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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, Curly Horse, 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, Thoroughbred, Unspecified, Welsh Mountain/Quarter Horse
Description
Malignant Hyperthermia (MH) is an inherited muscle disorder found in Quarter Horses and related breeds. It is caused by a mutation in the RYR1 gene (C7360G), which controls the release of calcium inside muscle cells. Horses with the mutation may show no signs at all until an episode is triggered by general anesthesia (especially inhalant anesthetics), extreme exercise or stress.
During an episode, signs develop rapidly and can include:
- High body temperature
- Increased heart rate
- High blood pressure
- Heavy sweating
- Acidosis (a buildup of acid in the blood)
- Muscle rigidity
Without prompt treatment, an MH episode can be fatal.
MH is inherited as an autosomal dominant trait, so a horse needs only one copy of the mutation to be at risk. A horse with one copy passes the mutation to about half of its foals, and a horse with two copies passes it to all of them. When two horses with one copy each are bred, about 75% of their foals will inherit at least one copy.
MH can occur together with Polysaccharide Storage Myopathy Type 1 (PSSM1). Horses with both mutations may have more severe signs, so testing for both is recommended, especially in Quarter Horses and related breeds.
Although MH is rare, testing is recommended before a horse undergoes anesthesia. If a horse is known to carry the mutation, your veterinarian can adjust the anesthetic plan and give preventive medication to reduce the risk of an episode.
Possible results
| Genotype | Description |
|---|---|
| MH/MH | At risk: Horse has two copies of the MH mutation and is at risk of MH episodes. It will pass one copy of the mutation to every offspring. |
| N/MH | At risk: Horse has one copy of the MH mutation and is at risk of MH episodes. Each offspring has a 50% chance of inheriting the mutation. |
| N/N | Clear: Horse does not carry the MH mutation and cannot pass it on to offspring. |
Breeds Affected
American Quarter Horse, American Paint Horse, Appaloosa and other Quarter Horse-related breeds
Reference
Malignant hyperthermia associated with ryanodine receptor 1 (C7360G) mutation in Quarter Horses.
[PMID: 19220734]