Romanino, Scene of a cat castration, 1531–32, Castello del Buonconsiglio, Trento.

Neutering, from the Latin neuter (of neither sex[1]), is the removal of an animal's reproductive organ, either all of it or a considerably large part. 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.

Neutering is the most common sterilizing method in animals. In the United States, most humane societies, animal shelters and rescue groups urge pet owners to have their pets spayed or neutered to prevent the births of unwanted litters, contributing to the overpopulation of unwanted animals in the rescue system.


Health and behavioral effects


Besides being a birth control method, and being convenient to many owners, neutering/spaying has the following health benefits:

  • 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 especially significant in male cats due to the extreme undesirability of male cat sexual behavior for many pet owners.[2]
  • Prevention of mammary tumors: Female cats and dogs are about seven times less likely to develop mammary tumors if they are spayed before their first heat cycle.[3] Mammary neoplasia, or breast cancer, is a very common disorder of female dogs, with a reported incidence of 3.4%. Of female dogs with mammary tumors, 50.9% have malignant tumors. Female dogs that have been spayed before their first heat have a lifetime chance of developing mammary tumors of about 99.5% less than that of intact females. If allowed to go through their first heat before spaying, then their risk is close to 92% less. Also, spaying female dogs more than two years before the removal of mammary tumors increases the dog's survival odds by 45%.[4]
  • Without its ability to reproduce, a female animal effectively has a zero risk of pregnancy complications, such as spotting and false pregnancies, the latter of which can occur in more than 50% of unspayed female dogs.[5]



  • 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.[6]
  • Spaying and neutering dogs and cats increases the risk of obesity.[7] In cats, a decrease in sex hormone levels seems to be associated with an increase in food intake.[8] In dogs, the effects of neutering as a risk factor for obesity vary between breeds.[9]
  • Neutered dogs of both sexes are at a twofold excess risk to develop osteosarcoma as compared to intact dogs.[10][11][12]
  • Studies of cardiac tumors in dogs showed that there was a 5 times greater risk of hemangiosarcoma, one of the three most common cancers in dogs, in spayed females than intact females and a 2.4 times greater risk of hemangiosarcoma in neutered dogs as compared to intact males.[13][14]
  • Spaying and neutering is associated with an increase in urinary tract cancers in dogs.[15]
  • Neutered dogs of both sexes 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%.[16]
  • Neutered dogs have also been known to develop hormone-responsive alopecia (hair loss).[17]
  • A 2004 study found that spayed and neutered dogs had a higher incidence of CCL rupture, a form of ACL injury.[18]

Specific to males

  • About 2% of neutered male dogs eventually develop prostate cancer, compared to less than 0.6% of intact males.[19][20] The evidence is most conclusive for Bouviers.[21]
  • 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.[22]
  • As compared to intact males, male neutered cats are at an increased risk for certain problems associated with feline lower urinary tract disease, including the presence of stones or a plug in the urethra and urethral blockage.[23]
  • Neutering also has been associated with an increased likelihood of urethral sphincter incontinence in males.[24]

Specific to females

Current research

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.[29] One study reported results of aggression towards familiar and strange people and other dogs reduced between 10 and 60 percent of cases,[30] while other studies reported increases in possessive aggression[31] and aggression towards familiar and strange people,[32] and yet another study reported no effect on territorial aggression, and only a reduction in dominance aggression that existed for at least 5 years.[33] For females with existing aggression, many studies reported increases in aggressive behavior[34][35][36][37] and some found increased separation anxiety behavior.[32][38] A report from the American Kennel Club Canine Health Foundation reported significantly more behavioral problems in castrated dogs. The most commonly observed behavioral problem in spayed females was fearful behavior and the most common problem in males was aggression.[39] Early age gonadectomy is associated with an increased incidence of noise phobias and undesirable sexual behaviors.[40]


Females (spaying)

Feline uterus

In female animals, spaying involves abdominal surgery to remove the ovaries and uterus (hystero-oophorectomy). Alternatively, it is also possible to remove only the ovaries (oophorectomy), 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.[41]

Surgical incision site of a female cat

The surgery is usually performed through a ventral midline incision below the umbilicus. 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 13th rib which may need to be broken so the ovaries can be identified. The ovarian arteries are then ligated with resorbable suture material and then the arteries transected. 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 and then the subcutaneous layer are closed with resorbable suture material. The skin is then stapled, sutured, or glued closed.

Spaying in female dogs removes the production of progesterone, which is a natural calming hormone and a serotonin uplifter. Spaying may therefore escalate any observable aggressive behaviour, either to humans or other dogs.

The risk of infections, bleeding, ruptures, inflammation and even reactions to the drugs given to the animal as part of the procedure are all possibilities that should be considered.

Males (castration)

In male animals, castration involves the removal of the testes, and is commonly practiced on both household pets (for birth control and behaviour modification) 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.[42]
  • 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.[43] 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,[44] 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[45][46]). 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.


  • Male mammals - Noninvasive vasectomy using ultrasound.[47]
  • Female mammals - orally administered phosphodiesterase 3 inhibitor ORG 9935 daily before and during ovulation which blocks the resumption of meiosis resulting in ovulation of a non-fertilizable, immature oocyte without rupturing the follicle.[48]

Surgical alternatives

Vasectomy: The cutting and tying of the vasa deferentia. 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 people who seek minimal infringement on the natural state of companion animals to achieve the desired 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.[49]

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%.[2] 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.[50][51][52]

While the age-unrelated risks and benefits cited above also apply to early-age neutering, various studies have indicated that the procedure is safe and not associated with increased mortality or serious health and behavioral problems when compared to conventional age neutering.[53][54][55][56][57] Anesthesia recovery in young animals is usually more rapid and there are fewer complications.[57][58] One study found that in female dogs there is an increasing risk of urinary incontinence the earlier the procedure is carried out; the study recommended that female dogs be spayed no earlier than 3 to 4 months of age.[54] A later study comparing female dogs spayed between 4 to 6 months and after 6 months showed no increased risk.[59]

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.[54] Another study showed no correlation between age of neutering and musculoskeletal problems.[56] A study of large breed dogs with cranial cruciate ligament rupture associated early-age neutering with the development of an excessive tibial plateau angle.[60] 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.[54][56]

In terms of behavior in dogs, separation anxiety, aggression, 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.[54] 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.[53]

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,[61] some Islamic associations 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 'maslaha' (general good)[62] or "choos[ing] the lesser of two evils".[63]


Traditional interpretations of Orthodox Judaism forbids the castration of both humans and non-human animals by Jews,[64] except in lifesaving situations.[65] In 2007, the Sephardic Chief Rabbi of Israel 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.[66]

See also


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