Euthanasia (Greek, "good death") is the practice of killing a person or animal, in a painless or minimally painful way, for merciful reasons, usually to end suffering.
An animal is euthanized when it is killed in a manner deemed to be humane, and in the case of a pet, in accordance with the emotional needs of the owner.
It can be done with inhalant agents, noninhalant pharmacologic agents (administered by a lethal injection), and physical methods. Pets are almost always euthanized via lethal injection, typically a very high dose of a barbiturate anaesthetic such as pentobarbital. Unconsciousness, respiratory and cardiac arrest follow rapidly, usually within 30 seconds to several minutes later. Pet owners generally consider it to be a quick and peaceful death.
Pet and livestock owners might decide to do this when the animal is suffering significantly due to injury or terminal illness, is overly aggressive, or when the owner/guardian is no longer able to keep or care for the animal and is unable or unwilling to find a new home for it. Additionally, many stray and feral animals, in particular, cats, are euthanized due to the lack of adoptive homes. In this latter case, the simple presence of the animals may be considered objectionable by those who live or work nearby, and who trap the animals (or request the assistance of animal control services to do so) and surrender them to the local animal shelter.
Euthanasia is typically performed in a veterinary clinic or hospital, or in an animal shelter, and is usually carried out by a veterinarian, or a veterinary technician working under the vet's supervision. Note that euthanasia is performed at the discretion of the attending veterinarian, who may refuse an animal owner's request to euthanize if he or she feels it is not medically or ethically necessary (so-called "convenience" euthanasia).
Some in the animal welfare or animal rights movements consider the use of the term "euthanasia", with its connotation of being done to ease suffering, to be a misnomer when applied to the euthanasia of homeless (or soon-to-be homeless) animals that are otherwise healthy and free of behavioral disorders. Some breeders also kill puppies that do not conform to the standard of the dog breed to prevent the perpetuation of the perceived flaws or faults. Most modern breeders simply spay or neuter the animals and place them in homes as pets.
Animal shelters often euthanize animals when they can't find a home for them, typically after a standard period of time (ranging from several days to several weeks for unclaimed stray animals). Some consider this immoral and cruel; others believe that euthanasia is a less objectionable alternative to having unwanted animals go to unsuitable homes or having them live out their lives in shelters which generally do not have the funding to give unlimited numbers of animals proper care and exercise indefinitely. Behavioral unsuitability for adoption (ie, aggression, house-breaking, etc) is a major non-clinical reason for euthanasia in animal shelters.
So-called "no kill" shelters exist, some run by private animal welfare organizations while others are subsidized wholly or in part by local government agencies. These shelters make it official policy to never euthanize animals for non-medical reasons. Overcrowding, lack of adoptive homes and underfunding are recognized problems. Supporters consider these minor compared to the ethics of euthanasia they consider unnecessary.
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