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Animal euthanasia (from Greek, meaning "good death") is the act of humanely putting an animal to death or allowing it to die as by withholding extreme medical measures. Reasons for euthanasia include incurable (and especially painful) conditions or diseases, lack of resources to continue supporting the animal, or laboratory test procedures. Euthanasia methods are designed to cause minimal pain and distress. Euthanasia is distinct from animal slaughter and pest control, which are performed for purposes other than an act of mercy, although in some cases the killing procedure is the same.
Intravenous anesthetic 
Pets are almost always euthanized by intravenous injection, typically using a very high dose of pentobarbital or sodium thiopental. Unconsciousness, respiratory then cardiac arrest follow rapidly, usually within 30 seconds. Observers generally describe the method as leading to a quick and peaceful death.
For companion animals euthanized in animal shelters, 14 states in the US now prescribe intravenous injection as the required method. These laws date to 1990, when Georgia's "Humane Euthanasia Act" became the first state law to mandate this method. Before that, gas chambers and other means were commonly employed. The Georgia law was resisted by the Georgia Commissioner of Agriculture, Tommy Irvin, who was charged with enforcing the act. In March 2007, he was sued by former State Representative Chesley V. Morton, who wrote the law, and subsequently ordered by the Court to enforce all provisions of the Act.
Some veterinarians perform a two-stage process: an initial injection that simply renders the pet unconscious and a second shot that causes death. This allows the owner the chance to say goodbye to a live pet without their emotions stressing the pet. It also greatly mitigates any tendency toward spasm and other involuntary movement which tends to increase the emotional upset that the pet's owner experiences.
For large animals, the volumes of barbiturates required are considered by some to be impractical, although this is standard practice in the United States. For horses and cattle, other drugs may be available. Some specially formulated combination products are available, such as Somulose (Secobarbital/Cinchocaine) and Tributame (Embutramide/Chloroquine/Lidocaine), which cause deep unconsciousness and cardiac arrest independently with a lower volume of injection, thus making the process faster, safer, and more effective.
Occasionally, a horse injected with these mixtures may display apparent seizure activity before death. This may be due to premature cardiac arrest. However, if normal precautions (e.g., sedation with detomidine) are taken, this is rarely a problem. Anecdotal reports that long term use of phenylbutazone increase the risk of this reaction are unverified.
After the animal has expired, it is not uncommon for the body to have posthumous body jerks, or for the animal to have a sudden bladder outburst. Unlike in films, the animal's eyes do not usually close when it dies, and this has been stated purely as a drama display for pet death in movies (such as Marley & Me).
Gas anesthetics such as isoflurane and sevoflurane can be used for euthanasia of very small animals. The animals are placed in sealed chambers where high levels of anesthetic gas are introduced. Death may also be caused using carbon dioxide once unconsciousness has been achieved by inhaled anaesthetic. Carbon dioxide is often used on its own for euthanasia of wild animals. There are mixed opinions on whether it causes distress when used on its own, with human experiments lending support to the evidence that it can cause distress and equivocal results in non-humans.
Carbon monoxide is often used, but some states in the US have banned its use in animal shelters: although carbon monoxide poisoning is not particularly painful, the conditions in the gas chamber are often not humane. Nitrogen has been shown to be effective, although some young animals are rather resistant and it currently is not widely used.
Cervical dislocation 
Cervical dislocation, or displacement of the neck, is a simple and common method of killing small animals such as mice. Performed properly, it causes instant death, and it requires no equipment other than a pair of gloves for protection while handling the animal.
Intracardiac or intraperitoneal injection 
While intraperitoneal injection is fully acceptable (although it may take up to 15 minutes to take effect in dogs and cats), an intracardiac (IC) injection may only be performed on an unconscious or deeply sedated animal. In California, performing IC injections on a fully conscious animal is a crime.
This can be an appropriate means of euthanasia for large animals (e.g., horses, cattle, deer) if performed properly. This may be performed by means of:
- Free bullet
- Traditionally used for shooting horses. The horse is shot in the forehead with the bullet directed down the spine through the medulla oblongata, resulting in instant death. The risks are minimal if carried out by skilled personnel in a suitable location.
- Captive bolt
- Commonly used for cattle and other livestock. The bolt is fired through the forehead causing massive disruption of the cerebral cortex. In cattle, this stuns the animal, though if left for a prolonged period it will die from cerebral oedema. However, this is a welfare problem. Death should be rapidly brought about by pithing or exsanguination. Horses are killed outright by the captive bolt, making pithing and exsanguination unnecessary.
Reasons for euthanasia 
- Terminal illness – e.g. cancer
- Behavioral problems (usually ones that cannot be corrected) – e.g. aggression
- Illness or broken limbs that would cause suffering for the animal to live with, or when the owner cannot afford (or has a moral objection to) treatment
- Old age – deterioration leading to loss of major bodily functions, resulting in severe impairment of the quality of life
- Lack of homes – Most shelters receive considerably more surrendered animals than they are capable of re-housing. This may be attributed to irresponsible owners who do not spay or neuter pets (which results in unwanted litters), as well the tendency of consumers to obtain pets via means other than adoption. Most pets turned in to animal shelters are not adopted.
- Animal testing – Before, during or after use in testing, animals may be euthanized. For example, an experiment may consist of breeding rats with a certain genetic defect. After behavioural tests, the rat is euthanised, dissected and examined.
Small animal 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 veterinarian's supervision. Often animal shelter workers are trained to perform euthanasia as well. Some veterinarians will perform euthanasia at the pet owner's home—this is virtually mandatory in the case of large animal euthanasia. In the case of large animals which have sustained injuries, this will also occur at the site of the accident, for example, on a racecourse.
In some instances, animals euthanized at shelters or animal control agencies have been sent to meat rendering facilities to be processed for use in cosmetics, fertilizer, gelatin, poultry feed, pharmaceuticals and pet food. The amount of pentobarbital in dog food may have caused dogs to become less responsive to the drug when being euthanized, though a 2002 FDA study found no dog or cat DNA in the foods they tested. They theorized that the drug found its way into dog food from euthanized cattle and horses. They also stated that the level of the drug found in the food was safe.
See also 
- Animal chaplains
- Animal loss
- Animal slaughter
- Animal welfare
- Chick culling
- Dysthanasia (animal)
- Rainbow Bridge (pets)
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