THE MERCK VETERINARY MANUAL
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Overview of Euthanasia

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Euthanasia is defined as an easy, painless death. In regard to animals, euthanasia is the act of killing an animal in a humane manner. The primary objectives of animal euthanasia are: 1) relieving pain and suffering of the animal(s) to be euthanized, 2) minimizing the pain, anxiety, distress, and fear the animal experiences before consciousness is lost, and 3) inducing a painless and distress-free death.

Observing the behavioral and physiologic responses of animals is beneficial in assessing whether the objectives of euthanasia are being met. A variety of behaviors and physiologic responses may be demonstrated by animals experiencing pain and/or fear, including (but not limited to) distress vocalizations, struggling, escape attempts, agitation, freezing, aggression, fearful postures or facial expressions, trembling, salivating, urinating, defecating, evacuation of anal sacs, pupillary dilation, panting, tachycardia, and sweating. Response to euthanasia procedures (handling, restraint, confinement, venipuncture, gas odors, etc) varies among species and among individuals, necessitating careful monitoring of every euthanasia.

Loss of physiologic function during euthanasia 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 function, and finally, 4) permanent loss of brain function. If loss of motor or respiratory and cardiac function precedes loss of consciousness, animals 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.

Before the carcass is disposed of, death must be verified by a means appropriate to the species and the method of euthanasia. Narcosis must not be confused with death. Determination of death in ectothermic animals may be more difficult due to differences in their physiology.

Euthanasia frequently results in chemical tissue residues, necessitating proper disposal to prevent contamination of the environment or other animals (eg, scavengers, predators).

The methods used for euthanasia of animals intended for consumption by humans or other animals in the USA must meet the requirements of the USDA; chemical agents that result in chemical tissue residues cannot be used unless approved by the FDA.

Other factors that must be taken into account in animal euthanasias include the safety of operators, observers, and other animals; human psychological responses to euthanasia, eg, sadness and grief; and fear and anxiety of other animals exposed to the behaviors, vocalizations, and pheromones of animals being euthanized. Counseling services and pet loss support hotlines are available for grieving pet owners in some communities and veterinary colleges. Personnel involved in euthanasias or animal slaughter may also experience negative psychological consequences. Workplace support programs and accessibility to counseling may help alleviate the stress felt by euthanasia personnel.

The operator must be knowledgeable regarding the agent, method, equipment, and behavior and physiology of the species and individual animal(s) to be euthanized; be trained in the technique; and have demonstrated skill in the euthanasia operation to be performed.

Selection of an appropriate method and agent of euthanasia is paramount in assuring a humane death. Selection is based on the behavior, physiology, and metabolism of the species, as well as any particular characteristics of the individual animal(s) that would influence the ability to use a particular method. The setting, the available means of animal restraint, the skill and knowledge of the operator, the number of animals to be euthanized, and the purpose for which the animal(s) is to be used are also determining factors in selection of the methods and the agent. Euthanasia in circumstances other than the clinical veterinary setting, eg, wildlife in the field, may limit the euthanasia options. However, in all circumstances, humaneness to the animal should be a prevailing concern. Slaughter of animals for food, fur, or fiber, and euthanasia of wildlife and feral animals should all adhere to the same humane standards for euthanasia.

For a list of acceptable methods and agents for euthanasia of different species of animals, see Agents and Methods of Euthanasia by SpeciesTables. It is the obligation of the operator to know the physiology and behavior of the species to be euthanized, and the specific technical information and safety precautions for the method and agent selected. The 2000 report of the American Veterinary Medical Association panel on euthanasia (revised in 2007) provides additional details, as well as references for specific technical and safety information.

Table 1

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Inhalant agents should not be used alone in animals <16 wk old because neonatal animals are more resistant to hypoxia and it takes longer for them to die. Reptiles, amphibians, diving birds, and diving and burrowing mammals may have a prolonged time to loss of consciousness with inhalant gases. Inhalant anesthetics are useful for small animals (<7 kg) in which injections are difficult. The order of preference of inhalant anesthetics for euthanasia is halothane, enflurane, isoflurane, sevoflurane, methoxyflurane, and desflurane, with or without nitrous oxide.

Injectable agents are the most rapid and reliable and are preferred when venipuncture can be accomplished without causing fear to the animal or unnecessary risk to the operator. All barbituric acid derivatives are acceptable IV euthanasia agents. Certain injectable agents (eg, strychnine, nicotine, caffeine, magnesium sulfate, cleaning agents, solvents, disinfectants, other toxins or salts, potassium chloride as a sole agent, and all neuromuscular blocking agents) are absolutely not acceptable and are condemned as agents of euthanasia.

Physical methods of euthanasia, including captive bolt, gunshot, cervical dislocation, decapitation, microwave irradiation, and thoracic compression can be humane methods of euthanasia when used properly by skilled operators with well-maintained equipment and when other means of euthanasia are impractical or contraindicated by the intended use of the animal. Exsanguination, stunning, and pithing should not be used as sole methods of euthanasia but as adjuncts to other methods.

Last full review/revision March 2012 by Elizabeth A. Shull, DVM, DACVIM (Neurology), DACVB

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