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Breeds: American Indian, Appaloosa, Appaloosa Cross, Appaloosa/Friesian, Appaloosa/Quarter Horse, Appaloosa/Thoroughbred, Appendix, AraAppaloosa, Arabian/Paint, Arabian/Quarter Horse, Australian Stock Horse, Azteca, Baroque Pinto, Canadian, Cleveland Bay, Clydesdale/Paint Cross, Colonial Spanish Horse, Crossbred, Grade Horse, Grade Pony, Miniature Appaloosa, Mixed Breed, Mixed Breed (Horse), Morgan Horse, Mustang, Origine Constatée, Paint Cross, Paint Horse, Paint Horse/Thoroughbred, Paint/Warmblood, Ponies of America, Pura Raza Espanola, Quarter Cross, Quarter Horse, Quarter Pony, Quarter/Paint Horse, Quarter/Stock Horse, Quarter/Thoroughbred, Quarter/Warmblood, Spanish Barb, Stock Horse, Stonewall Sporthorse, Sugarbush Harlequin Draft, Unspecified, Welsh Mountain/Quarter Horse
Malignant Hyperthermia, or MH, is a genetic muscle disorder that affects Quarter Horses and related breeds. Horses with the MH mutation may not show any physical signs of the disorder until triggered by exposure to anesthesia or extreme exercise or stress. Symptoms include:
- high body temperature
- increased heart rate
- high blood pressure
- muscle rigidity
Symptoms develop rapidly. If not treated quickly, this condition can be fatal.
MH is inherited as an autosomal dominant trait. This means that the disorder can be passed on even if only one parent has the defective gene. The mutation can be present along with another genetic disorder, Polysaccharide Storage Myopathy (PSSM). If a horse also has PSSM, the symptoms associated with MH can become more severe. Therefore, testing for both PSSM and MH is recommended, especially for Quarter Horse breeds.
Although this condition is rare, testing for MH is recommended in case a horse must undergo anesthesia. Horses that are known to have the MH mutation can be given medication prior to administering anesthesia to help reduce the severity of the symptoms. If a horse with one copy (carrier) of MH is bred to a horse with no copies, there is a 50% chance per horse of them developing MH symptoms. If two carriers are bred, there is a 75% chance per horse born that they will develop MH. If a horse has two copies of the MH mutation, there is a 100% chance per every horse born that they will develop MH.
Aleman M, Nieto JE, Magdesian KG. J Vet Intern Med. 2009 Mar-Apr;23(2):329-34. doi: 10.1111/j.1939-1676.2009.0274.x. Epub 2009 Feb 11. [PMID: 19220734]
|MH/MH||Affected: Horse has two copies of the MH gene mutation and is likely to exhibit signs of the disorder. Horse will pass a copy of the gene on to all offspring.|
|n/MH||Affected: Horse has one copy of the MH gene mutation and is likely to exhibit signs of the disorder. Horse has a chance to pass the gene on to any offspring.|
|n/n||Clear: Horse is negative for the MH gene mutation.|