Discovered at UC Davis Malignant Hyperthermia (MH)

Quick Summary

Malignant hyperthermia (MH) is an inherited disease in which affected horses can be triggered by halogenated anesthetics, succinylcholine, stress, or excitement, which can induce a hyper-metabolic state characterized by symptoms including muscle contracture, elevated temperature, and an irregular heart rhythm.

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Phenotype: Malignant hyperthermia (MH) is an inherited disease in which affected horses can be triggered by halogenated anesthetics, succinylcholine, stress, or excitement, which can induce a hyper-metabolic state characterized by symptoms including muscle contracture, elevated temperature, and an irregular heart rhythm.

Mode of Inheritance: Autosomal dominant

Alleles: N = Normal/Unaffected, MH = Malignant hyperthermia

Breeds appropriate for testing: Quarter Horse and related breeds

Explanation of Results:

  • Horses with N/N genotype will not have malignant hyperthermia and cannot transmit this malignant hyperthermia variant to their offspring.
  • Horses with N/MH genotype will be affected by malignant hyperthermia. They can transmit this malignant hyperthermia variant to  their offspring. Matings with N/N genotype will result in a 50% chance of producing a malignant hyperthermia-affected foal.
  • Horses with MH/MH genotype will have malignant hyperthermia.

Turnaround Time
At least 15 business days; may be delayed beyond 15 business days if sample requires additional testing, or a new sample is requested.

Sample Collection

Horse DNA tests are carried out using cells from the roots of a hair sample (roughly 20-30 hairs).

1. Grab about 10 hairs at the base.

2. Wrap the hairs around your finger and give it a quick pull.

3. Check the ends to make sure the pulled hairs have roots.

4. Repeat the process until you have collected about 20-30 hairs with intact roots.

5. You can choose different places on the mane or tail. NOTE: For foals, we recommend pulling all hairs from the tail only. 

6. Tape the hairs to the submission form and fold the form along the dotted line to protect the sample. Do not use ziploc bags as they can cause condensation that allows mold to grow on the hair.

Hairs with roots

7. Place the folded form containing the sample in a paper envelope and mail it to the laboratory.

 

Additional Details

Malignant hyperthermia (MH) is an inherited disease that causes a life-threatening condition in susceptible horses triggered by exposure to halogenated anesthetics or succinylcholine, and occasionally by stress or excitement. This genetic condition has been identified in Quarter Horses and American Paint Horses.

MH is inherited as an autosomal dominant trait so that only one copy of the MH allele is required to cause disease. A missense mutation (c.7360C>G) in the RyR1 gene results in an altered protein product (p.R2454G). The RYR1 gene provides instructions for making the protein called ryanodine receptor 1.  Ryanodine receptors form channels that when activated release positively charged calcium ions from storage sites within skeletal muscle cells. The defective ryanodine receptor 1 of MH horses results in excessive release of calcium ions inside skeletal muscle cells, which then triggers a hyper-metabolic state that can be fatal.

Signs of MH episodes include muscle contracture (rigidity), elevated body temperature (> 40°C), elevated heart rate, irregular heart rhythm, excessive sweating, and shallow breathing. Presence of the MH mutation also results in more severe clinical tying-up phenotype in horses that have the PSSM1 mutation.

This diagnostic DNA test for MH allows identification of horses that have this mutation and are at risk of developing clinical signs of the disease, especially if subjected to anesthesia. This test is recommended for Quarter Horses, Paints and related breeds. This DNA test will also assist veterinarians to use appropriate medication for surgical procedures and make the correct diagnosis of the cause of a tying-up event.