Skip to main content
« Back to the lexicon overview

Malignant Hyperthermia (MH Horse)

Malignant hyperthermia (MH) causes anesthesia- or stress-induced muscle rigidity with fever and respiratory distress. MH results in death of the horse in approximately 34% of cases.

The disease occurs in many breeds. The inheritance is autosomal dominant.


Genetic Test: available in Shop


caused by halogenated anesthetics or stress:


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

General Information

  • Malignant Hyperthermia (MH) causes skeletal muscles to degenerate, which is associated with low blood pH (acidemia).
  • 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.
  • MH may worsen symptoms of PSSM (Polysaccharide Storage Myopathy).

Breeds affected

Most breeds

Test Information

This test detects a single basepair change (c.7360C>G) in exon 46 of the RYR1 gene.


Test in Shop

Genotype and Lab Report

Inheritance: autosomal dominant

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




n/n = normal

The horse has no copies of the genetic variant causative for MH and therefore cannot pass it on to its offspring.


n/MH = 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 (n/MH or MH/MH) should not be used for breeding.


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.

« Back to the lexicon overview