A Gentle End: Humane Equine Euthanasia

Although euthanizing a horse can be a difficult decision, a recent study found that the widely used euthanasia method is effective and humane. | © Paula da Silva/arnd.nl

Anyone who faces the difficult decision to put down a horse has to wonder: Will euthanasia really end his life quickly and painlessly or will he suffer? A new study offers some peace of mind.

A research team led by Monica Aleman, MVZ Cert., PhD, DACVIM, a faculty member in the Large Animal Medicine Service at the UC Davis Veterinary Medical Teaching Hospital, set out to determine how quickly brain death occurs in the euthanasia method most veterinarians use—a lethal overdose of the anesthetic sodium pentobarbital delivered rapidly through an intravenous infusion. A horse’s death is typically determined by physical signs: The heart stops and the horse stops breathing. Brain death is different. Defined as the complete loss of brain function, it’s measured by the absence of electrical activity in the cerebral cortex, the “gray matter” responsible for processing sensory information, orchestrating voluntary movement and maintaining awareness, and of reflexes in the brain stem, which is responsible for involuntary functions such as breathing. Although brain death is an accepted way of determining death in people, less is known about how it occurs in horses.

Dr. Aleman’s study tracked changes in 15 horses who required euthanasia for a variety of conditions. The horses were hooked up to a range of instruments for recording brain activity and other body functions, including electroencephalogram (EEG) and electrocardiogram (ECG) equipment, and they were monitored as the sodium pentobarbital took effect. The results:

• Electrical activity in the cerebral cortex vanished within 52 seconds of the administration of the drug, in some cases while the infusion was still in progress.

• Loss of brain-stem activity followed quickly, as shown by a lack of blink reflexes and pupil response to light and by a brainstem auditory-evoked response (BAER) test. The total time from administration of the drug to brain death ranged from about a minute to just over four minutes.

• Physical signs mirrored the neurological tests. Heart sounds and pulse were undetectable within a minute.

The ECG continued to show traces of electrical activity in the heart for five to 16 minutes after the end of the infusion. But at this point “brain death has already occurred,” Dr. Aleman notes. Her study provides reassurance that the most-used euthanasia method is effective, fast and humane.

This article originally appeared in the August 2015 issue of Practical Horseman.

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