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. 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.
- 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. 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%.
- 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.
- Pyometra, Uterine cancer, Ovarian cancer and Testicular cancer are prevented as the susceptible organs are removed, though Stump pyometra may still occur in spayed females.
- 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.
- Spaying and neutering cats may increase the risk of obesity. In cats, a decrease in sex hormone levels seems to be associated with an increase in food intake. In dogs, the effects of neutering as a risk factor for obesity vary between breeds.
- Neutered dogs of both sexes are at a twofold excess risk to develop osteosarcoma as compared to intact dogs. The risk of osteosarcoma increases with increasing breed size and especially height.
- 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.
- Spaying and neutering is associated with an increase in urinary tract cancers in dogs.
- 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%.
- Neutered dogs have also been known to develop hormone-responsive alopecia (hair loss).
- A 2004 study found that spayed and neutered dogs had a higher incidence of cranial cruciate ligament (CCL) rupture, a form of anterior cruciate ligament (ACL) injury.
Specific to males
- About 2% of neutered male dogs eventually develop prostate cancer, compared to less than 0.6% of intact males. The evidence is most conclusive for Bouviers.
- 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.
- 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.
- Neutering also has been associated with an increased likelihood of urethral sphincter incontinence in males.
Specific to females
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. 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. Early age gonadectomy is associated with an increased incidence of noise phobias and undesirable sexual behaviors.
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 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.
Surgical incision site of a female cat
The surgery can be performed using a traditional open approach or by laparoscopic "keyhole" surgery. Open surgery is more widely available as laparoscopic surgical equipment costs are expensive.
Traditional open 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 body wall which may need to be broken down 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.
Laparoscopic surgery is performed using a camera and instruments placed through small incisions (ports) in the body wall. The patient is under anaesthesia and lying on their back. The incisions are between 5mm and 10mm and the number varies according the equipment and technique used. The surgeon watches on a screen during the operation. The first port is made just behind the umbilicus and the camera inserted. The abdomen is inflated with carbon dioxide gas to create a space to operate in. A second port is introduced a few centimeters in front of the navel and a long grasping instrument called a Babcock forceps is inserted. The surgeon finds the ovary with the instrument and uses it to suspend the ovary from a needle placed through the abdominal wall. This lifts the ovary and uterus safely away from other organs. The surgeon then removes the grasping instrument and replaces it with an instrument that cauterizes and cuts tissue. This instrument uses electricity to heat the blood vessels to seal them and to cut them. No sutures are placed inside. The ovary is separated from the uterus and round ligament. The cautery instrument is removed and replaced by the grasping instrument which is used to pull the ovary out through the small abdominal incision (port). This is repeated on the other side and the small holes are closed with a few sutures.
The benefits of laparoscopic surgery are less pain, faster recovery, smaller wounds to heal. A study has shown that patients are 70% more active in the first three days post surgery compared to open surgery. The reason open surgery is more painful is that larger incisions are required, and the ovary needs to be pulled out of the body which stretches and tears tissue in the abdomen (it is not uncommon for patients to react under anaesthesia by breathing faster at this point).
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.
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)
- Male dogs – Neutersol (Zinc gluconate neutralized by arginine). Cytotoxic; produces irreversible 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 mice - injection of a solution of the JQ1 molecule to bind to a pocket of BRDT necessary for chromatin remodeling which gives the proteins that regulate how genes act access to the genetic material
- 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.
- Male mammals - Noninvasive vasectomy using ultrasound.
- Male mice - reversible regulation of the KATNAL1 gene in Sertoli Cell Microtubule Dynamics of the testes.
- 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.
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.
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.
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. Anesthesia recovery in young animals is usually more rapid and there are fewer complications. 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. A later study comparing female dogs spayed between 4 to 6 months and after 6 months showed no increased risk.
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, 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. 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
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:
Religious views on neutering
While there are differing views in Islam with regard to neutering animals, 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) or "choos[ing] the lesser of two evils".
Traditional interpretations of Orthodox Judaism forbids the castration of both humans and non-human animals by Jews, except in lifesaving situations. 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.
- ^ University of Notre Dame online Latin dictionary
- ^ a b c Determining the optimal age for gonadectomy of dogs and cats JAVMA
- ^ Morrison, Wallace B. (1998). Cancer in Dogs and Cats (1st ed.). Williams and Wilkins. ISBN 0-683-06105-4.
- ^ "Mammary Tumors: Introduction". The Merck Veterinary Manual. http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/112300.htm. Retrieved 13 April 2010.
- ^ C Gobello et al. (23 Aug 2001). "Canine Pseudopregnancy: A Review". International Veterinary Information Service. http://www.ivis.org/advances/concannon/gobello/ivis.pdf. Retrieved 13 April 2010.
- ^ Brodbelt; Blissitt, KJ; Hammond, RA; Neath, PJ; Young, LE; Pfeiffer, DU; Wood, JL (2008). "The risk of death: the confidential enquiry into perioperative small animal fatalities.". Veterinary Anaesthesia and Analgesia 35 (5): 365–73. doi:10.1111/j.1467-2995.2008.00397.x. PMID 18466167.
- ^ Colliard L, Paragon BM, Lemuet B, Bénet JJ, Blanchard G (February 2009). "Prevalence and risk factors of obesity in an urban population of healthy cats". J. Feline Med. Surg. 11 (2): 135–40. doi:10.1016/j.jfms.2008.07.002. PMID 18774325.
- ^ Cave NJ, Backus RC, Marks SL, Klasing KC (October 2007). "Oestradiol, but not genistein, inhibits the rise in food intake following gonadectomy in cats, but genistein is associated with an increase in lean body mass". J Anim Physiol Anim Nutr (Berl) 91 (9-10): 400–10. doi:10.1111/j.1439-0396.2006.00667.x. PMID 17845247.
- ^ McGreevy PD, Thomson PC, Pride C, Fawcett A, Grassi T, Jones B (May 2005). "Prevalence of obesity in dogs examined by Australian veterinary practices and the risk factors involved". Vet. Rec. 156 (22): 695–702. PMID 15923551.
- ^ Priester; McKay, FW (1980). "The Occurrence of Tumors in Domestic Animals". National Cancer Institute monograph 23 (54): 1–210. PMC 1790092. PMID 7254313. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=1790092.
- ^ Ru, G; Terracini, B; Glickman, L (1998). "Host related risk factors for canine osteosarcoma". The Veterinary Journal 156 (1): 31–9. doi:10.1016/S1090-0233(98)80059-2. PMID 9691849.
- ^ Cooley, D. M., Beranek, B. C. et al. (1 November 2002). "Endogenous gonadal hormone exposure and bone sarcoma risk". Cancer Epidemiol Biomarkers Prev 11 (11): 1434–40. PMID 12433723. http://cebp.aacrjournals.org/cgi/content/full/11/11/1434.
- ^ Prymak C, McKee LJ, Goldschmidt MH, Glickman LT. (1988). "Epidemiologic, clinical, pathologic, and prognostic characteristics of splenic hemangiosarcoma and splenic hematoma in dogs: 217 cases (1985)". J Am Vet Med Assoc. 193 (6): 706–712. PMID 3192450.
- ^ Ware, Wendy A.; Hopper, David L. (1999). "Cardiac Tumors in Dogs: 1982–1995". Journal of Veterinary Internal Medicine 13 (2): 95–103. doi:10.1892/0891-6640(1999)013<0095:CTID>2.3.CO;2. PMID 10225598.
- ^ Sanborn, L.J. (2007). "Long-Term Health Risks and Benefits Associated with Spay / Neuter in Dogs". http://www.naiaonline.org/pdfs/LongTermHealthEffectsOfSpayNeuterInDogs.pdf.
- ^ Moore GE, Guptill LF, Ward MP, Glickman NW, Faunt KF, Lewis HB, Glickman LT. (2005). "Adverse events diagnosed within three days of vaccine administration in dogs". J Am Vet Med Assoc. 227 (7): 1102–1108. doi:10.2460/javma.2005.227.1102. PMID 16220670.
- ^ Ettinger, Stephen J.;Feldman, Edward C. (1995). Textbook of Veterinary Internal Medicine (4th ed.). W.B. Saunders Company. ISBN 0-7216-6795-3.
- ^ Slauterbeck JR, Pankratz K, Xu KT, Bozeman SC, Hardy DM. Canine ovariohysterectomy and orchiectomy increases the prevalence of ACL injury. Clin Orthop Relat Res. 2004 Dec;(429):301-5.
- ^ Teske, E; Naan, EC; Van Dijk, EM; Van Garderen, E; Schalken, JA (2002). "Canine prostate carcinoma: epidemiological evidence of an increased risk in castrated dogs". Molecular and Cellular Endocrinology 197 (1–2): 251–5. doi:10.1016/S0303-7207(02)00261-7. PMID 12431819.
- ^ Sorenmo, K. U.; Goldschmidt, M.; Shofer, F.; Goldkamp, C.; Ferracone, J. (2003). "Immunohistochemical characterization of canine prostatic carcinoma and correlation with castration status and castration time". Veterinary and Comparative Oncology 1 (1): 48–56. doi:10.1046/j.1476-5829.2003.00007.x. PMID 19379330.
- ^ Laura J. Sanborn, MS (14 May 2007). "Long-Term Health Risks and Benefits Associated with Spay / Neuter [sic] in Dogs". http://www.naiaonline.org/pdfs/LongTermHealthEffectsOfSpayNeuterInDogs.pdf. Retrieved 13 April 2010.
- ^ Hart (2001). "Effect of gonadectomy on subsequent development of age-related cognitive impairment in dogs". Journal of the American Veterinary Medical Association 219 (1): 51–6. doi:10.2460/javma.2001.219.51. PMID 11439769.
- ^ Lekcharoensuk; Osborne, CA; Lulich, JP (2001). "Epidemiologic study of risk factors for lower urinary tract diseases in cats". Journal of the American Veterinary Medical Association 218 (9): 1429–35. doi:10.2460/javma.2001.218.1429. PMID 11345305.
- ^ Aaron A, Eggleton K, Power C, Holt PE. Urethral sphincter mechanism incompetence in male dogs: a retrospective analysis of 54 cases. Vet Rec. 139:542-6, 1996
- ^ Thrusfield, M. V.; Holt, P. E.; Muirhead, R. H. (1998). "Acquired urinary incontinence in bitches: its incidence and relationship to neutering practices". Journal of Small Animal Practice 39 (12): 559–66. doi:10.1111/j.1748-5827.1998.tb03709.x. PMID 9888109.
- ^ Arnold S, Arnold P, Hubler M, Casal M, Rŭsch P (1989). "Urinary incontinence in spayed bitches: prevalence and breed disposition". Europ J of Compan Anim Pract. 131 (5): 259–263.
- ^ Thrusfield Mv (1985). "Association between urinary incontinence and spaying in bitches". Vet Rec. 116 (26): 695. doi:10.1136/vr.116.26.695. PMID 4024434.
- ^ Panciera DL (1994). "Hypothyroidism in dogs: 66 cases (1987–1992)". J Amer Vet Med Assoc 204 (5): 761–767.
- ^ Kobelt A. J., Hemsworth P. H., Barnett J. L., Coleman G. J. (2003). "A survey of dog ownership in suburban Australia-conditions and behaviour problems". Appl. Anim. Behav. Sci. 82 (2): 137–148. doi:10.1016/S0168-1591(03)00062-5.
- ^ The Effects of Spaying and Neutering on Canine Behavior James O’Heare, Association of Animal Behavior Professionals
- ^ Guy N. C., Luescher U. A., Dohoo S. E., Spangler E., Miller J. B, Dohoo I. R., Bate L. A. (2001). "A case series of biting dogs: characteristics of the dogs, their behaviour, and their victims". Appl. Anim. Behav. Sci. 74: 15–57. doi:10.1016/S0168-1591(01)00155-1.
- ^ a b Takeuchi Y., Ogata N., Houpt J. A., Scarlett J. M. (2001). "Differences in background and outcome of three behavior problems of dogs". Appl. Anim. Behav. Sci. 70 (4): 297–308. doi:10.1016/S0168-1591(00)00156-8. PMID 11179553.
- ^ Neilson J., Eckstein R., Hart B. (1997). "Effects on castration on problem behaviors in male dogs with reference to age and duration of behavior". JAVMA 211 (2): 180–182. PMID 9227747.
- ^ Polsky R. H. (1996). "Recognizing dominance aggression in dogs". Vet. Med. 91: 196–201.
- ^ Blackshaw, J.K. (1991). "An overview of types of aggressive behavior in dogs and methods of treatment". Appl. Anim. Behav. Sci. 30 (3–4): 351–361. doi:10.1016/0168-1591(91)90140-S.
- ^ Wright J. C. (1991). "Canine aggression toward people. Bite scenarios and prevention". Vet. Clin. North. Am. Small. Anim. Pract. 21 (2): 299–314. PMID 2053252.
- ^ Crowell-Davis S. L. (1991). "Identifying and correcting human-directed dominance aggression of dogs". Vet. Med. 86: 990–998.
- ^ Podberscek A. L., Serpell J. A. (1996). "The English Cocker Spaniel: preliminary findings on aggressive behaviour". Appl. Anim. Behav. Sci. 47: 75–89. doi:10.1016/0168-1591(95)01012-2.
- ^ Meuten DJ. Tumors in Domestic Animals. 4th Edn. Iowa State Press, Blackwell Publishing Company, Ames, Iowa, p. 575
- ^ Spain CV, Scarlett JM, Houpt KA. Long-term risks and benefits of early-age gonadectomy in dogs. JAVMA 2004;224:380-387.
- ^ Hooper RN, Taylor TS, Varner DD, Blanchard TL (October 1993). "Effects of bilateral ovariectomy via colpotomy in mares: 23 cases (1984–1990)". J. Am. Vet. Med. Assoc. 203 (7): 1043–6. PMID 8226251.
- ^ "Neutersol and Esterilsol". http://www.acc-d.org/Neutersol. Retrieved 2008-11-28.
- ^ "Small-Molecule Inhibition of BRDT for Male Contraception". Cell 150 (4): 673-684. 2012-08-17. doi:10.1016/j.cell.2012.06.045. http://www.cell.com/fulltext/S0092-8674%2812%2900929-4?switch=standard.
- ^ Jones, Inke; Lucas Ricciardi, Leonard Hall, Hedley Hansen, Vijay Varadan, Chris Bertram, Simon Maddocks, Stefan Enderling, David Saint, Said Al-Sarawi, Derek Abbott (2008-01-17). "Wireless RF communication in biomedical applications" (pdf). Smart Materials and Structures (IOP Publishing Ltd) 17: 8–9. doi:10.1088/0964-1726/17/1/015050. http://www.eleceng.adelaide.edu.au/personal/dabbott/publications/SMS_jones2008.pdf. Retrieved 2008-06-25.
- ^ SpayVac. Retrieved on early 2003.
- ^ 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.
- ^ DeNicola, Anthony; Lowell A. Miller, James P. Gionfriddo, Kathleen A. Fagerstone (2007-03-16). "Status of Present Day Infertility Technology". Northeast Association of Fish and Wildlife Agencies. Archived from the original on 29 August 2007. http://web.archive.org/web/20070829131847/http://www.neafwa.org/html/wildlifeabs.shtml#d. Retrieved 2007-03-16.
- ^ Fried NM, Sinelnikov YD, Pant BB, Roberts WW, Solomon SB (December 2001). "Noninvasive vasectomy using a focused ultrasound clip: thermal measurements and simulations". Biomedical Engineering, IEEE Transactions on 48 (12): 1453–9. doi:10.1109/10.966604. PMID 11759926.
- ^ Lee B. Smith, L. Milne, N. Nelson, S. Eddie, P. Brown, N. Atanassova, M. K. O’Bryan, L. O’Donnell, D. Rhodes, S. Wells, D. Napper, P. Nolan, Z. Lalanne, M. Cheeseman and J. Peters (May 2012). "KATNAL1 Regulation of Sertoli Cell Microtubule Dynamics is Essential for Spermiogenesis and Male Fertility". PLoS Genetics. http://www.eurekalert.org/pub_releases/2012-05/uoe-gdp052412.php.
- ^ Jeffrey T. Jensen, Mary B. Zelinski, Jessica E. Stanley, John W. Fanton, and Richard L. Stouffer (April 2008). "The phosphodiesterase 3 inhibitor ORG 9935 inhibits oocyte maturation in the naturally selected dominant follicle in Rhesus macaques". Contraception 77 (4): 303–7. doi:10.1016/j.contraception.2008.01.003. PMC 2505347. PMID 18342656. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2505347.
- ^ "Penectomized Teaser Bull". The Drost Project. http://www.drostproject.org/en_bovrep/2-8/itemtop56.html. Retrieved 2011-08-24.
- ^ Early-Age (Prepubertal) Spay/Neuter of Dogs and Cats
- ^ Early Neutering of Companion Animals Position Statement American Animal Hospital Association
- ^ Dog and Cat Spay/Castration
- ^ a b Long-term risks and benefits of early-age gonadectomy in cats
- ^ a b c d e Long-term risks and benefits of early-age gonadectomy in dogs
- ^ Howe; Slater, MR; Boothe, HW; Hobson, HP; Fossum, TW; Spann, AC; Wilkie, WS (2000). "Long-term outcome of gonadectomy performed at an early age or traditional age in cats". Journal of the American Veterinary Medical Association 217 (11): 1661–5. doi:10.2460/javma.2000.217.1661. PMID 11110455.
- ^ a b c Howe; Slater, MR; Boothe, HW; Hobson, HP; Holcom, JL; Spann, AC (2001). "Long-term outcome of gonadectomy performed at an early age or traditional age in dogs". Journal of the American Veterinary Medical Association 218 (2): 217–21. doi:10.2460/javma.2001.218.217. PMID 11195826.
- ^ a b Howe (1997). "Short-term results and complications of prepubertal gonadectomy in cats and dogs". Journal of the American Veterinary Medical Association 211 (1): 57–62. PMID 9215412.
- ^ Kustritz, M (2002). "Early spay-neuter: Clinical considerations". Clinical Techniques in Small Animal Practice 17 (3): 124–8. doi:10.1053/svms.2002.34328. PMID 12476815.
- ^ De Bleser, B.; Brodbelt, D. C.; Gregory, N. G.; Martinez, T. A. (2009). "The association between acquired urinary sphincter mechanism incompetence in bitches and early spaying: A case-control study". The Veterinary Journal 187 (1): 42–47. doi:10.1016/j.tvjl.2009.11.004. PMID 20004121.
- ^ Duerr; Duncan, CG; Savicky, RS; Park, RD; Egger, EL; Palmer, RH (2007). "Risk factors for excessive tibial plateau angle in large-breed dogs with cranial cruciate ligament disease". Journal of the American Veterinary Medical Association 231 (11): 1688–91. doi:10.2460/javma.231.11.1688. PMID 18052804.
- ^ Islam Question and Answer – De-clawing a cat so that it won’t do any damage, and neutering/spaying cats
- ^ What some religions say about sterilisation.
- ^ http://www.spca.org.my/neuter.htm#5 Spaying/Neutering Information
- ^ What does Jewish law say about neutering male pets?
- ^ Feinstein, Moshe. Igrot Moshe.
- ^ CHAI – Why Spay/Neuter is Crucial