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→ Here you’ll find the IMM information for downloading and printing:

→ Immune-mediated muscle wasting often triggered by infections or vaccination

Also known as MYH1 Myopathy.


  • Muscle wasting (especially hindquarters)
  • Muscle stiffness
  • Weakness
  • Lymphocytes in muscular fibers

General Information:

  • IMM is triggered by environmental factors. In about 39% of reported IMM cases, horses had a history of recent infection or vaccination.
  • Mainly found in Quarter Horses and related breeds.
  • Affected horses usually show symptoms either before the age of 8 years or after the age of 17 years.
  • Muscle wasting may progress very quickly and last for several months.

Inheritance: autosomal dominant with incomplete penetrance.

→ Horses with one or two copies of the variant (My/n or My/My) are at high risk to develop IMM. Symptoms are triggered by environmental factors, such as infection or vaccination.

Possible Genotypes:

Genotype: The horse is: Effects:
n/n normal. The horse has no predisposition for IMM and therefore cannot pass it on to its offspring.
My/n at risk-affected (heterozygous). The variant will be passed on to its offspring with a probability of 50%.
My/My high-risk affected (homozygous). The variant will be passed on to all its offspring. All offspring will be affected (My/n).



  • This test is best used to confirm a diagnosis of IMM.  Because of the variable penetrance of this mutation (not all horses with the mutation will become sick), it is not yet clear how or if this test should be used for making breeding decisions.  If a horse has become sick, it should not be used for breeding.

Test information: This tests detects the change of a single basepair (chr11:52993878T>C; E321G) in the MYH1 gene.

Gianino 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. 33:897-901.

Valberg, S.J. et al. (2018). An E321G MYH1 mutation is strongly associated with nonexertional rhabdomyolysis in Quarter Horses. J Vet Intern Med.1-8.

Durward-Akhurst, S.A., Valberg, S.J. : Immune-Mediated Muscle Diseases of the Horse. Vet Pathol 55:68-75, 2018. Pubmed reference: 28129093. DOI: 10.1177/0300985816688755.

Finno et al. (2018). A missense mutation in MYH1 is associated with susceptibility to Immune-mediated myositis in Quarter Horses. Skelet Muscle 8(1):7.

Further information is available at Online Mendelian Inheritance in Animals.

Breeds affected:

Test #: H124

Samples: Blood, Hair

Special Price: 30 € (net)

Time required: 10-14 days

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