Malignant Hyperthermia (MH)

MH is a genetic mutation that causes a life-threatening condition triggered by certain anesthesia drugs such a halothane or isoflurane. MH can also be triggered by stress or excitement. During an episode a horse with MH will release uncontrolled amounts of calcium into the bloodstream, which results in painful muscle cramps, extremely high temperature up to 113 degrees Fahrenheit, irregular heart rhythm, excessive sweating and shallow breathing. A horse that has PSSM or MYHM in conjunction with MH will likely have much more severe episodes.

MH is a dominant mutation. This means that your horse only needs one copy of MH to be affected.

The percentage of horses affected is unknown, though it is most commonly found in halter horses.


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