MH
Malignant Hyperthermia
recessive traitWhat is MH?
Malignant Hyperthermia (MH) is a potentially fatal muscle disorder triggered by exposure to certain volatile anesthetic agents (such as halothane or isoflurane) and depolarizing muscle relaxants. When a susceptible horse is exposed to these drugs, muscle cells undergo uncontrolled calcium release, causing rapid muscle rigidity, extreme hyperthermia (dangerously high body temperature), and metabolic crisis. Without rapid intervention, death can occur during or shortly after anesthesia.
How is it inherited?
MH in horses is caused by a mutation in the RYR1 gene and follows an autosomal recessive pattern for triggering — though some heterozygotes may also be at elevated risk. The condition is triggered by specific anesthetic agents, not by normal activity, so affected horses appear completely healthy under ordinary conditions.
What do the results mean?
| Result | Status | What it means |
|---|---|---|
| N/N | Normal | No MH mutation. Horse can receive standard anesthetic agents without elevated risk from this mutation. |
| N/MH | Carrier | One MH allele. Horse may be at increased risk with triggering agents. Veterinarians should be informed so anesthetic protocols can be adjusted. |
| MH/MH | Homozygous (high risk) | Two MH alleles. Horse carries significant risk of a life-threatening MH episode under triggering anesthetics. Trigger-free anesthetic protocols are required for any procedure. |
No MH mutation. Horse can receive standard anesthetic agents without elevated risk from this mutation.
One MH allele. Horse may be at increased risk with triggering agents. Veterinarians should be informed so anesthetic protocols can be adjusted.
Two MH alleles. Horse carries significant risk of a life-threatening MH episode under triggering anesthetics. Trigger-free anesthetic protocols are required for any procedure.
What should buyers know?
MH rarely affects day-to-day life — horses with the mutation are normal under all ordinary conditions. However, if the horse ever needs surgery, anesthesia, or sedation, the veterinarian must know the MH status in advance so they can use trigger-free agents. Always share test results with your vet.
Which breeds are affected?
Found in Quarter Horses and related stock breeds. Prevalence is low compared to other equine genetic conditions, but the consequences of an unknown MH episode under anesthesia can be severe.
This page is for general education only. Always consult a veterinarian or equine geneticist before making purchasing or breeding decisions based on genetic test results.
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