→ 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
- 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.
|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.