THE MERCK VETERINARY MANUAL
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Overview of Euthanasia

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Euthanasia is the term used to describe ending the life of an animal in a way that eliminates or minimizes pain and distress. Animal slaughter, depopulation, and humane killing are distinguished from euthanasia, because they are performed for reasons different than sparing an animal from unresolvable painful or distressful conditions. Euthanasia of animals is a common procedure performed by veterinary professionals, and because of the seriousness of the action, it deserves appropriate consideration. Some of the most difficult euthanasia decisions that veterinarians are required to make involve the euthanasia of healthy animals when no other alternative for their care can be identified. A veterinarian must be fully prepared to speak frankly about the animal's condition and be knowledgeable about all possible alternative care resources when interacting with animal owners, caretakers, and control professionals. Recognizing the importance of a "good death" in the humane termination of an animal's life, many countries and professional organizations have developed guidelines and recommendations for animal euthanasia; some are more specific for certain species and environmental settings. Most recommendations emphasize certain factors that personnel performing euthanasia should consider when selecting the best method of euthanasia. These factors include: 1) ability of the method to induce loss of consciousness and death with minimum pain and distress; 2) time required to induce loss of consciousness; 3) reliability; 4) safety of personnel; 5) irreversibility; 6) compatibility with intended animal use and purpose; 7) documented emotional effect on observers or operators; 8) compatibility with subsequent evaluation, examination, or use of tissue; 9) drug availability and human abuse potential; 10) compatibility with species, age, and health status; 11) ability to maintain equipment in proper working order; 12) safety for predators or scavengers should the animal's remains be consumed; 13) legal requirements; and (14) environmental impacts of the method or disposition of the animal's remains.

To be able to understand and help others to understand euthanasia, personnel performing euthanasia should be well informed of established definitions of the more important physiologic and psychological states that may be observed during euthanasia. The American Veterinary Medical Association Panel on Euthanasia accepts the following important definitions: 1) anesthetic-induced unconsciousness is the loss of the righting reflex; 2) pain is a conscious experience; 3) distress is an animal's response to stimuli that interferes with its well-being and comfort. Thus, biological functions (urination, defecation), unintentional movements (limb paddling), and vocalizations after administration of an appropriate euthanasia method, and subsequent recumbency are not necessarily signs of perceived pain or distress. Loss of physiologic function and death should occur in the following order to help prevent fear and distress: 1) rapid loss of consciousness; 2) loss of motor function; 3) arrest of respiratory and cardiac functions; and finally 4) permanent loss of brain function. If loss of motor or respiratory and cardiac function precedes loss of consciousness, as might be the case if paralytic agents are used, animals may become fearful and experience distress. In some species, particularly rabbits and chickens, tonic immobility may be induced by fear, and care must be taken to not confuse this behavioral response with loss of consciousness.

In addition to the selection of the method of euthanasia, the euthanasia administrator should consider the natural behavior of the species. For virtually all species, being placed in a novel environment before euthanasia is stressful. Therefore, a euthanasia approach that can be applied in familiar surroundings may help reduce distress. Gentle restraint, careful handling, and talking during euthanasia often have a calming effect on animals accustomed to human contact. These techniques may also be effective coping strategies with personnel and owners. In some species, sedation may assist in achieving the best conditions. The emotional attachment between animals and their owners or caretakers requires an additional layer of professional respect and care. Discussing euthanasia openly with personnel allows everyone the opportunity to understand that the best interests of the animal, owners, and caretakers have been considered.

Regardless of the method of euthanasia selected, personnel performing euthanasia must confirm death and dispose of the carcass in a legal manner that does not contaminate food sources or the environment. Confirming death may best be accomplished by an adjunctive method (thoracotomy, decapitation) performed after the loss of consciousness. Using an adjunctive method may be especially important when euthanizing ectothermic animals, because their heartbeat and respiration are difficult to assess.

For a list of acceptable methods and agents for euthanasia of different species of animals, see Table 1: Agents and Methods of Euthanasia by SpeciesTables. The AVMA Guidelines for Euthanasia 2013 (view the guidelines) provide additional details, as well as references for specific technical and safety information.

Table 1

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Last full review/revision May 2015 by Samuel C. Cartner, DVM, PhD, DACLAM

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