Merck Manual

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Pet Owner Version

Malignant Hyperthermia in Horses


Hui-Chu Lin

, DVM, DACVAA, Department of Clinical Sciences, Auburn University

Reviewed/Revised Oct 2019 | Modified Oct 2022

Malignant hyperthermia is seen mostly in swine, but it has also been reported in horses, dogs, and cats. This syndrome is characterized by abnormally high body temperature, muscle rigidity, a very rapid and irregular heartbeat, increased breathing rate, bluish tinge to skin and mucous membranes, unstable blood pressure, fluid buildup in the lungs, impaired blood coagulation, kidney failure, and death.

Malignant hyperthermia is consistently triggered in susceptible animals by excitement, apprehension, exercise, or environmental stress. Giving certain anesthetics or specific drugs that affect the neurologic and muscular systems also consistently triggers malignant hyperthermia in susceptible animals.

Diagnosis is based on development of signs in an animal that has been given an anesthetic agent or is participating in a stressful event. Signs can develop slowly or rapidly. They include muscle stiffness, twitching, a rapid heartbeat, and an increased breathing rate. Body temperature increases rapidly and can reach 113°F (45°C).

Usually, malignant hyperthermia episodes occur suddenly and are severe. If the condition is recognized early in an animal under anesthesia, supportive measures may be able to save the animal. Unfortunately, regardless of treatment, malignant hyperthermia is usually fatal.

Stress must be minimized to prevent malignant hyperthermia episodes in individual animals. If an animal that is suspected to be susceptible to malignant hyperthermia (or that has survived a previous episode) needs anesthesia and surgery, certain precautions should be taken. These include administering a drug called dantrolene 1 to 2 days before anesthesia and avoiding certain anesthetic agents.

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