non-exertional rhabdomyolysis (ER) with CK > 5000 U/L, 5-10% of the affected horses had the mutation. What is striking is that of the horses with non-ER and atrophy, 90% were positive for the mutation. Sixty percent of horses with severe rhabdomyolysis—but no atrophy—were also positive for the mutation.
In summary, Valberg stressed that this genetic mutation might be a very common cause of immune-mediated myositis and non-exertional rhabdomyolysis in Quarter Horses.
The genetic test can be performed on hair sent to the University of California, Davis.