horses running in field

Malignant Hyperthermia (MH)


  • MH is a rare inherited autosomal dominant disease in Quarter Horse and related breeds.
  • It causes a life-threatening condition primarily triggered by anesthesia drugs.
  • A genetic mutation has been identified and a DNA test is available.
  • Breeders should utilize the genetic test to avoid producing affected animals. Horses with the MH mutation should not be bred.

What is malignant hyperthermia?

Malignant hyperthermia (MH) is a rare inherited autosomal dominant disease. It causes a life-threatening condition in susceptible horses triggered by anesthesia drugs (such as halothane, isoflurane, and succinylcholine), and occasionally by stress or excitement. This genetic condition has been identified in Quarter Horses and American Paint Horses. A mutation in the ryanodine receptor 1 (RYR1) gene results in an excessive release of calcium inside skeletal muscle cells. The increase in muscle metabolism raises the body temperature and triggers a potentially fatal hyper-metabolic state.  Presence of the MH mutation can result in more severe clinical tying-up in horses that also have the PSSM1 mutation.

What are the clinical signs of malignant hyperthermia?

Signs of MH episodes include muscle contracture (rigidity), elevated body temperature, elevated heart rate, irregular heart rhythm, excessive sweating and shallow breathing.

How is malignant hyperthermia diagnosed?

Malignant hyperthermia can be difficult to diagnose because it resembles other conditions seen in horses under anesthesia. Quarter Horses and related breeds should be DNA tested prior to general anesthesia for surgery or other procedures so appropriate steps can be taken to prevent or minimize episodes.

How is malignant hyperthermia treated?

Dantrolene (4 mg/kg) may be administered to susceptible horses 30-60 min prior to anesthesia. During MH episodes, horses can be treated with alcohol, chilled intravenous fluids with sodium bicarbonate and mechanical ventilation.

What is the prognosis for malignant hyperthermia?

The prognosis for horses that have an MH episode while under anesthesia is poor. For horses that also have PSSM1 and/or HYPP, clinical signs of typing-up can be greatly exacerbated with the MH mutation.

How can malignant hyperthermia be prevented?

Breeders can prevent malignant hyperthermia by avoiding mating horses with the causative mutation. Since this disease has an autosomal dominant mode of inheritance, only one copy of the disease allele is needed for a horse to be affected. DNA samples can be submitted to the UC Davis Veterinary Genetics Laboratory for testing. This genetic test is also part of the AQHA 5-panel testing requirements for breeding animals.

For more information:

UC Davis Veterinary Genetics Laboratory MH DNA testing:

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to