Neutering, from the Latin neŭter (of neither type), is the removal of an animal's reproductive organ, either all of it or a considerably large part of it. It is the most drastic surgical procedure with sterilizing purposes. The process is also referred to as castration, or gelding in male horses; while the process in females is also called spaying.
Unlike in humans, neutering is the most common sterilizing method in animals. While many agree on the advantages of neutering as a method of birth control, the necessity and humanity of this method (as opposed to alternative methods of birth control) and the political agendas within the debate are a subject of some controversy. In the USA, most humane societies, animal shelters, and rescue groups (not to mention numerous commercial entities) urge pet owners to have their pets "spayed or neutered" to prevent the births of unwanted litters, contributing to the overpopulation of animals. In Europe, the procedure is less commonly performed, especially in dogs.
In addition to being a birth control method, neutering has health benefits. Hormone-associated diseases such as benign prostatic hypertrophy are prevented. Female cats and dogs are seven times more likely to develop mammary tumors if they are not spayed before their first heat cycle.  A dangerous common uterine infection known as pyometra is also prevented. Uterine, ovarian, and testicular cancer are also prevented for obvious reasons, although these types of cancer are uncommon to begin with.
The procedure may end or curb such behaviors as roaming in search of a mate, and sexual mounting. Depending on their environment, dogs which are less prone to escaping from their yard and/or roaming are also less likely to be lost, stolen, or hit by a car.
As with any surgical procedure, immediate complications of neutering include the usual anesthetic and surgical complications, such as bleeding and infection. These risks are relatively low in routine spaying and neutering; however, they may be increased for some animals due to other pre-existing health factors.
Neutered dogs and cats of both genders have an increased risk of obesity. Theories for this include reduced metabolism, reduced activity, and eating more due to altered feeding behavior.
Neutered dogs of both genders are at a twofold excess risk to develop osteosarcoma as compared to intact dogs , as well as an increased risk of hemangiosarcoma.
Neutered dogs of both genders have an increased risk of adverse reactions to vaccinations.
Neutered dogs have also been known to develop hormone-responsive alopecia (hair loss).
In addition, neutered male dogs are at higher risk than intact males of developing moderate to severe geriatric cognitive impairment (geriatric cognitive impairment includes disorientation in the house or outdoors, changes in social interactions with human family members, loss of house training, and changes in the sleep-wake cycle) .
Despite the risk of pyometra being greatly reduced in spayed females, Stump pyometra may still occur in this group.
Obviously, most animals lose their libido due to the hormonal changes involved with both genders, and females no longer experience heat cycles, which are sometimes considered a major nuisance factor, especially in female cats. Minor personality changes may occur in the animal. Neutering is often recommended in cases of undesirable behavior in dogs, although studies suggest that while roaming, urine marking, and mounting are reduced in neutered males, it has little effect on aggression and other important behavioral issues.  Intact male cats are more prone to urine spraying, while many common behavioral causes of urine marking remain in castrated cats.
In female animals, spaying involves abdominal surgery to remove the ovaries and uterus (ovariohysterectomy). Alternatively, it is also possible to remove the ovaries and leave the uterus inside (oophorectomy), which is mainly done in cats and young bitches. It is commonly practiced on household pets such as cats and dogs as a method of birth control, but is rarely performed on livestock.
The surgery is usually performed through a ventral (belly) midline incision below the umbilicus (belly button). The incision size varies depending upon the surgeon and the size of the animal. The uterine horns are identified and the ovaries are found by following the horns to their ends.
There is a ligament that attaches the ovaries to the kidneys which may need to be broken so the ovaries can be identified. The ovarian arteries are then ligated twice (tied-off) with resorbable suture material and then the arteries transected (cut). The uterine body (which is very short in litter bearing species) and related arteries are also tied off just in front of the cervix (leaving the cervix as a natural barrier). The entire uterus and ovaries are then removed. The abdomen is checked for bleeding and then closed with a 3 layer closure. The linea alba (muscle layer) and then the subcutaneous layer (fat under skin) are closed with resorbable suture material. The skin is then stapled, sutured, or glued closed.
In male animals, castration involves the removal of the testes, and is commonly practiced on both household pets (for birth control) and on livestock (for birth control, as well as to improve commercial value).
Male dogs - Neutersol (Zinc gluconate neutralized by arginine). Cytotoxic; produces infertility by chemical disruption of the testicle. It is no longer produced.
Male rats - Adjudin (analogue of indazole-carboxylic acid), induces reversible germ cell loss from the seminiferous epithelium by disrupting cell adhesion function between nurse cells and immature sperm cells, preventing maturation.
Female mammals - Vaccine of antigens (derived from purified Porcine zona pellucida) encapsulated in liposomes (cholesterol and lecithin) with an adjuvant, latest US patent RE37,224 (as of 2006-06-06), CA patent 2137263 (issued 1999-06-15). Product commercially known as SpayVac, a single injection causes a treated female mammal to produce antibodies that bind to ZP3 on the surface of her ovum, blocking sperms from fertilizing it for periods from 22 months up to 7 years (depending on the animal).
Vasectomy: The snipping and tying of the vasa deferentia (plural of vas deferens). Failure rates are insignificantly small. This procedure is routinely carried out on male ferrets and sheep to manipulate the estrus cycles of in-contact females. It is uncommon in other animal species.
Tubal Ligation: Snipping and tying of fallopian tubes as a sterilization measure can be performed on female cats and dogs. Risk of unwanted pregnancies is insignificantly small. Only a few veterinarians will perform the procedure.
Like other forms of neutering, vasectomy and tubal ligation eliminate the ability to produce offspring. They differ in that they leave the animal's levels and patterns of sex hormone unchanged. Females will still get in heat, want to mate, and the related issues will still be present. This method is favored by some of the people who want to infringe on the natural state of companion animals as little as necessary to achieve the reduction of unwanted births of cats and dogs.
Penile translocation is sometimes performed in cattle to produce a "teaser bull", which retains its full libido, but is incapable of intromission. This is done to identify estrous cows without the risk of transmitting veneral diseases. 
Terminology for neutered animals
Neutered males of given animal species sometimes have specific names:
A specialized vocabulary in animal husbandry and fancy has arisen for spayed females of given animal species:
Chicken - Poulard
Ferret - Sprite
TV celebrity Bob Barker helped to popularize the spay-or-neuter drive by closing every episode of The Price Is Right with a request for people to help control the pet population by spaying or neutering their pets. In the movie Shrek 2, Donkey proposed that Puss in Boots be given the "Bob Barker Treatment", an indirect reference to neutering.
Orthodox Judaism forbids the castration of both humans and animals by Jews, except in lifesaving situations. 
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^ * Horses - "In the fall of 2002 and the spring of 2003 we initiated a study to compare the long-term efficacy of a single-shot contraceptive vaccine directed at gonadotropin releasing hormone (GonaCon) with that of a single-shot vaccine directed at the zona pellucida (SpayVac) with the use of intrauterine contraceptive devices (IUD).
Multiyear contraceptive efficacy was greatest for SpayVac, followed by GonaCon and the IUD.
All mares in the SpayVac group were infertile ... during the first breeding season. In years two and three, 80% of the SpayVac-treated mares ... were infertile. It is noteworthy that the drop in contraception rate was greatest between years one and two with only minimal decrease from year two to year three. This suggests that considering the immunological response, there are two sub-populations of mares. One population responded with antibody titers adequate for contraception that were maintained over several years, versus the other population that lasted no more than one year.
The average titer for SpayVac contracepted mares progressively declined during each year of study. However, the average titer in year 3 for contracepted SpayVac mares was still nearly 8-fold greater than the average “breakthrough” titer for all SpayVac-treated mares that became pregnant. There was a 37% decline in titer between year 1 and 2 and a 33% decline between years 2 and 3. If we assume an average annual rate of decline in titer of 35%, this suggests that on average, the majority of SpayVac-treated mares will remain contracepted for four additional years before the breakthrough titer is reached. This projection of a total of 7 years of contraception for SpayVac-treated mares is supported by the literature report of long-term efficacy of SpayVac use in Grey seals (Brown et al., 1997)." Gary Killian, Nancy K. Diehl, Lowell Miller, Jack Rhyan, David Thain (2007). "Long-term Efficacy of Three Contraceptive Approaches for Population Control of Wild Horses". Cattlemen's Update: 48-63.
Deer - "We treated 20 does with SpayVac (zp-based vaccine) in March 2003, 33 does with GonaCon-KLH (GnRH-based vaccine) in February 2004, and 27 does with a modified SpayVac formulation in August 2004 in Princeton, NJ. We also administered GonaCon-KLH to 29 does in Madison, NJ (July 2005) and 15 does received GonaCon-Blue in Newark, DE (August 2005). After one year (2004), only one of 20 (5%) does gave birth after receiving SpayVac. None of the remaining 14 SpayVac-treated does gave birth the second year (2005), and 7 of the 13 (54%) remaining does gave birth the third year (2006). None (n = 16) of the does administered modified SpayVac reproduced in spring 2005. Three of 12 (25%) remaining does in this same treatment reproduced spring 2006." DeNicola, Anthony; Lowell A. Miller, James P. Gionfriddo, Kathleen A. Fagerstone (2007-3-16). Status of Present Day Infertility Technology. Northeast Association of Fish and Wildlife Agencies. Retrieved on 2007-3-16.
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