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Neutering, from the Latin neuter (of neither sex), is the removal of an animal's reproductive organ, either all of it or a considerably large part. It is the most drastic surgical procedure with sterilizing purposes. The process is often used in reference to males whereas spaying is often reserved for females. Colloquially, both terms are often referred to as fixing. While technically called castration for males, in male horses, the process is referred to as gelding.

Unlike in humans, neutering is the most common sterilizing method in animals. In the United Statesmarker, 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.

Health and behavioral effects


Besides being a birth control method, neutering[males]/spaying[females] has the following health benefits:

  • Prevention of mammary tumors: Female cats and dogs are seven times more likely to develop mammary tumors if they are not spayed before their first heat cycle. The risk is generally estimated at 25% over a lifetime in unspayed females, meaning that the risk is reduced to ca. 3.6% over a lifetime in females spayed before their first heat.
  • Pyometra is prevented, either due to the removal of the organ (when ovariohysterectomy is performed) and/or because of the lack of female sex hormones (oestrogen and progesterone) after spaying.
  • Uterine cancer, ovarian cancer and testicular cancer are prevented due to the removal of the susceptible organs. These cancers are uncommon in dogs and cats, with only a 1% risk of obtaining testicular or ovarian cancer. In female rabbits, however, the rate of uterine cancer may be as high as 80%.



A cat under anesthesia
  • 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. In one study the risk of anesthetic-related death (not limited to neutering procedures) was estimated at 0.05% for healthy dogs and 0.11% for healthy cats. The risk for sick dogs and cats were 1.33% and 1.40% respectively.
  • 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 and urinary tract cancer.

  • Neutered dogs of both genders have a 27% to 38% increased risk of adverse reactions to vaccinations. The incidence of adverse reactions for neutered and intact dogs combined is 0.32%

  • Neutered dogs have also been known to develop hormone-responsive alopecia (hair loss).

Specific to Males

  • Neutered male dogs display a fourfold increased incidence of prostate cancer over intact males.

  • In a study of 29 intact male dogs and 47 castrated males aged 11–14, the neutered males were significantly more likely to progress from one geriatric cognitive impairment condition (out of the four conditions - 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) to two or more conditions. Testosterone in intact males is thought to slow the progression of cognitive impairment, at least in dogs that already have mild impairment.

Specific to Females

  • Spayed female dogs can develop urinary incontinence. Studies report incidence rates of 4.9% to 20%.
  • Spayed female dogs are at an increased risk of hypothyroidism
  • Despite the risk of pyometra being greatly reduced in spayed females, Stump pyometra may still occur in this group.

Behavioral effects

Sexually dimorphic behaviors such as mounting, urine spraying and some forms of male aggression (relating to females in estrus) may be reduced due to the decrease in hormone levels brought about by neutering. This is an especially significant benefit in male cats due to the extreme undesirability of male cat sexual behavior for pet owners. Most animals lose their libido and females no longer experience heat cycles, which are sometimes considered a major nuisance factor. Personality changes may occur in the animal.

Various studies of the effects neutering has overall on male and female dog aggression have been unable to arrive at a consensus. A possible reason for this according to one study is changes to other factors have more of an effect than neutering. One study reported results of aggression towards familiar and strange people and other dogs reduced between 10 and 60 percent of cases, while other studies reported increases in possessive aggression and aggression towards familiar and strange people, and yet another study reported no effect on territorial aggression, and only a reduction in dominance aggression that existed for at least 5 years. For females with existing aggression, many studies reported increases in aggressive behavior and some found increased separation anxiety behavior.


Females (spaying)

Feline uterus
In female animals, spaying involves abdominal surgery to remove the ovaries and uterus (ovario-hysterectomy). Alternatively, it is also possible to remove only the ovaries (ovariectomy), which is mainly done in cats and young female dogs. Spaying is performed commonly on household pets such as cats and dogs, as a method of birth control. It is performed less commonly on livestock, as a method of birth control or for other reasons. In mares, these other reasons include behavior modification.
Incision wound of a female cat
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 (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.

See also oophorectomy and hysterectomy.

Males (castration)

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).

For more information, see castration and gelding (specific to horses)

Nonsurgical alternatives


  • Male dogs - Neutersol (Zinc gluconate neutralized by arginine). Cytotoxic; produces infertility by chemical disruption of the testicle. It is now produced as Esterilsol in Mexico.
  • 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.
  • Male sheep and pigs - Wireless Microvalve. Using a piezoelectric polymer that will deform when exposed to a specific electric field broadcast from a key fob (like a car alarm) the valve will open or close, preventing the passage of sperm, but not seminal fluid. Located in a section of the vas deferens that occurs just after the epididymis, the implantation can be carried out by use of a hypodermic needle.
  • 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 sperm from fertilizing it for periods from 22 months up to 7 years (depending on the animal). This will not prevent the animal from going into heat (ovulating) and other than birth control, none of the above mentioned advantages or disadvantages apply.


  • Noninvasive vasectomy using ultrasound.

Surgical alternatives

Vasectomy: The cutting 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 from neutering in that they leave the animal's levels and patterns of sex hormone unchanged. Both sexes will retain their normal reproductive behavior, and other than birth control, none of the advantages and disadvantages listed above apply. 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 venereal diseases. [55085]

Early-age neutering

Early-age neutering (or prepubertal gonadectomy - the removal of the ovaries or testes before the onset of puberty) is typically performed in dogs and cats between 8 and 16 weeks of age, as compared to the conventional 6 to 8 months. It is used mainly in animal sheltering and rescue where puppies and kittens can be neutered before being adopted out, eliminating non-compliance with sterilisation agreement, which is typically above 40%. The American Veterinary Medical Association, American Animal Hospital Association and the Canadian Veterinary Medical Association support the procedure for population control, provided that the veterinarian uses his/her best knowledge when making the decision about the age at neutering. As is with conventional age neutering.

While the non-age specific risks and benefits cited above also apply to early-age neutering, various studies indicate that the procedure is safe and not associated with increased mortality or serious health and behavioral problems when compared to conventional age neutering. Anesthesia recovery in young animals is usually more rapid and there are fewer complications. One serious downside is that in female dogs, there is an increasing risk of urinary incontinence the earlier the procedure is carried out. It is recommended that female dogs be spayed no earlier than 3 to 4 months of age.

One study showed the incidence of hip dysplasia increased to 6.7% for dogs neutered before 5.5 months compared to 4.7% for dogs neutered after 5.5 months, although the cases associated with early age neutering seems to be of a less severe form. There was no association between age of neutering and arthritis or long-bone fractures. Another study showed no correlation between age of neutering and musculoskeletal problems. A study of large breed dogs with cranial cruciate ligament rupture associated early-age neutering with the development of an excessive tibial plateau angle Female dogs neutered early are much more likely to develop cystitis although the risk does not appear to be chronic. Two studies showed an increased risk of canine parvovirus infection, which one of the study attributed to the increased susceptibility of young dogs rather than long term immune suppression.

In terms of behavior in dogs, separation anxiety, escape behavior and inappropriate elimination are reduced while noise phobia and sexual behavior was increased. In males with aggression issues, earlier neutering may increase barking. In cats, asthma, gingivitis, and hyperactivity were decreased, while shyness was increased. In male cats, occurrence of abscesses, aggression toward veterinarians, sexual behaviors, and urine spraying was decreased, while hiding was increased.

Terminology for neutered animals

Male animals

Neutered males of given animal species sometimes have specific names:

Female animals

A specialized vocabulary in animal husbandry and fancy has arisen for spayed females of given animal species:

Religious views on neutering



While there are differing views in Islam with regard to neutering animals, some Islamic association have stated that when done to maintain the health and welfare of both the animals and the community, neutering is allowed on the basis of 'maslahat' (general good) or "choos[ing] the lesser of two evils".


Traditional interpretations of Orthodox Judaism forbids the castration of both humans and animals by Jews, except in lifesaving situations. In 2007, the Sephardic Chief Rabbi of Israelmarker Rabbi Shlomo Amar issued a ruling stating that it is permissible to have companion animals spayed or neutered on the basis of the Jewish mandate to prevent cruelty to animals.

See also

  • AB 1634 - A California bill proposing mandatory spay-neuter laws.


External links

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