Malignant hyperthermia


Good news!   

Your horse is clear of malignant hyperthermia and does not have this gene mutation.  

Your horse cannot pass on the malignant hyperthermia genetic mutation. There are no breeding or performance implications.

What is malignant hyperthermia?

Malignant hyperthermia (MH) is a genetic muscle disorder that causes acute tying up episodes, paralysis, and cardiac arrest, as well as dangerously high fevers. If an affected horse also has polysaccharide storage myopathy (which also causes tying up, constant twitching and muscle weakness) the symptoms of malignant hyperthermia may be more serious. 

Episodes are sometimes brought about after stressful events or exercise.

Genomic Details

Malignant hyperthermia is influenced by a mutation in the RYR1 gene. This gene is responsible for making a protein called ryanodine receptor 1, which releases calcium into muscle cells. When a horse has the disease gene version of RYR1 too much calcium is released leading symptoms such as muscle rigidity and irregular heart rate.

The RYR1 gene does not cause malignant hyperthermia on its own. The gene interacts with anesthesia drugs, stress, and excitement to cause the disease.

Malignant hyperthermia is a dominant genetic disease. This means only one gene mutation from one parent is needed for the disease to occur.

The mutation that causes malignant hyperthermia is a switch in one of the four base pairs that make up a DNA molecule. Cytosine (C) is replaced by Guanine (G) changing the RYR1 gene. Therefore, a horse’s genetic result for malignant hyperthermia is one of three possibilities:

  • C/C: Normal. Does not have the disease. Does not have the mutation.

  • C/G: Affected. Has one disease mutation from one parent. This is enough to cause the disease. It might be passed to their foal.

  • G/G: Affected. Has two disease gene mutations, one from each parent. One mutation will be passed to their foal.