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→ Fever, muscle rigidity and respiratory distress triggered by anaesthetics or other stress.

Symptoms caused by halogenated anesthetics or stress:

  • Hyperthermia (fever)
  • Muscle rigidity
  • Respiratory distress
  • Excessive sweating
  • Increased hematocrit
  • Increased heart rate

General Information:

  • MH causes skeletal muscles to degenerate, which is associated with low blood pH (acidemia).
  • MH may worsen symptoms of PSSM (Polysaccharide Storage Myopathy).
  • In 34% of cases, MH results in death of the affected horse. Therefore, horses should be tested, especially prior to surgery or medical treatment.
  • Symptoms are not only triggered by anesthetics but also by stress and/or exercise.

Inheritance: autosomal dominant

→ Horses with one or two copies of the variant (MH/n or MH/MH) are affected.

Possible Genotypes:

Genotype The horse is:
n/n normal. The horse has no copies of the genetic variant causative for MH and therefore cannot pass it on to its offspring.
MH/n affected (heterozygous). The genetic variant will be passed on to the offspring with a probability of 50%. These 50% will be affected (heterozygous).
MH/MH affected (homozygous). The genetic variant will be passed on to all offspring. All offspring will be affected (heterozygous).



  • Affected horses (MH/n or MH/MH) should not be used for breeding.

Test information: This test detects a single basepair change in exon 46 of the RYR1 gene.

Aleman, M., Riehl, J., Aldridge, BM., LeCouteur, RA., Stott, JL., Pessah, IN.: Association of a mutation in the ryanodine receptor 1 gene with equine malignant hyperthermia. Muscle Nerve 30:356-65, 2004. Pubmed reference: 15318347. Doi: 10.1002/mus.20084.

Aleman, M., Nieto, JE., Magdesian, KG.: Malignant hyperthermia associated with ryanodine receptor 1 (C7360G) mutation in Quarter Horses. J Vet Intern Med 23:329-34, 2009. Pubmed reference: 19220734. Doi: 10.1111/j.1939-1676.2009.0274.x.

McCue, ME., Valberg, SJ., Jackson, M., Borgia, L., Lucio, M., Mickelson, JR.: Polysaccharide storage myopathy phenotype in quarter horse-related breeds is modified by the presence of an RYR1 mutation. Neuromuscul Disord 19:37-43, 2009. Pubmed reference: 19056269. Doi: 10.1016/j.nmd.2008.10.001

Further information is available at Online Mendelian Inheritance in Animals.

Breeds affected:

Most breeds

Test #: H110

Samples: Blood, Hair

Price: 35 € (net)

Time required: 10-14 days

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