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MH is a muscle disorder that causes an increased rate of metabolic activity usually associated with administration of certain types of anaesthetic gases

Full name

Malignant hyperthermia

Breeds known to be affected

Quarter Horse and related breeds

Inheritance pattern

Autosomal dominant


MH is a muscle disorder that may only become apparent if the horse is subjected to an extreme stress or exposed to a halogenated anaesthetic. When exposed, the mutation triggers the release of excess calcium in skeletal muscle cells causing a high temperature (hence the name), increased heart rate and blood pressure, sweating and muscle rigidity. MH is frequently fatal. MH sometimes occurs in horses which are also positive for PSSM1, causing them to have more severe tying up symptoms.

MH is rare and only found in some Quarter Horse and paint families; however, because it is potentially fatal it is recommended all possible carriers be tested before undergoing anaesthesia. MH is associated with a mutation in the RyR1 gene and is a dominant trait, meaning a horse only needs 1 copy of the mutation (MH/n) to be affected.

Interpretation of results

n/n: Horse does not carry the mutation associated with MH.

MH/n: Horse has one copy of the mutation associated with MH.

MH/MH: Horse has two copies of the mutation associated with MH.


Aleman et al. Association of mutation in the ryanodine receptor 1 gene with equine malignant hyperthermia. Muscle Nerve 2004; 30:356-65 doi:10.1002/mus.20084

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