English
Malignant hyperthermia (MH) is a rare inherited muscle disorder in horses that affects how muscles regulate calcium. When calcium regulation is disrupted, the muscles become overactive and produce excessive heat, carbon dioxide, and acid within the body.
The condition is most often triggered during general anesthesia, especially after exposure to certain inhaled anesthetic drugs such as halothane. Other medications and stressful events may also play a role. Once triggered, the reaction can progress very quickly and may affect multiple body systems within a short period of time.
Horses affected by malignant hyperthermia usually appear healthy until they are exposed to a triggering event. This means the condition may not be recognized until anesthesia is already underway. In some horses, inherited changes involving the ryanodine receptor (RyR1) gene are believed to increase susceptibility.
Because signs can develop and worsen rapidly, careful anesthetic planning and awareness of breed-related risk factors are important for reducing the chance of a severe episode.
Malignant Hyperthermia in Horses
Malignant hyperthermia (MH) is a rare inherited pharmacogenetic skeletal muscle disorder associated with abnormal calcium regulation within muscle cells. [1][2][3][4]
A pharmacogenetic disorder is a condition in which an individual’s genetic makeup affects how their body responds to certain medications or drugs. [4][5][6]
In MH, uncontrolled calcium release causes muscles to become overactive and produce dangerous amounts of heat. [6] It also triggers the production of excess waste products, including carbon dioxide and lactic acid. [4][6]
In horses, the condition can trigger a rapid and potentially life-threatening increase in body temperature when susceptible individuals are exposed to certain anesthetic agents, particularly inhaled drugs, or other stressors. [1][2][3] Malignant hyperthermia has been reported in multiple horse breeds undergoing general anesthesia. [7]
Table 1. Malignant hyperthermia at-a-glance
| Category | Key Information |
|---|---|
| Definition |
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| Primary Cause |
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| Risk Factors |
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| Onset |
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| Symptoms |
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| Severity |
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| Treatment |
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| Recovery Time |
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| Prognosis |
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| Prevention |
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Pathophysiology & Abnormal Thermoregulation
As with all mammals, the horse’s body temperature is tightly regulated within a narrow range despite changes in environmental conditions.
Small variations occur between individuals and throughout the day, and younger horses tend to have slightly higher baseline temperatures. The average resting temperature for adult horses is approximately 38.0°C (100.5°F). [1]
Thermoregulation relies on coordinated mechanisms that promote heat loss when body temperature rises, including: [1]
- Increased blood flow to the skin surface
- Sweating
- Heat exchange via respiration and the skin
Malignant hyperthermia occurs when skeletal muscle responds abnormally to triggering agents, most often certain anesthetic drugs. In affected horses, mutations involving calcium-regulating channels in skeletal muscle can disrupt normal calcium control inside muscle cells.
The ryanodine receptor, encoded by RYR1, is one of the key channels involved in calcium release from the sarcoplasmic reticulum, the internal storage site for calcium in muscle cells. [2][3]
Under normal conditions, calcium is released in a controlled way to allow muscle contraction and is then pumped back into storage so the muscle can relax.
In malignant hyperthermia, exposure to a trigger can cause excessive calcium release inside skeletal muscle cells. This keeps the muscle in a state of sustained contraction and forces the cells into a hypermetabolic state. [2][3]
That uncontrolled muscle activity rapidly increases oxygen consumption, carbon dioxide production, energy use, and heat generation.
As the reaction progresses, the horse may develop: [6][7][8]
- Muscle rigidity or twitching
- Rising carbon dioxide levels
- Increased heart rate
- Increased respiratory effort
- Sweating
- Acidosis
- Electrolyte disturbances
- Rapidly rising body temperature
In malignant hyperthermia, normal cooling mechanisms become overwhelmed because heat is being generated faster than the body can dissipate it. The resulting hyperthermia is part of a wider metabolic crisis driven by uncontrolled skeletal muscle activity.
Without rapid intervention, this process can progress to severe muscle damage, cardiovascular instability, organ dysfunction, and death. [6][8]
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Symptoms
Clinical signs of malignant hyperthermia typically develop suddenly and progress rapidly, often during the administration of anesthesia. The condition produces a severe hypermetabolic reaction within the muscle and may quickly become life-threatening. [1][2][3]
Symptoms of MH in horses may include the following: [1][2][3][4][6]
- Rapid increase in body temperature
- Dark urine
- Third eyelid protrusion
- Muscle rigidity
- Elevated heart rate
- Circulatory shock in severe cases
- Death in severe untreated cases
Because early signs can resemble other anesthetic complications, malignant hyperthermia requires rapid recognition. Early recognition allows a veterinary team to stop triggering anesthetic agents and begin treatment before severe muscle damage, systemic complications, or death occur.
Monitoring Rectal Temperature
Rectal temperature is one of the most important measurements during a suspected malignant hyperthermia episode because affected horses can develop a rapid and dangerous rise in body temperature. Temperature is monitored by the veterinary team during anesthesia, recovery, and treatment whenever malignant hyperthermia is suspected.
In a stable horse, rectal temperature can be taken with a digital thermometer and lubricant. The person taking the temperature should stand beside the horse’s hindquarters, not directly behind the horse. If the horse is unstable, distressed, recumbent, or recovering from anesthesia, temperature monitoring should only be performed by a veterinarian.
Use one hand to gently move the tail to the side, then insert the lubricated thermometer into the rectum according to the manufacturer’s instructions. Hold the thermometer in place until the reading is complete. Digital thermometers typically beep once the reading is ready.
In malignant hyperthermia, rectal temperature is only one part of the clinical picture. Veterinarians also monitor heart rate, breathing, carbon dioxide levels, muscle rigidity or twitching, sweating, acid-base status, electrolytes, and other indicators of a developing metabolic crisis.
Rectal Temperature Check
| Urgency | Observation |
|---|---|
| Expected Range Healthy Horse |
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| Caution Seek Veterinary Care Promptly |
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| Emergency Seek Emergency Veterinary Care |
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Severity
Malignant hyperthermia in horses can vary in severity, ranging from rapidly developing clinical signs to a fatal systemic crisis. [1] The condition may begin with rapid increase in muscle activity and heat production, then progress quickly as body temperature rises and normal body functions become increasingly disrupted. [1][2][3]
In more severe cases, the disorder can lead to significant complications, including: [1][2][3][7]
- Acidosis
- Muscle damage
- Circulatory collapse
- Severe rise in carbon dioxide levels
- Cardiovascular disturbances, including tachycardia and hypertension
- Cardiopulmonary arrest
- Organ failure
Without prompt recognition and intervention, malignant hyperthermia may progress to death. [1][2][3]
When to Call the Veterinarian
Malignant hyperthermia is most likely to be recognized by the veterinary team during anesthesia or shortly after a procedure. However, owners should tell their veterinarian before any planned anesthetic event if a horse has a history of complications related to anesthesia, unexplained severe reactions to stress or exertion, or known breed-related susceptibility.
Call a veterinarian promptly if a horse develops concerning signs after anesthesia or a stressful event, including:
- Rapidly rising body temperature
- Severe muscle stiffness, trembling, weakness, or collapse
- Labored or unusually rapid breathing
- Dark urine or signs of muscle pain
- Abnormal heart rate, agitation, depression, or signs of shock
Veterinary evaluation is important because these signs may have several possible causes, and treatment depends on identifying the underlying problem quickly. For suspected malignant hyperthermia, prompt intervention is critical to help limit muscle damage, acid-base disturbances, organ complications, and death.
Risk Factors
Certain factors may increase the likelihood of malignant hyperthermia developing in certain horses. Malignant hyperthermia is a pharmacogenetic disorder, meaning an inherited genetic risk affects how muscle responds to certain triggering drugs, particularly those used during anesthesia. [1][2][4]
Since risk factors for malignant hyperthermia have a genetic component, predisposition to the condition exists in certain horse breeds.
Breeds that may be at higher risk of MH include: [4]
Beyond genetic susceptibility, risk factors associated with malignant hyperthermia episodes in affected horses include: [1][2][3][5][6]
- Exposure to inhaled anesthetic gases such as halothane, isoflurane, sevoflurane, or desflurane
- Certain drugs or procedures performed during anesthesia, including nerve stimulation or caffeine administration
- Use of muscle-relaxing drugs such as succinylcholine
- Physical or emotional stress, particularly in high-risk horses
- Intense exercise in predisposed horses
- Conditions that increase muscle activity and heat production, contributing to an extreme metabolic reaction
These risk factors do not mean a horse will develop malignant hyperthermia, but they are important considerations when planning anesthesia or other procedures in a horse with known or suspected risk factors.
Table 2. Malignant hyperthermia risk factors in horses
| Risk factor | Examples | Practical takeaway |
|---|---|---|
| Breed predisposition |
|
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| Inherited genetic risk |
|
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| Inhaled anesthetic exposure |
|
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| Muscle-relaxing or triggering drugs |
|
|
| Stress and high muscle activity |
|
|
Causes
Malignant hyperthermia is caused by an abnormal skeletal muscle response to specific triggering conditions. In affected horses, normal calcium regulation within muscle cells is disrupted, causing sustained muscle contraction, excessive heat production, and a rapidly developing metabolic crisis. [1]
While rare, malignant hyperthermia in horses most commonly occurs during or shortly after general anesthesia and is particularly associated with the use of inhaled anesthetics. Once triggered, the reaction can develop very quickly and may become fatal within a short time. [8]
Triggers associated with malignant hyperthermia in horses include: [1][2][3]
- Inhaled halogenated anesthetics
- Muscle relaxants
- Local anesthetics
- Physical or physiological stress
When a reaction begins, it leads to an uncontrolled cascade within skeletal muscle. Ongoing calcium release drives muscle contraction and energy use, producing heat faster than the horse can dissipate it. This can lead to rising body temperature, increased carbon dioxide production, acidosis, electrolyte disturbances, muscle damage, and cardiovascular instability. [1][2][3]
It’s important to note that not every horse exposed to triggering conditions develops malignant hyperthermia. The risk depends on whether the horse has underlying risk factors, which may be genetic or otherwise related to abnormal muscle calcium handling.
Diagnosis
Diagnosis of malignant hyperthermia is based on characteristic symptoms during or shortly after exposure to triggering agents, most commonly general anesthesia. [2] Although originally attributed to inhaled anesthetics alone, malignant hyperthermia has more recently been linked to several types of anesthetic. [3]
Because the condition can progress quickly, diagnosis is often made based on the horse’s clinical signs during anesthesia rather than waiting for test results to confirm diagnosis.
Veterinarians monitor changes such as rising body temperature, increased carbon dioxide levels, muscle rigidity or twitching, elevated heart rate, abnormal breathing patterns, and changes in acid-base status. These findings are interpreted alongside the anesthetic protocol and the timing of symptom onset.
In some cases, genetic testing may be used to identify at-risk horses before anesthesia, although not all affected horses carry genetic variants associated with MH. [2][3]
Genetic testing is most useful for horses with a known family history, previous suspicious anesthetic reaction, or breed-related concern, but a negative test does not always rule out predisposition.
Early recognition during anesthesia is critical, as malignant hyperthermia can be mistaken for insufficient dosing of anesthesia. [8] Rapid escalation of signs, particularly when combined with rising body temperature, are key indicators of malignant hyperthermia. [8]
If malignant hyperthermia is suspected, the veterinary team must act quickly while continuing to evaluate other possible causes of abnormal anesthetic responses.
Other issues with anesthesia may also be involved, including: [8]
- Inadequate anesthesia
- Pain
- Excitement during recovery
- Ventilation problems
- Equipment issues
- Metabolic disturbances
Because signs can overlap with other anesthetic complications, malignant hyperthermia is treated as an emergency until another cause of symptoms is found. Early recognition allows the veterinary team to stop triggering agents, begin cooling and supportive care, and reduce the risk of severe complications.
Treatment
Treatment is most effective when preventive steps are taken before anesthesia is administered in at-risk horses. Preventive administration of oral dantrolene before anesthesia can reduce the risk of a severe episode. [2][3]
Prompt removal of triggering agents, especially inhaled anesthetics, is considered the first essential step in limiting progression of the reaction. [8] Once triggering agents are removed, treatment focuses on stabilizing the horse and correcting the internal changes. [8]
Because malignant hyperthermia is a rapidly developing metabolic crisis, treatment must begin as soon as the condition is suspected.
The immediate goals of stabilization are: [8]
- Reduce ongoing muscle contraction
- Lower body temperature
- Support oxygen delivery
- Correct internal imbalances
Horses affected during surgery or anesthesia require close monitoring by the veterinary team throughout treatment and recovery.
Supportive care after stabilization may include: [2][3]
- Active external cooling: Helps reduce body temperature when heat production exceeds the horse’s ability to dissipate heat
- Mechanical ventilation: Helps manage carbon dioxide levels and support oxygenation during the metabolic crisis
- Intravenous fluids: Support circulation, hydration, and metabolic correction
- Correction of acid-base imbalance: Helps address acidosis and related metabolic changes that can develop during severe episodes
- Intensive care hospitalization: Allows ongoing monitoring of body temperature, heart rhythm, blood gases, electrolyte status, muscle damage, and organ function
In severe cases, abnormalities such as hypercapnia (excess carbon dioxide), acidosis, and electrolyte disturbances may develop quickly and require intensive correction. [7][8] Even with appropriate intervention, outcomes can remain guarded once the condition has progressed significantly. [2][3][8]
After the immediate crisis is controlled, continued monitoring remains important because complications can persist beyond the anesthetic event. Horses may require ongoing supportive care for muscle injury, metabolic abnormalities, weakness, pain, or delayed recovery from anesthesia.
Prognosis
If detected early and managed properly, most horses recover from malignant hyperthermia. [7] However, severe episodes may develop rapidly during anesthesia and progress to life-limiting hyperthermia. [2][3]
The best outcomes are associated with early recognition, immediate removal of triggering agents, and rapid supportive treatment before severe metabolic changes become established. Horses that respond quickly to intervention may recover from the acute episode, but they should still be treated as at risk during future anesthetic procedures.
In advanced cases, the condition can lead to cardiac arrest and death, particularly when the metabolic crisis is not controlled early. [2][3] Overall, the prognosis is guarded to poor in severe cases. [3]
The outcome depends largely on how quickly the condition is recognized and treated, as established cases may be difficult to control once severe metabolic changes have developed. [2][3] Horses that recover may experience permanent muscle damage. [7]
Recovery from malignant hyperthermia does not always mean the horse’s overall medical problem has been resolved. Generally speaking, horses are put under anesthesia because they require surgery, diagnostic imaging, emergency treatment, or another procedure for a separate condition.
If the anesthetic procedure must be stopped or delayed because of malignant hyperthermia, the original injury or illness may still require treatment once the horse is stable.
Long-term prognosis also depends on whether the horse has ongoing muscle injury, complications from the anesthetic episode, an unresolved underlying medical condition, or genetic risk for future episodes. Any horse suspected of malignant hyperthermia should have the reaction clearly documented in its medical record so future anesthetic planning can account for the risk.
Prevention
Prevention of malignant hyperthermia focuses primarily on reducing the risk of an episode during general anesthesia in susceptible horses.
In addition to completing bloodwork and a physical examination to assess the horse’s overall health and readiness for surgery, administration of oral dantrolene before anesthesia can significantly reduce the risk of a severe reaction. [2][3] Genetic testing may help identify at-risk horses before anesthesia. [4]
Preventive planning is especially important for horses with a known mutation, a previous suspicious anesthetic reaction, or a close relative affected by malignant hyperthermia. Owners should inform their veterinarian of any previous anesthesia complications, unexplained severe muscle reactions, or known genetic test results before any procedure requiring sedation or general anesthesia.
Additional preventive measures focus on minimizing the horse’s exposure to known triggers and ensuring close supervision during procedures that carry risk. Careful selection and planning of anesthetic protocols in horses with known risk can help reduce the likelihood of an episode. [2][3]
Whenever possible, known triggering agents should be avoided, and vulnerable horses should be closely monitored throughout anesthesia to allow for rapid recognition and intervention if signs develop. [2][3]
Frequently Asked Questions
Here are some frequently asked questions about malignant hyperthermia in horses:
Malignant hyperthermia in horses is a rare inherited muscle disorder that affects how muscle cells regulate calcium. When a susceptible horse is exposed to certain triggering drugs or stressors, the muscles can become overactive and produce excessive heat, carbon dioxide, and acid. This can overwhelm the horse's normal cooling mechanisms and quickly affect multiple body systems. [1][2][3][4]
Malignant hyperthermia in horses is caused by an inherited susceptibility that leads to an abnormal muscle response after exposure to certain triggers. The reaction is most commonly associated with general anesthesia, especially inhaled anesthetic gases and some muscle-relaxing drugs. Physical or physiological stress may also contribute in predisposed horses. [1][2][3]
Malignant hyperthermia in horses is a veterinary emergency because it can progress very quickly and become life-threatening. Once triggered, the condition can cause severe overheating, breathing and heart problems, muscle damage, collapse, and shock. Prompt recognition and treatment are critical to improving the horse's chance of recovery. [1][2][3][8]
Signs of malignant hyperthermia in horses often appear suddenly during or shortly after anesthesia. Affected horses may develop a rapid rise in body temperature, increased heart rate, muscle rigidity, sweating, trembling, weakness, labored breathing, dark urine, or collapse. Severe cases may also involve acidosis, electrolyte imbalance, circulatory shock, organ failure, or death if treatment is not started quickly. [1][2][3][4][6]
Malignant hyperthermia usually happens in horses during or soon after exposure to a triggering agent, most often during general anesthesia. Susceptible horses may appear healthy before anesthesia, so the condition may not be obvious until a procedure is already underway. This is why anesthetic planning, risk awareness, and close monitoring are important for horses with known or suspected susceptibility. [1][2][3]
Anesthetic drugs that can trigger malignant hyperthermia in susceptible horses include inhaled halogenated anesthetics such as halothane, isoflurane, sevoflurane, and desflurane. Some muscle relaxants, including succinylcholine, and certain anesthetic-related drugs or procedures may also be associated with risk. Using multiple potential triggers together can increase concern in a horse with known or suspected susceptibility. [1][2][3][5][6]
Stress or intense exercise may contribute to malignant hyperthermia in predisposed horses, although anesthesia is the most common setting for recognized episodes. Physical or emotional stress can increase muscle activity and heat production, which may add risk in a susceptible horse. Owners should tell their veterinarian about any unexplained severe reactions to stress, exertion, or previous anesthesia before a planned procedure. [1][2][3][5][6]
Horse breeds most at risk for malignant hyperthermia include Quarter Horses, Thoroughbreds, Appaloosas, Arabians, and ponies. Quarter Horses are identified as having the highest reported susceptibility among these groups. Breed risk does not mean an individual horse will develop malignant hyperthermia, but it can help guide veterinary planning before anesthesia. [4]
A horse can look healthy before developing malignant hyperthermia because the condition is usually triggered only after exposure to specific drugs or stressors. Susceptible horses may show no obvious signs in day-to-day life. The first warning signs may appear only once anesthesia is underway, which can make early recognition challenging. [2][3][8]
Malignant hyperthermia in horses is diagnosed based on characteristic signs that develop during exposure to triggering agents, especially under general anesthesia. Veterinarians may suspect the condition when rising carbon dioxide levels, increased heart rate, muscle activity, abnormal breathing, and increasing body temperature occur rapidly during a procedure. Genetic testing may help identify some susceptible horses, but not all affected horses carry known variants. [2][3][8]
Genetic testing cannot always rule out malignant hyperthermia in horses because not every affected horse carries a known testable variant. Testing may still be useful for horses with a family history, suspicious anesthetic reaction, or breed-related risk. A negative genetic test should be interpreted with veterinary guidance, especially when the horse's history suggests possible susceptibility. [2][3][4]
Before your horse has anesthesia, tell your veterinarian about any previous anesthetic complications, unexplained severe stress reactions, intense exercise reactions, family history of malignant hyperthermia, or relevant genetic test results. This information helps the veterinary team assess whether the horse may need special anesthetic planning. Horses from higher-risk breeds may also warrant extra discussion before procedures involving general anesthesia. [2][3][4]
Malignant hyperthermia in horses is treated by immediately removing triggering agents and stabilizing the horse. Treatment may include active cooling, mechanical ventilation, intravenous fluids, correction of acid-base and electrolyte abnormalities, and intensive supportive care. The veterinary team may also monitor body temperature, blood gases, heart rhythm, muscle damage, and organ function during recovery. [2][3][8]
Malignant hyperthermia in horses can sometimes be prevented, or the risk reduced, with careful planning before anesthesia. Preventive measures may include identifying at-risk horses, avoiding known triggering agents when possible, using appropriate anesthetic protocols, and closely monitoring susceptible horses during procedures. Oral dantrolene may be given before anesthesia in known or suspected susceptible horses to reduce the risk of a severe episode. [2][3][4]
Malignant hyperthermia can cause serious complications in horses because excessive muscle activity disrupts temperature regulation, breathing, circulation, and metabolism. Severe cases may involve acidosis, high carbon dioxide levels, electrolyte imbalance, muscle damage, dark urine, circulatory collapse, organ failure, or cardiopulmonary arrest. These complications can develop quickly, which is why suspected cases require immediate veterinary intervention. [1][2][3][4][6][7]
A horse can recover from malignant hyperthermia if the condition is recognized early and treated quickly. Recovery is more likely when triggering agents are removed promptly and supportive care begins before severe metabolic changes develop. [2][3][7] However, recovery from the malignant hyperthermia episode does not always mean the horse's original medical problem has been resolved. Horses are put under anesthesia because they need surgery, diagnostic imaging, or emergency treatment for another condition, and that issue may still need to be addressed once the horse is stable. Horses that survive a severe episode may also have lasting muscle damage and should be treated as at risk during future anesthetic procedures.
The prognosis for a horse with malignant hyperthermia depends on how quickly the condition is recognized and treated. Horses detected early and managed appropriately may recover, but severe cases can progress to cardiopulmonary arrest, organ failure, or death. The prognosis becomes guarded to poor once severe metabolic changes are established. [2][3][7]
Summary
Malignant hyperthermia is a rare inherited muscle disorder in horses that causes abnormal calcium regulation within muscle cells, leading to excessive muscle activity, heat production, carbon dioxide buildup, and acidosis.
- The condition is most often triggered during general anesthesia, especially after exposure to inhaled anesthesia
- Clinical signs can develop suddenly and progress rapidly, including rising body temperature, increased heart rate, muscle rigidity and breathing difficulties
- Risk factors may be higher in certain breeds such as Quarter Horses, Thoroughbreds, Appaloosas, Arabians, and ponies
- Prevention relies on assessing and identifying horses before anesthesia and mitigating risk factors as needed
References
- Reed. S. M. et al. Equine Internal Medicine. 3rd ed. Saunders Elsevier. 2010.
- Finno. C. J. et al. Equine Diseases Caused by Known Genetic Mutations. The Veterinary Journal. 2009. View Summary
- Finno. C. J. Genetics of Muscle Disease. Veterinary Clinics of North America: Equine Practice. 2025. View Summary
- Aleman. M. et al. Equine Malignant Hyperthermia. AAEP. 2004.
- Aleman. M. A Review of Equine Muscle Disorders. Neuromuscular Disorders. 2008. View Summary
- Aleman. M. et al. Association of a Mutation in the Ryanodine Receptor 1 Gene with Equine Malignant Hyperthermia. Muscle & Nerve. 2004. View Summary
- Deutsch. J. and Taylor. P. M. Mortality and Morbidity in Equine Anaesthesia. Equine Veterinary Education. 2022.
- Castro. D. and Clark-Price. S. Anesthesia and Myopathies of Horses. Veterinary Clinics of North America: Equine Practice. 2025. View Summary
