Neutering, from the Latinneuter ('of neither sex'), is the removal of an animal's reproductive organ, either all of it or a considerably large part. "Neutering" is often used incorrectly to refer only to male animals, but the term actually applies to both sexes. The male-specific term is castration, while spaying is usually reserved for female animals. Colloquially, both terms are often referred to as fixing. In male horses, castrating is referred to as gelding. Modern veterinary practice tends to use the term de-sexing.
Closed spaying incision on a female dog, taken 24 hours after surgery.
In female animals, spaying (more technically termed ovo-hysterectomy or ovariohysterectomy) involves abdominal surgery to remove the ovaries and uterus (in humans, this is called a hystero-oophorectomy). Another option is to remove only the ovaries (oophorectomy or ovariectomy), which is mainly done in cats and young dogs. Another, less commonly performed method is an "ovary-sparing spay" in which the uterus is removed but one (or both) ovaries are left. Traditional spaying (removal of uterus and ovaries) 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.
A complete ovariohysterectomy may involve removal of the ovaries, uterus, oviducts, and uterine horns.
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 three-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. For suturing the feline linea alba, the most appropriate suture bite and stitch interval size was suggested to be 5 mm.
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 the back. The incisions are between 5 and 10 millimetres (0.20 and 0.39 in) and the number varies according to 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 is inserted. The abdomen is inflated with carbon dioxide gas to create a space in which to operate. 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. Another method uses ligatures and even the uterus is removed. In female dogs only removing the ovaries and not the uterus is not state of the art because this way the risk of Pyometra persists.
The benefits of laparoscopic surgery are less pain, faster recovery, and 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 reactions to the drugs given to the animal as part of the procedure are all possibilities that should be considered.
Closed castration incision on a male dog, taken 12 hours after surgery.
In male animals, castration involves the removal of the testes (testicles), 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). Often the term neuter[ing] is used to specifically mean castration, e.g. in phrases like "spay and neuter".
There is a not-uncommon belief, especially among dog fanciers, that castrated male animals psychologically suffer from the loss of their testicles. This has fueled a small but growing market for prosthetic (artificial) testicles, especially for dogs.
Vasectomy: In a more delicate procedure than castration, the vasa deferentia - ducts that run from the testes to the penis - are cut then tied or sealed, to prevent sperm from entering into the urethra. Failure rates are insignificantly small. Breeders routinely have this procedure carried out on male ferrets and sheep to manipulate the estrus cycles of in-contact females. It is uncommon in other animal species. Because a vasectomy is usually a more expensive procedure, among pet-keepers it is more often performed on show animals, to cosmetically preserve their appearance (though depending upon the fancier organization, the procedure may invalidate the animal's candidacy for certain awards, or relegate it to a non-pedigree, generic "household pet" competition division, just as with full castration).
Tubal ligation: Snipping and tying of fallopian tubes as a sterilization measure can be performed on female cats, dogs, and other species; it is essentially the female equivalent of vasectomy, but a more invasive procedure. Risk of unwanted pregnancies is insignificantly small. Only a few veterinarians 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.
"Gomerization" is breeders' informal term for surgical techniques by which male livestock, such as bulls, retain their full libido (and related effects like sex pheromones that would be lost through castration), but are rendered incapable of copulation. This is done to stimulate and identify estrous females without the risk of transmitting venereal diseases or causing a pregnancy by a male other than the one intended for selective breeding. Animals altered for this purpose are referred to as teasers (teaser bulls, etc.), or gomers. Several methods are used. Penile translocation surgically alters the penis to point far enough away from its normal direction that it cannot manage vaginal penetration. Penile fixation permanently attaches the penis to the abdomen so that it cannot be lowered for penetration. Penectomy is the partial or complete removal of the penis.
Male dogs - Two intratesticular injectable formulations are known to sterilize male dogs. Zeuterin was approved by the United States Food & Drug Administration for permanent sterilization of male dogs ages three months and older by causing necrosis of the testicle. It is not currently available commercially. Calcium chloride dissolved in a variety of diluents have also been studied, with the majority of research and most promising results using calcium chloride dissolved in ethyl alcohol. Calcium chloride formulations can be purchased for use in animals from compounding pharmacies, but the use of calcium chloride for sterilization of males is not approved by the FDA or any other international regulatory agency.
Male cats - Calcium chloride formulations have also been studied in male cats.
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 piezoelectricpolymer 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 mice - reversible regulation of the KATNAL1 gene in the 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.
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 and 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.
Of particular note are two recent studies from Lynette Hart's lab at UC Davis. The first study from 2013, published in a well-known interdisciplinary peer-reviewed journal  demonstrated "no cases of CCL (cruciate ligament tear) diagnosed in intact males or females, but in early-neutered males and females the occurrences were 5 percent and 8 percent, respectively. Almost 10 percent of early-neutered males were diagnosed with LSA (lymphosarcoma), 3 times more than intact males. The percentage of HSA (hemangiosarcoma) cases in late-neutered females (about 8 percent) was 4 times more than intact and early-neutered females. There were no cases of MCT (mast cell tumor) in intact females, but the occurrence was nearly 6 percent in late-neutered females"
The second study from 2014  highlighted significant difference in closely related breeds (retrievers), suggesting that inter-breed variability is quite high and that sweeping legal measures and surgical mandates are not the best solutions to canine welfare and health. Specifically the study states: "In Labrador Retrievers, where about 5 percent of gonadally intact males and females had one or more joint disorders, neutering at 6 months doubled the incidence of one or more joint disorders in both sexes. In male and female Golden Retrievers, with the same 5 percent rate of joint disorders in intact dogs, neutering at 6 months increased the incidence of a joint disorder to 4-5 times that of intact dogs. The incidence of one or more cancers in female Labrador
Retrievers increased slightly above the 3 percent level of intact females with neutering. In contrast, in female Golden
Retrievers, with the same 3 percent rate of one or more cancers in intact females, neutering at all periods through 8 years of
age increased the rate of at least one of the cancers by 3-4 times. In male Golden and Labrador Retrievers neutering had
relatively minor effects in increasing the occurrence of cancers."
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.
Health and behavioral effects
This article contains a pro and con list, which is sometimes inappropriate. Please help improve it by integrating both sides into a more neutral presentation, or remove this template if you feel that such a list is appropriate for this article.(April 2017)
Besides being a birth control method, and being convenient to many owners, castrating/spaying has the following health benefits:
Sexually dimorphic behaviors such as mounting and urine spraying are reduced due to the decrease in hormone levels brought about by neutering. In species other than dogs, certain forms of male aggression are also reduced. Sexual behavior in cats seems to make them especially undesirable by pet owners.
Early spaying significantly reduces the risk of development of mammary tumours in female dogs. The incidence of mammary tumours in un-spayed female dogs is 71% (of which approximately 50% will be malignant and 50% will be benign), but if a dog is spayed before its first heat cycle, the risk of developing a mammary tumour is reduced to 0.35%--a 99.5% reduction. The positive effects of spaying on reduction of later mammary tumours decreases with each heat the dog has (backing up the contention that the greatest benefit to reduce future mammary tumour development is to spay before the first heat), and there is no added benefit to spaying to reduce recurrence of a mammary tumour once it has been diagnosed.
Neutering increases life expectancy in cats: one study found castrated male cats live twice as long as intact males, while spayed female cats live 62% longer than intact females. Non-neutered cats in the U.S. are three times more likely to require treatment for an animal bite. Having a cat neutered confers health benefits, because castrated males cannot develop testicular cancer, spayed females cannot develop uterine, cervical or ovarian cancer, and both have a reduced risk of mammary cancer.
Without the ability to reproduce, a female necessarily has zero risk of pregnancy complications, such as spotting and false pregnancy, the latter of which can occur in more than 50% of unspayed female dogs.
Pyometra (or a pus filled womb) ('Pyo' = pus; 'metra' = uterus or womb) is a life-threatening condition that requires emergency veterinary treatment. The risk of a non-spayed bitch developing pyometra by age 10 is 25% across all breeds, but can be as high as 54% in some breeds. The treatment of choice for a closed-pyometra (where the cervix is closed and the pus cannot drain) is admission to hospital, commencement on intravenous fluids and appropriate antibiotics and, once stable enough for the anaesthetic and surgery, emergency removal of the infected pus-filled uterus. Medical management can be attempted if the animal's condition allows (for example in the case of an 'open' pyometra where the pus drains per-vaginum from the uterus via the open cervix) or dictates (where the animal is too old or otherwise unwell to withstand surgery), if the owner wishes to keep the dog entire to breed or if the owner is unable to afford the veterinary fees associated with surgery. Emergency removal of the infected uterus carries a much higher degree of risk of death than a routine 'spay' operation. The risk of death from in dogs undergoing surgical treatment for pyometra is up to 17%. Thus the risk of death in entire female dogs from a pyometra, even if given correct veterinary attention can be up to 9% by 10 years of age (17% of 54%). This risk is reduced to virtually zero if spayed.
As with any surgical procedure, immediate complications of neutering include the usual anesthetic and surgical complications, such as bleeding, infection, and death. These risks are relatively low in routine 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 risks for sick animals were 1.33% for dogs and 1.40% for cats.
Spaying and castrating cats and dogs may increase the risk of obesity if nutritional intake is not reduced to reflect the lower metabolic requirements of neutered animals. 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 among breeds.
Neutered dogs of both sexes are at a twofold excess risk to develop osteosarcoma (bone cancer) 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 (cancer of blood vessel lining), 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 castrated dogs as compared to intact males.
Spaying and castrating is associated with an increase in urinary tract cancers in dogs, however the risk is still less than 1%.
Neutered dogs of both sexes have a 27% to 38% increased risk of adverse reactions to vaccinations. However, the incidence of adverse reactions for neutered and intact dogs combined is only 0.32%.
Neutered dogs have also been known to develop hormone-responsive alopecia (hair loss).
A 2004 study found that neutered dogs had a higher incidence of cranial cruciate ligament (CCL) rupture, a form of anterior cruciate ligament (ACL) injury.
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.
Neutering also has been associated with an increased likelihood of urethral sphincter incontinence in male dogs.
There is evidence that spaying can increase the risk of urinary incontinence in dogs, especially when done before the age of three months. Up until 12 months of age, the risk decreases as the age at spaying increases. Urinary incontinence can affect up to one out of five spayed female dogs, and develops an average of 2.9 years after the dog has been spayed.
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 two studies 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 more studies reported there was no significant difference in aggression risk between neutered and non-neutered males. 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[clarify].
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".
Orthodox Judaism forbids the castration of both humans and non-human animals by Jews, except in lifesaving situations. In 2007, the SephardicChief Rabbi of Israel Rabbi Shlomo Amar issued a ruling stating that it is permissible to have companion animals neutered on the basis of the Jewish mandate to prevent cruelty to animals.
^Hooper R. N.; Taylor T. S.; Varner D. D.; Blanchard T. L. (October 1993). "Effects of bilateral ovariectomy via colpotomy in mares: 23 cases (1984-1990)". Journal of the American Veterinary Medical Association. 203 (7): 1043-6. PMID8226251.
^ abSpain, C. Victor; Scarlett, Janet M.; Houpt, Katherine A. (1 February 2004). "Long-term risks and benefits of early-age gonadectomy in cats". Journal of the American Veterinary Medical Association. 224 (3): 372-379. doi:10.2460/javma.2004.224.372. PMID14765796.
^ abHowe, Lisa M.; Slater, Margaret R.; Boothe, Harry W.; Hobson, H. Phil; Holcom, Jennifer L.; Spann, Angela C. (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. CiteSeerX10.1.1.204.173. doi:10.2460/javma.2001.218.217. PMID11195826.
^ abHowe (1997). "Short-term results and complications of prepubertal gonadectomy in cats and dogs". Journal of the American Veterinary Medical Association. 211 (1): 57-62. PMID9215412.
^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. PMID20004121.
^Duerr; Duncan, C. G.; Savicky, R. S.; Park, R. D.; Egger, E. L.; Palmer, R. H. (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. PMID18052804.
^Colliard L.; Paragon B. M.; Lemuet B.; Bénet J. J.; Blanchard G. (February 2009). "Prevalence and risk factors of obesity in an urban population of healthy cats". Journal of Feline Medicine and Surgery. 11 (2): 135-40. doi:10.1016/j.jfms.2008.07.002. PMID18774325. S2CID25617418.
^Cave N. J.; Backus R. C.; Marks S. L.; Klasing K. C. (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". Journal of Animal Physiology and Animal Nutrition. 91 (9-10): 400-10. doi:10.1111/j.1439-0396.2006.00667.x. PMID17845247.
^McGreevy P. D.; Thomson P. C.; 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". Veterinary Record. 156 (22): 695-702. doi:10.1136/vr.156.22.695. PMID15923551. S2CID36725298.
^Priester; McKay, F. W. (1980). "The Occurrence of Tumors in Domestic Animals". National Cancer Institute Monograph (54): 1-210. PMID7254313.
^Prymak C.; McKee L. J.; Goldschmidt M. H.; Glickman L. T. (1988). "Epidemiologic, clinical, pathologic, and prognostic characteristics of splenic hemangiosarcoma and splenic hematoma in dogs: 217 cases (1985)". Journal of the American Veterinary Medical Association. 193 (6): 706-712. PMID3192450.
^Teske, E.; Naan, E. C.; Van Dijk, E. M.; Van Garderen, E.; Schalken, J. A. (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. PMID12431819. S2CID7080561.
^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. PMID19379330.
^Lekcharoensuk; Osborne, C. A.; Lulich, J. P. (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. PMID11345305.
^Aaron, A.; Eggleton, K.; Power, C.; Holt, P. E. (1996). "Urethral sphincter mechanism incompetence in male dogs: a retrospective analysis of 54 cases". Veterinary Record. 139 (22): 542-6. doi:10.1136/vr.139.22.542. PMID8961524. S2CID5642622.
^Panciera D. L. (1994). "Hypothyroidism in dogs: 66 cases (1987-1992)". Journal of the American Veterinary Medical Association. 204 (5): 761-767. PMID8175472.
^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". Applied Animal Behaviour Science. 82 (2): 137-148. doi:10.1016/S0168-1591(03)00062-5.
^ abCasey R. A.; Loftus B.; Bolster C.; Richards G. J.; Blackwell E. J. (March 2014). "Human directed aggression in domestic dogs (Canis familiaris): Occurrence in different contexts and risk factors". Applied Animal Behaviour Science. 152: 52-63. doi:10.1016/j.applanim.2013.12.003.
^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". Applied Animal Behaviour Science. 74: 15-57. doi:10.1016/S0168-1591(01)00155-1.
^ abTakeuchi Y.; Ogata N.; Houpt J. A.; Scarlett J. M. (2001). "Differences in background and outcome of three behavior problems of dogs". Applied Animal Behaviour Science. 70 (4): 297-308. doi:10.1016/S0168-1591(00)00156-8. PMID11179553.
^Neilson J.; Eckstein R.; Hart B. (1997). "Effects on castration on problem behaviors in male dogs with reference to age and duration of behavior". Journal of the American Veterinary Medical Association. 211 (2): 180-182. PMID9227747.
^Podberscek A. L.; Serpell J. A. (1996). "The English Cocker Spaniel: preliminary findings on aggressive behaviour". Applied Animal Behaviour Science. 47 (1-2): 75-89. doi:10.1016/0168-1591(95)01012-2.
^Meuten D. J. Tumors in Domestic Animals. 4th Edn. Iowa State Press, Blackwell Publishing Company, Ames, Iowa, p. 575