Malignant hyperthermia
A horse can carry an RYR1 variant that makes a malignant hyperthermia episode more likely while appearing entirely healthy. This increased inherited risk is called genetic susceptibility. During an episode, the muscles become overactive, use excessive energy and generate dangerous heat. Certain anaesthetic drugs can trigger that reaction. DNA testing identifies the known equine variant. It cannot promise that an episode will occur, but provides information the veterinary team needs when planning a procedure.
Dominant inheritance
Malignant hyperthermia can make muscles overactive and generate dangerous heat
Muscle contraction uses a calcium signal. A protein called the ryanodine receptor controls calcium release from a store inside the cell, and RYR1 supplies the instructions for this protein. The equine variant associated with MH can disrupt that control, allowing excessive calcium to be released and keeping the muscle abnormally active.
The muscle consumes large amounts of energy and generates heat. Severe muscle injury and disturbances elsewhere in the body can follow. Hyperthermia describes the temperature increase, while malignant refers to the syndrome’s potential severity, not cancer. This is also more specific than a horse becoming hot on a summer day: inherited susceptibility and triggering circumstances are central to the explanation.
Some anaesthetic drugs can trigger malignant hyperthermia in a susceptible horse
Certain agents used during anaesthesia can trigger MH in a susceptible horse. Episodes have also been associated with major stress or excitement. This does not mean every anaesthetic exposure inevitably causes an episode, or that owners should choose drugs themselves. The result informs an individual veterinary plan, including an assessment of which procedures and agents are appropriate.
A horse can carry the variant without any recognised signs. A previous uneventful procedure does not erase its susceptibility. Testing is therefore useful for anticipating risk, not only for explaining illness already seen. It is also important to separate inherited MH susceptibility from an anaesthetic complication caused by something else, even when outward signs overlap.
A malignant hyperthermia episode can cause stiffness, overheating and breathing difficulties
Possible signs include muscle rigidity, rising body temperature, heavy sweating, a rapid or irregular heartbeat and breathing difficulties. Muscle damage can be severe. A suspected episode requires urgent veterinary care and may be fatal. Temperature alone cannot identify or exclude the syndrome, particularly when several changes are developing together.
During anaesthesia, the veterinary team considers multiple monitored measurements and how they change over time. In a horse that is not anaesthetised, similar signs can have other urgent explanations. A high temperature does not establish MH by itself, and a positive DNA result does not prove that every later illness or episode of muscle damage is caused by MH.
Which horses have been found to carry the malignant hyperthermia variant?
The known equine variant is documented in Quarter Horses and related breeds. A large study published in 2025 identified it in Quarter Horses, Paint Horses and Appaloosas among the breeds examined. This describes the sampled populations; it cannot guarantee absence from every other breed or from a horse with mixed ancestry.
The study separated screening data from horses referred for muscle disease. That distinction matters because a selected clinical group cannot provide a death risk for all positive horses. A rare variant can still be highly important to the individual horse carrying it, particularly before anaesthesia. Population frequency, individual genotype and the course of a clinical episode answer different questions.
How malignant hyperthermia DNA testing complements examination of the horse
Testing targets the specific RYR1 variant associated with the known equine form. One copy is sufficient to confer susceptibility: inheritance is autosomal dominant. The result can therefore be useful before any episode occurs. If an episode is suspected, the veterinarian combines it with circumstances, clinical findings and appropriate tests rather than diagnosing from DNA alone.
A negative result is not a general guarantee of anaesthetic safety. Other complications and other causes of muscle injury still need consideration. The additional presence of the PSSM1 variant can increase the severity of muscle problems; these tests identify different inherited changes. The MH and PSSM1 DNA results therefore need to be interpreted alongside an examination of the horse and an assessment of the planned procedure.
Planning veterinary care for a horse at risk of malignant hyperthermia
The DNA test result for the RYR1 variant associated with MH should be shared with the veterinarian before a procedure and kept in the horse’s record. It helps the team assess risk, choose medications, plan monitoring and prepare for an emergency. The veterinary team should provide any instructions about medication, fasting or feeding; owners should not devise them from the test result themselves.
An MH episode can be extremely serious despite intensive care. The chances of recovery depend on the circumstances, how quickly the episode is recognised and its complications. A horse with the RYR1 variant but no signs therefore has a different outlook from a horse experiencing a severe episode. Treatment does not remove the variant, so the DNA test report and details of any suspected episode should remain available to future veterinary teams.
How a horse can pass the malignant hyperthermia variant to its foal
When one parent has one copy of the RYR1 variant linked to malignant hyperthermia and the other has none, every conception has a 50% chance of a foal with one copy of that variant and a 50% chance of a foal without it. These are genetic transmission probabilities, not a prediction of an episode at birth or the same clinical risk for every offspring.
The consequences of two copies in horses remain insufficiently documented. The large 2025 study found no horse with two copies of the tested RYR1 variant; absence of observations does not, on its own, prove this combination is lethal. The test therefore cannot establish what would happen to a foal that inherited two copies. Breeding decisions need to consider the transmissible susceptibility, future foal welfare and both parents’ results.
Understanding malignant hyperthermia DNA test results
Tested malignant hyperthermia variant not detected
The tested MH variant was not found. Other anaesthetic risks and causes of muscle disease still need assessment.
One MH variant copy: increased risk of malignant hyperthermia episodes
An episode can occur in triggering circumstances. The result does not mean an episode is happening or is inevitable.
Two copies of the malignant hyperthermia variant: effects remain poorly known
The effects of two copies in horses are not established. Such a report needs checking and interpretation with the laboratory and veterinarian.
N/MH means one copy without the tested variant, written N, and one with the variant, written MH. The 2025 study found no two-copy horse, so the consequences of that combination remain insufficiently documented.
Questions about the condition: Malignant hyperthermia
Does a positive malignant hyperthermia DNA test mean the horse is having an episode?
Not necessarily. It identifies genetic susceptibility, and a horse can appear normal without an episode. The result still matters when planning veterinary procedures and considering breeding.
Does an uneventful anaesthetic rule out malignant hyperthermia during a later procedure?
No. It is useful history but does not remove an identified susceptibility. The veterinary team needs the result and the circumstances of each procedure to plan appropriately.
Does a high temperature mean malignant hyperthermia?
No. Infection, heat, exercise and other illnesses can increase body temperature. MH is a specific syndrome interpreted with its context and other findings. An emergency needs assessment without waiting for a home genetic diagnosis.
Scientific sources about the condition: Malignant hyperthermia
This article brings together the research and the university and laboratory resources listed below. Numbers after each section link to the references used. Sources consulted in November 2026.
- Malignant Hyperthermia (MH)
UC Davis Veterinary Genetics Laboratory · Test documentation
- Malignant hyperthermia: signs, diagnosis and clinical implications
UC Davis Center for Equine Health · University resource
- Aleman, McCue and Bellone (2025) — RYR1 genotypes and malignant hyperthermia in horses
Journal of Veterinary Internal Medicine · Research
- MH: genetic diagnostic testing
Racing Australia — Equine Genetics Research Centre · Test documentation