In cats, there seems to be a link to certain types of anesthetic agent, and other triggers include stress, excitement, and apprehension. Unfortunately, this condition usually responds poorly to treatment and fatalities are likely.

Malignant hyperthermia is a serious, potentially life-threatening increase in the cat's body temperature resulting not from excessive external heat (such as a hot day) but an internal mechanism. The cat is unable to restore their correct internal temperature and a significant rise of core temperature leads to breathing difficulties, shock, and ultimately multi-organ failure.

Signs the cat has a problem can develop slowly, over an hour or so, or swiftly in a matter of minutes. In conscious cats (those not undergoing anesthesia) the first signs are muscular stiffness, followed by rapid breathing. 

The underlying cause of malignant hyperthermia is a faulty gene which codes for an abnormal response to the movement of calcium inside muscle cells. This causes errors of metabolism and muscle contraction, which result in an abnormally high core temperature.

The inciting cause of an episode may be linked to a sensitivity to certain anesthetic agents (mainly halothane) or stress. There also seems to be a link with caffeine, so caffeine containing beverages and chocolate are best avoided. 

A presumptive diagnosis is made given the combination of clinical signs, extremely high body temperature, and a recent history of anesthesia. The cat is often presented in a collapsed state and a physical exam quickly reveals the fever, which is enough to initiate treatment to reduce it. 

In less severely ill cases, the vet may run screening blood tests and perform imaging to seek out and eliminate other causes of fever which have a direct treatment. 

Unfortunately, there is no treatment that resets the cat's internal thermostat. The vet gives supportive care, such as intravenous fluids, to try and physically cool the cat's core temperature. Other measures include showering the cat in cool water and placing the paws on cool packs. The administration of 100% oxygen helps to support cats with breathing difficulties. Some clinicians may use steroids for shock, and diazepam to control secondary seizures.

Unfortunately, in all but a few rare cases, treatment is rarely effective. This is due to changes in the thickness of the blood as a result of the high fever, which causes a reduction in blood flow to vital organs, multi-system failure, and death. 

Recovery rates of malignant hyperthermia are poor. In rare cases that recovered as a result of prompt action for an anesthetized cat, avoidance of that anesthetic agent in the future is advisable. 

Unfortunately, there is no one screening test which is effective at predicting which cats may be affected by this condition, although mercifully it is rare.