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REPRODUCTION: SPAY AND NEUTER: RISKS AND BENEFITS OF SPAY AND NEUTER by DR MCKENZIE

 Dr Mckenzie reviewed all the available scientific literature on this subject for this article, and he summarizes the information in a non-biased way. In our opinion, this is the most authoritative article on this subject that is currently available (2011). If you want to know what the available science is now, this is it. 

We have a much shorter article that directly addresses our opinion on the optimal age for neutering. It is here in this YVCipedia Reproduction section.   

Evaluating the Benefits & Risks of Neutering by Brennan Mckenzie  DVM

What Is Neutering?
Neutering involves removing the source of the hormones that control reproduction and 
that determine the typical physical and behavioral characteristics that distinguish males 
and females. In dogs and cats, this is usually done by surgically removing the testicles in 
males (castration) and the ovaries in females (spaying). In the United States, the uterus is 
typically removed along with the ovaries in females, and virtually all dogs and cats are 
spayed or castrated. In Europe and some other parts of the world neutering is far less 
common, and when females are spayed only the ovaries are usually removed.
The primary purpose of neutering is to prevent reproduction, but like most medical 
interventions the procedure has a variety of other effects, both beneficial and undesirable. 
The decision whether or not to neuter a pet involves comparing the benefits and the risks 
in the context of the circumstances in which the pet lives. Veterinarians, breeders, and pet 
owners often have strong opinions about neutering, and unfortunately these are all too 
often based on tradition, habit, rumor, or misconceptions. There is a large and complex 
scientific literature addressing the pros and cons of neutering, and this review is an 
attempt to extract from it some sound information with which to formulate rational 
guidelines for making decisions about neutering. 
A number of reviews of the pros and cons of neutering have appeared in scientific 
publications and posted on the Internet by interested laypersons.[70,93,148-153]  Some 
of these are excellent summaries of the issues, others are inaccurate or misleading. This 
survey is an attempt to look at the totality of the scientific information currently available 
and after reviewing it to draw some pragmatic conclusions about the benefits and risks of 

neutering dogs and cats.

Interpreting Scientific Studies
Scientific studies are superior to tradition, intuition, and personal experience as a form of 
evidence. However, they have limitations which affect the applicability of the data and 
conclusions they generate to individual patients. The ideal study of the risks and benefits 
of neutering would involve taking a large number of dogs of many breeds, dividing them 
into two identical groups, neutering one group but not the other, and then ensuring they 
live otherwise identical lives and examining the differences between the neutered and 
intact groups. No such study will ever be done for practical, scientific, and ethical 
reasons. 
Most of the studies which do examine the possible influence of neutering on health and 
disease are retrospective. They examine animals that have already developed  a condition 
of interest and then look at whether more of the affected animals are neutered or intact 
than a control group of similar animals without the condition. While this is useful 
information, it can be misleading for many reasons. Groups studied in one location may 
be very different from pets in other areas, in terms of age, breed, and other risk factors, 
and conclusions about the study group may not apply to pets in general. 2
For example, a study of a small group of dogs of a single breed which has a high rate of a 
certain type of cancer might show neutered dogs more likely than intact dogs to have that 
cancer. But this might have nothing to do with the risks for a different breed which rarely 
gets that type of cancer. Or neutered pets studied in an affluent city may get more 
veterinary care, and better care in general, than intact dogs roaming loose in a rural area, 
so differences between the groups in some disease might be due to factors other than 
neutering.
A good scientific study will try to control for such factors and the authors will identify 
potential problems with their data. Whenever possible, I have tried to include such 
considerations in evaluating the evidence in this review, but it is often unclear what if any 
confounding factors might influence the results of a given study, so conclusions should 
ideally be based on multiple studies of large numbers of animals by different 
investigators in different places. It is this need for replication and the accumulation of 
data over time that leads to periodic re-evaluations of medical practices. Contrary to the 
impression often given by the media, this is not because science frequently makes radical 
and erratic changes of direction but because the process of deepening our understanding 
and modifying our practices accordingly is complex and never-ending.
Another factor to consider in interpreting studies about the risks and benefits of neutering 
is how we measure and describe risk. Differences in the risk of a given condition between 
groups are often described in terms of relative risk. For example, intact animals may be 
seen to have a given disease 5 times as often as neutered animals (500% greater risk). If 
this difference is determined to be statistically significant (meaning it is unlikely to be 
due to chance, not that it is significant or important in a more ordinary sense), then we 
can say intact animals have a greater relative risk of the disease (5 times greater). 
However, this says nothing about the absolute risk involved. If the disease that intact 
animals are more likely to get occurs in only 1 out of every 1,000 neutered animals
(0.001%), then the extra risk of being intact makes the absolute risk  for an intact animal 
0.005%, still a vanishingly small number. Thus, while differences in relative risk may 
sound dramatic, if the condition is uncommon then the real chances of an individual 
getting it may not be meaningfully different regardless of neuter status. 
In contrast, if the condition is common, then even a small change in the relative risk may 
mean very significant changes in the absolute risk. If 75% of neutered animals get a 
certain disease, and intact animals have a relative risk of 0.5 (50% lower risk), then the 
real chances of getting the disease are much lower (35%) even though the difference in 
relative risk is much less than in the previous example.
Again I have attempted, whenever possible, to assess both the relative and absolute risk 
associated with neuter status when discussing specific conditions. Unfortunately, the true 
incidence of many conditions (how common they are) isn't always known in veterinary 
populations, and when a figure is reported in one study, it may or may not be relevant to 

the patients from a different study, region, breed, and so on. 3

Benefits of Neutering

General BenefitsThe primary benefit of neutering is the prevention of unwanted reproduction. Though the 
number of unwanted cats and dogs euthanized at animal shelters has decreased form an 
estimated 23.4 million in 1970 to about 4.5 million by the year 2000 [1], this still 
represents a significant problem. Reducing the number of unwanted puppies and kittens 
has been and remains an important part of reducing the relinquishment and euthanasia of 
these animals.[2] Failure to neuter is an important component to the pet population 
problem. [55] Furthermore, being intact is a significant risk factor for both cats and dogs 
being given up by their owners[3,4], so neutering can reduce both the number of 
unwanted puppies and kittens and reduce the risk of owned animals being relinquished.
The feral or stray cat population, though notoriously difficult to assess, contains an 
estimated 30-40 million animals, most the product of unplanned breeding.[1] There is a 
great deal of controversy over the welfare of feral cats and the impact they may have on 
wildlife and public health.[5-8] It is generally agreed, however that feral cats suffer more 
disease and parasitism and have shorter lives than owned cats and that reducing the 
number and reproduction of unowned cats is a worthwhile goal.[9] Neutering of owned, 

and likely also feral cats, promotes this goal.[57]

Health Benefits of Neutering-
1. Risks of ReproductionReproduction  itself has potential risks which can be eliminated by neutering. Dogs of 
both sexes are susceptible to infection with Brucella canis, a bacterium which can cause 
disease in dogs and humans. This bacterium can be transmitted during breeding or 
acquired from contact with aborted fetuses and other material from infected females. The 
incidence of this disease varies by country and region, from 1-18% in the United States to 
upwards of 25% in some other countries. Clinical symptoms other than infertility are 
uncommon, though some dogs can experience serious infectious of the bone, eyes, or 
nervous system. [9]
The most common complication of pregnancy for females is dystocia, when the normal 
process of labor and delivery fails. Rates of dystocia in dogs vary greatly by breed, from 
as low as 5% of whelpings to over 85% in breeds with large heads.[10] One large study 
in Sweden, where most dogs are intact, found that 2% of female dogs in the sample 
experienced a dystocia and the overall incidence was 5.7 cases/1000 dog years at risk, 
though some breeds were at much greater risk and some experienced no dystocias. [11]
This study was of dogs covered by health insurance, which about half of dogs in that 
country are, so it may or may not be applicable to dogs in other countries or those whose 
owners do not utilize pet health insurance.
In cats, the risk also varies by breed, with one study reporting an overall dystocia rate of 
5.8% of deliveries, ranging from 0.4% in a colony of mixed breed cats to 18.2% for 
Devon rex cats.[12]4
Though dystocia can sometimes be treated medically, allowing natural delivery to 
proceed, the majority of dogs and cats with dystocia require surgical treatment.[10-13]
Most females recovery fully from c-sections, though the risks of such surgery are likely 
greater than those of a planned spay surgery due to the emergency nature of the 
procedure and the often compromised health of the female due to the dystocia. 
Much less common risks of pregnancy, such as pregnancy toxemia, diabetes mellitus, 
uterine torsion, uterine rupture, and pregnancy-associated pyelonephritis (kidney 
infection) can all be prevented by neutering.[13]
2. Mammary TumorsMammary tumors are very common in intact female dogs. Incidence is reported in a
number of different ways, which makes comparison between studies difficult. A study in 
Norway, where almost all female dogs are intact, found an overall incidence of malignant 
mammary tumors of 53.3%, with significant variation in risk by breed.[14] A UK study 
found mammary tumors to be the second most common type of tumor, with an incidence 
205 tumor per 100,000 dogs per year.[15] A Swedish study found an incidence in intact 
females of 1% at 6yrs of age, 6% at 8yrs, and 13% at 10 years when the study was 
terminated.[16] The incidence of mammary tumors in female cats is roughly half that 
seen in dogs.[17] Mammary cancer is extremely rare in male dogs. [17]
In dogs, the chances of developing a mammary tumor increase with age and vary with 
breed.[13,17] There is no apparent protective effect of having a litter for dogs or cats. 
[17,20]
About half of canine mammary tumors are malignant, whereas 85-90% of feline 
mammary tumors are malignant.[17,18] Mammary cancer is usually treated with surgery 
and often chemotherapy, and it is often fatal despite treatment, with 59% of dogs with 
malignant tumors in one study eventually dying of causes related to their. [21] 
Spaying dramatically reduces the risk of mammary cancer in both dogs and cats. In dogs 
the risk has been reported as 0.5% when spayed before the first heat, 8% if spayed before 
the second heat, and 26% if spayed after the second heat.[19] One study in cats found 
those spayed prior to 6 months of age had a 91% reduction in mammary cancer risk, and 
the risk was reduced 86% in those spayed before 1 year.[20] Spaying dogs later than the 
second heat does not reduce the risk of developing mammary cancer, but spaying at the 
time of surgical removal of the mammary tumor or within 2 years before diagnosis of 
mammary cancer is associated with longer survival.[21]
3. PyometraPyometra is a bacterial infection of the uterus. It occurs as a consequence of changes in 
the uterine environment brought about by repeated estrus (heat) cycles.[13] Pyometra can 
be treated medically, though with a very high rate of recurrence in the following heat 
cycle. [22,23] It is more commonly and successfully treated by spaying the affected 
dog.[13] 5
A study in Sweden, where elective spaying is rarely practiced, found that overall 25% of 
the females in the study developed pyometra by 10 years of age, and it is expected the 
risk would continue to increase in even older females. The risk varied considerably by 
breed, with some breeds having a 10% rate of pyometra and others up to 50%. Risk 
increased with age for all breeds.[24] Pyometra has been reported in cats, but no 
published figures regarding the incidence are available. Mortality from pyometra treated 
surgically is variable, from 4.2-17% in dogs and 8% in cats.[13]
Spaying essentially eliminates the risk of pyometra in dogs and cats. Infections in the 
small portion of the uterus not removed during ovariohysterectomy do occur if some 
ovarian tissue or other source of progesterone is present, but this is rare.[13] In Europe, it 
is common to remove only the ovaries and leave the uterus. This effectively protects 
against pyometra since the hormones responsible for the condition are not present.[25]
4. Cancer of Reproductive OrgansTumors of the ovaries are uncommon in dogs and cats with reported incidences of 6.25% 
in dogs and between 0.7%-3.6% in cats. There are several different types of ovarian 
tumors with variable degrees of malignancy. Little reliable information exists regarding 
the mortality associated with these tumors.[26]
Uterine tumors are very rare in dogs and cats, accounting for <2% of feline tumors and 
<0.5% of all canine tumors. Tumors of the uterus can generally be successfully removed 
by spaying the animal, though recurrence and spread to other organs has been 
reported.[26]
Tumors of the vulva or vagina in female dogs are not common, though they represent 2-
3% of all canine tumors. They occur primarily in intact females, often have receptors for 
ovarian hormones present in the tumor cells, and they are less likely to recur in dogs 
spayed at the time of tumor removal. [26-28,144] This suggests that the risk of such 
tumors is decreased in spayed females. Most vulvar and vaginal tumors are benign and 
can be cured by surgical removal, though the minority that are malignant have a poor 
prognosis and often recur or metastasize.[26]
Various rates of occurrence have been reported for testicular tumors, but random samples 
of testicles from dogs autopsied for reasons not related to testicular disease have shown 
that 16-27% of dogs had tumors, and many of these had more than one.[29] The testicles 
are the second most common site for cancer in intact male dogs.[145] Testicles which do 
not descend into the scrotum but remain in the abdomen or inguinal ring (cryptorchid 
testicles) are more likely to develop tumors, especially in dogs under 10 years of 
age.[30,31]
There are several types of testicular tumors. Most are slow to metastasize, with fewer 
than 15% of affected dogs showing spread to other organs. Some testicular tumors 
produce hormones, including estrogen which can cause feminization and bone marrow 
disease. Castration is the treatment of choice for testicular cancer, and it is usually 
curative. [13,26]6
5. Prostate DiseaseThe most common disease of the canine prostate is benign hyperplasia(BPH), an 
overgrowth of tissue that causes enlargement of the gland.[13,32] This incidence of this 
disorder increases with age, from 15-40% for dogs under 7 years of age and 60-100% of 
dogs over 7 years of age.[32-34] While most dogs have few symptoms from BPH, some 
will experience difficulty urinating or defecating or bloody preputial secretions. BPH is 
predisposing and complicating factor for prostatitis, a bacterial infection of the prostate. 
[13] Prostatitis has been reported to occur in up to 28.5% of intact male dogs.[32,35] It is 
a serious and uncomfortable, though rarely life-threatening disease. Both BPH and 
prostatitis are rare in intact dogs and both are effectively prevented and treated by 
castration.[13,32]
6. Behavioral BenefitsBehavioral problems are an important reason for relinquishment of pet dogs and cats by 
owners. [1,5,44] The most common problematic behaviors include aggression towards 
people or other animals, inappropriate elimination, and fearful behaviors.[45] To the 
extent that neutering increases or reduces the risks for these behaviors it can have an 
important impact on the relationship between pet and owner and ultimately on the pet's 
survival.
The biological and environmental influences on animal behavior are complex and 
difficult to unravel. Specific behavior patterns are influenced by many environmental and 
individual factors which all interact, so epidemiological correlations are often unreliable 
in predicting the outcome of interventions in individual cases. however, there are some 
consistent patterns that emerge from studies on normal and problematic behaviors in dogs 
and cats which illustrate the potential behavioral benefits and risks associated with 
neutering.
Some studies have reported intact male dogs to be disproportionately involved in 
aggressive behavior. [46,47]. Others have reported marked reductions in aggression and 
other problem behaviors in male dogs as an effect of castration. In one study, roaming 
behavior decreased 90%, aggression between males decreased 62%, urine marking 
decreased 50%, and mounting decreased 80% following castration,[48] and several other 
studies have found similar results.[49,50,54] Some studies have also reported intact dogs 
to be more likely to bite humans than neutered animals.[52]
Castration also dramatically reduces fighting, urine spraying, and roaming in male 
cats.[52-54] One study has found intact cats to be more aggressive and less affectionate 
than neutered cats.[102]
7. MiscellaneousAlmost every epidemiologic study of any disease examines differences in incidence 
between males and females and intact and neutered animals. If a significant correlation is 
found, this may or may not have meaningful clinical implications. There are likely many 
more such associations reported than I have listed here, but these are some that seem to 7
have clear significance when considering whether or not to neuter and about which pet 
owners often have questions and concerns.
Perineal hernias are protrusions of abdominal organs through a weakened area in the 
pelvic muscles. The disease is not uncommon, but no precise incidence has been 
reported. However, in one study 93% of cases were intact males, and an association with 
prostatic disease is suspected, so neutering clearly is protective for this problem.[41,56]
Perineal hernias can usually be successfully treated with surgery, and castration at the 
time of hernia repair is recommended.[42]
Perianal fistulas are a chronic immune-mediated disease seen most commonly in German 
Shepherds and Irish Setters and rarely in other breeds. It occurs predominately in intact 
male dogs, which suggests some hormonal influence, though a specific causal connection 
has not been identified. In one study, males outnumbered females 2:1 and intact dogs 
were 86% of affected patients. [43] The disease is chronic and often causes significant 
discomfort. It can frequently be controlled with medical therapy, though sometimes 
surgical treatment may also be necessary.
There is some suggestion in research on laboratory animals as well as epidemiologic 
studies of dogs and cats that neutered animals may live longer than intact animals.[58-
61,63,146,147] However, there are also studies which suggest that the longer females of 
some breeds retain their ovaries the more likely they are to achieve unusual longevity for 
their breed.[62] The possible effects of differences in the care neutered and intact animals
receive have not been examined, and this complicates any interpretation of differences in 

longevity.

Risks of Neutering
1.Neutering Surgery RisksLike all surgeries, neutering involves some risks. Total complication rates for routine 
castration or spaying have been reported from 2.6%-20% of cases.[64-67]. The majority 
of these are minor and require no treatment. [64,67] Complication rates vary considerably 
from practice to practice to practice and are generally reported to be higher in studies of 
surgeries performed by students in training.[64,67] Reported death rates are less than 
0.1%.[64]
2.CancersProstate cancer in dogs has previously been reported to have a very low incidence of less 
than 1%[13], but several recent studies have suggested it may be more common, though 
not always clinically recognized, and these papers have reported rates of 3.6-13%.[32,35]
Most such cancers are malignant, with metastases reported in 40-80% of cases at the time 
of diagnosis.[13,36]. There is some uncertainty about the role of castration in prostate 
cancer development. While some reports have found fewer prostate cancers in castrated
dogs than in intact dogs [36-38], most recent studies have found either no effect of 
castration on the rate of prostate cancers [39] or an increased risk for castrated 8
dogs.[35,40] Most canine prostate cancers examined seem to lack receptors for male 
hormones, so it appears that unlike in humans these hormones are not responsible for the 
initiation or progression of prostatic cancers, but it is unclear whether castration is overall 
beneficial, neutral, or a risk factor for their development.[30,40] Prostate cancer is an 
aggressive cancer with a poor long-term prognosis.[30]
Osteosarcoma is a bone tumor usually seen in large breed dogs.[68,69] Overall incidence 
has been reported as 0.2%, but for at risk breeds rates of 4.4%-6.2% are often
reported.[70,72] A rate of 12.5% was reported in one study, though the authors suggested 
this might have been an overestimate. [71] Neutered dogs have been reported to be at 
higher risk for osteosarcoma than intact dogs.[68,71]. In one study, no difference was 
found in overall risk for intact versus neutered animals of either sex, but neutering before 
1 year of age was found to increase the risk, and it was found that the longer an 
individual had been intact the lower their osteosarcoma risk.[71] However, the neutered 
animals in this study (especially the spayed females) lived longer than the intact animals, 
which may have contributed to an increased incidence of cancer in the neutered group. 
It is possible that neutering, especially before sexual maturity, raises the risk of 
osteosarcoma, at least in predisposed breeds. Osteosarcoma is an aggressive cancer with a 
poor long-term prognosis, and it is generally treated with surgery and chemotherapy.[69]
Hemagiosarcoma is a cancer of the cells that normally form blood vessels.[73] The 
overall incidence has not been reported, but it makes up 5% of all non-skin cancers in 
dogs.[73] It is less common in the cat, found in 0.5% of cats autopsied and 2% of cancers 
in this species.[73] It most commonly occurs in the spleen, and certain breeds (such as 
German Shepherds, Labrador Retrievers, and Golden Retriever) are at greater risk than 
others.[73,74,76] Hemangiosarcoma can also develop in the heart, with a reported 
incidence of 0.19%.[75] 
Spayed females have been reported to have 2 times the risk of splenic hemangiosarcoma 
and 5 times the risk of cardiac hemangiosarcoma of intact females.[74,75] Castrated 
males have either been found to have no increased risk of splenic hemangiosarcoma[74] 
and only a slightly higher risk (1.55 times) that of intact males for cardiac 
hemangiosarcoma.[75] Hemangiosarcoma is an aggressive cancer with a poor long-term 
prognosis, and it is usually treated with removal of the spleen (if this is the primary site) 
and chemotherapy.[73]
Transitional cell carcinoma is a cancer of the lower urinary tract, usually found in the 
bladder and uncommonly in the urethra of dogs.[77] It represents 1%-2% of canine 
cancers and is rare in the cat.[77,78] It is more common in females than males, 
prevalence varies by breed, and neutered animals have been reported to be at 2-4 times 
greater risk than intact animals.[78,79] Transitional cell carcinoma is an aggressive 
cancer with a fair long-term prognosis, and it is usually treated with chemotherapy and 
sometimes surgery or radiation therapy.[77]9
3. Orthopedic DiseaseRupture of the cranial cruciate ligament in the knee is a common problem of large breed 
dogs, with a reported incidence of 1.8%-4.5%, though the incidence in predisposed 
breeds has been reported to be as high as 8.9%.[80-82,86] In addition to breed and 
obesity, neutering increases the risk of cranial cruciate ligament rupture, [80,81,82,86]
One study suggested neutering may increase the angle between the bones in the knee in a 
way that promotes cruciate rupture, but this effect was only seen in dogs neutered earlier 
than 6 months of age.[85] Cruciate ligament rupture is treated with a variety of surgical 
approaches, and it has an excellent long-term prognosis.[83,84]
Hip dysplasia is a developmental abnormality of the hip joint that can result in arthritis 
and clinical discomfort. It is rare in small breeds, with rates of affected dogs less than
1%, but it can be seen in as many as 40%-75% of large breed dogs.[86-89] Hip dysplasia 
is estimated to lead to clinically significant arthritis is fewer than 5% of affected dogs, but 
there are many factors involved including breed, weight, and the degree of anatomic 
abnormality of the hip joint, which makes predicting the outcome for any individual 
difficult.[89] The incidence of hip dysplasia is most strongly associated with breed and 
family history.[86,90,91]. 
Some studies have identified neutering as increasing the risk of hip dysplasia.[86,92]. As 
discussed below, the age at neutering may also be a factor influencing the development of 
hip dysplasia.[93] It is unclear if the increased risk is directly due to the effects of 
neutering or due to an increased incidence of obesity in neutered dogs. Hip dysplasia can 
be treated if detected early with surgical therapies that reduce the chances of clinically 
significant arthritis later in life.[94,95] In older dogs who have already developed arthritis 
and clinical symptoms, these can be managed surgically or medically, with medications, 
weight reduction, and other therapies.[96-98] Because of the genetic basis of the disorder, 
the ideal approach to eliminating it is to neuter those dogs that carry the predisposing 
genes to eliminate the disease from the population.[99,100]
4. Behavioral RisksThough neutering has been associated with a decreased incidence of some kinds of 
aggression, there is limited evidence that it may sometimes be associated with an increase 
in aggressive behavior. There is one study that identified more owner-directed aggression 
reported in Springer Spaniels that were neutered than in intact Springers.[101] How 
reliable such an owner survey might be or how applicable to other breeds is unclear. 
Similarly, one study found evidence of an increase in aggression towards owners among 
spayed female dogs who were spayed before 11 months of age and who had already 
showed some aggressive behaviors before neutering.[103] However, there were some 
differences between the control group and the spayed dogs in addition to having surgery, 
and these make the results less reliable. 
One study found female German Shepherds who were neutered were more reactive to the 
presence of unfamiliar humans and dogs than were intact dogs.[104]. Another study 
found neutered dogs to be more active than intact dogs and castrated males to be more 
excitable than intact males but found no other measurable behavioral differences between 10
the groups.[105] The clinical significance or applicability of these findings to behavior 
problems is unclear.
One study has examined the relationship between neutering and the development of agerelated behavioral changes thought to be similar to Alzheimer's disease or other forms of 
senile dementia in humans.[106] Such changes are relative common, being reported in 
28% of dogs 10-12 years old and 68% of dogs 15-16 years old.[107] When multiple 
comparisons were made between intact males, castrated males, and spayed females (no 
intact females were included in the study), the only association found was for castrated 
males who had already shown signs of behavioral impairment when first assessed to 
progress to more severe impairment at a higher rate than intact males or spayed females. 
The significance of this finding is not clear.
5. MiscellaneousUrinary incontinence is common in middle-aged to older female dogs associated with 
spaying, with a reported incidence of 5-30%. Rates are lower in small dogs and higher in 
large breed dogs.[93,108-111] It can usually be successfully treated with 
medication.[108,112]
Several reports have found spayed females to be at increased risk for urinary tract 
infections compared to intact females[113,114], but other studies have not found such a 
relationship.[115] No associated with urinary tract infections has been found for 
neutering of male dogs.[113] Most urinary tract infections can be successfully treated 
with antibiotics. 
Feline Lower Urinary Tract Disease (FLUTD) is a collection of symptoms ranging from
mild  bloody urine and straining while urinating to potentially life-threatening urinary 
tract obstruction.[116] Causes include bladder inflammation (cystitis), urinary tract 
infection, urinary tract stones, tumors, and others.[116] FLUTD has been reported to 
occur in 1.3%-4.6% of cats in private practice and 7%-8% of cats in veterinary teaching 
hospitals.[117,118] While some studies have found no association between FLUTD 
conditions and neutering [70,119], and it does not appear that neutering affects the size of 
the urethra in male cats (a possible risk factor for obstruction)[120], several 
epidemiologic studies have found that neutering status does raise the risk of some causes 
of FLUTD.[121,122]. Castrated males were at an increased risk compared to intact males 
for all causes of FLUTD except infection and urinary incontinence. Spayed females had 
an increased risk for urinary tract stones, urinary tract infections, and urinary tract tumor, 
but not other causes of FLUTD. Intact females had a decreased risk for most causes.[122] 
While most cases of FLUTD are treatable and not life-threatening, urinary tract 
obstruction in males is a very serious condition. This occurred in about 12% of cats with 
FLUTD symptoms, and the risk is higher in castrated males cats.[122,123]
Hypothyroidism is a condition in which the thyroid gland atrophies or is damaged by the 
immune system and fails to produce adequate amounts of thyroid hormone.[124,125] It 
occurs in an estimated 0.2%-0.3% of dogs.[126,127] Some studies have found that 11
neutered dogs are at higher risk than intact dogs for developing 
hypothyroidism.[126,127] However, other studies have not found any such 
association.[129] Supplementation of thyroid hormone resolves the disease in most 
cases.[127,128]
Diabetes mellitus is a complex disease that comes in a variety of forms and has a variety 
of causes. Briefly, an affected animal will have blood sugar levels that are too high and 
will usually need insulin injections to control their blood sugar and prevent the many 
serious secondary problems associated with uncontrolled diabetes.[130] Incidence in cats 
has been reported from 0.08%-2%, with Burmese cats having a higher rate of occurrence 
than other breeds or mixed-breed cats.[131-134] Incidence in dogs is estimated at 0.19-
0.64%, with significant breed variations.[135,136] Diabetes is more common in male cats 
than females, and neutering is associated with an increased risk of diabetes in both male 
and female cats in some studies.[132] However when age and weight are controlled for 
no effect of neutering is seen in others.[134] For dogs, diabetes is usually believed to be 
more common in females than males [130,136] though this is not found in all 
populations.[133] 
Castrated males were at higher risk for diabetes than intact males in one study, though 
weight was not controlled for.[137] Some authors have suggested that intact females may 
be at greater risk of diabetes due to the antagonistic effects of ovarian hormones on 
insulin, and spaying is an important part of regulating diabetes in female dogs. [133] 
Weight is clearly a risk factor for diabetes in cats, thought there is some debate about 
whether this is true in dogs, and since neutered animals are prone to be heavier than intact 
animals matched by breed and age, this may be a confounding factor creating the 
appearance of a direct effect of neutering on diabetes risk.[130-133,136] Diabetes is a 
serious chronic disease that can often be managed for long periods but cannot be cured.
Pancreatitis is an inflammatory condition of the pancreas, an organ involved in digestion 
and also insulin production.[138] It can occur as a sudden severe disease or as a longterm chronic, waxing and waning disease. The true incidence of pancreatitis is unknown, 
and although autopsy surveys have found evidence of inflammation in anywhere from 
less than 1% to more than 50% of dog pancreases, no study has yet examined how 
common clinical pancreatitis is.[139-141] In dogs, there is some evidence that neutered 
animals may be at higher risk than intact animals for sudden-onset form.[142,143]
Obesity is a common and growing clinical problem in dogs and cats. Though clear and 
consistent definitions do not exist, various reports have suggested that among dogs 18%-
44% are overweight and 2.9%-7.6% are obese.[154-156] Among cats, an estimated 19%-
40% are overweight and 7.8% are obese.[157-159] Being overweight is a significant risk 
factor for many serious diseases.[134,160-162] Almost all studies agree that neutered cats 
and dogs are more likely to be overweight or obese than intact cats and dogs.[154-
156,158-160,163-169] However, the exact relationship between neutering and excess 
body weight has not been clearly established. 12
Some studies have indicated that neutered animals have a lower metabolic rate and so 
burn fewer calories regardless of activity, which would make them prone to being 
overweight.[170-173] But other studies, which controlled for the proportion of an 
animal's weight made up of fat, which is not very metabolically active, have found 
comparable metabolic rates in intact and neutered animals.[167-169,174] There is 
evidence that the reason neutered animals gain excess weight is that they eat more and 
expend less energy than intact animals despite having the same resting metabolic 
rate.[53,165,169,171,172] There are also many other risk factors for obesity, including 
sex, breed, and variables associated with owners and their habits, that affect the chances 
of an animal becoming overweight regardless whether it is neutered or 
intact.[154,159,160,175]
It is clear that obesity is preventable. Proper restriction of the amount of food, and hence 
the number of calories available to dogs and cats is all that is necessary to prevent obesity 

regardless of neuter status. [154,160]

Optimal Age for Neutering
For decades, the traditional age for neutering dogs and cats has been 6-9 months. There is 
no clear scientific basis for choosing this age, and it has been suggested that this practice 
arose as a response to anesthetic mortality in very young animals in the first half of the 
20th century.[70] Anesthetic procedures have evolved dramatically since that time, and it 
has since been demonstrated that not only is the procedure safe in puppies and kittens 7-
12 weeks of age, but these younger patients actually recover faster and have fewer 
complications than those neutered at the traditional age.[65,102,176]
A large scale trial found no significant differences in the week immediately after surgery 
between patients neutered at the traditional age and earlier, apart from more minor 
surgical complications in the traditional age group.[65] Another study followed cats 
neutered at 7 weeks and at 7 months for 1 year and found no differences in any 
outcome.[102] Two large studies followed puppies and kittens neutered before and after 
24 weeks of age for approximately 3 years.[177,178] For cats, of the numerous measures 
of health, behavior, and relationship with owner, the only difference detected was a 
greater incidence of urinary tract problems in the cats neutered at the traditional age.[177] 
In the dog study, puppies neutered earlier than 24 weeks did have a higher rate of 
infections, primarily parvovirus. This may have been due to differences in the 
management policies of the two shelters in which the subjects were neutered since the 
rate of parvovirus infections was higher at the shelter where most of the early neutering 
animals were spayed or castrated.[178] Dogs in the traditional age group had more 
gastrointestinal problems than dogs in the early neuter group.[178] Interestingly, there 
was no difference in the incidence of urinary incontinence in female dogs in this study, 
which contrasts with another paper that found urinary incontinence occurred twice as 
often in females spayed after their first heat as those spayed before having a heat 
cycle.[111]13
By far the largest, best designed studies in dogs and cats involved following over 1800 
dogs and 1600 cats after neutering (either before or after 5.5 months of age) for an 
average of 4-4.5 years, but as long as 11 years in some cases.[93,149] For dogs, 7 out of 
14 behavioral measures appeared affected by age at neutering, with early-neutering 
worsening 3 problem behaviors and improving 4. Animals in the early-neuter group 
exhibited higher rates of noise phobia and sexual behaviors. The early-neutered group 
also exhibited less separation anxiety, fearful urination in the house, and escaping. Earlycastrated males (but not females) showed more aggression towards humans in the 
household and more barking. When only problems considered by owners to be serious 
were analyzed, the reduced risk of escaping for the early-neuter group was the only
behavior still significantly associated with age at neutering.[93]
For medical conditions, 4 were significantly associated with age at neutering. Dogs 
neutered early had higher rates of hip dysplasia, though the dysplasia seen in the 
traditional-age group was clinically worse and this group was far more likely to be 
euthanized for the problem than the early-neuter group. Rates of cystitis and urinary 
incontinence were higher for females neutered before 5.5 months of age. The early-neuter 
group had lower rates of respiratory infections but higher rates of parvoviral infection. 
And finally, the early-neuter group had a lower rate of obesity than those dogs neutered at 
the traditional age. The remaining 43 outcome measures studied showed no difference 
between the two groups.[93]
For cats, early neutering increased shyness around strangers for both sexes, and it 
increased hiding behavior for males but not females. Early-neutered cats were showed 
less hyperactivity, and early-neutered males showed less aggression towards 
veterinarians, less urine spraying, and fewer sexual behaviors. There may also have been 
a decreased rate of scratching furniture in early-neutered cats, but these cats were more 
likely to be declawed so the effect may be an artifact. When only problems considered 
serious were analyzed, none of these behaviors was significantly associated with age at 
neuter.[149]
Early-neutered cats experienced lower rates of asthma and gingivitis, and males 
experienced fewer abscesses in the first 5-6 years after neutering. Cats neutered early 
may have experienced lower rates of cancer, but when only malignancies confirmed by a 
veterinarian were considered this effect was not significant. For the other 38 outcome 

measures studied, no difference between the groups was observed.[149]

Conclusions
It is apparent that spaying and castration have clear benefits for the pet population in 
general and both benefits and risks for individual dogs and cats. When the totality of the 
evidence is considered, it is generally the case that common, serious problems in females 
are reduced by spaying and that less common or less serious problems may be
exacerbated. It is, of course, impossible to predict for a particular pet what the medical or 
behavioral results of spaying or leaving her intact will be. However, the scientific 
evidence supports routine spaying of female dogs not intended for breeding because 
overall it is more likely to prevent than cause serious disease. The evidence is mixed 14
regarding the risks and benefits of spaying dogs before 5-6 months of age, so no strong
recommendation for or against the practice can be made. However, it is clear that spaying 
female dogs before their first heat is preferable to spaying them after this event. For cats, 
early spaying seems to have more benefits than risks. 
In male dogs, the individual benefits of castration are not so clearly greater than the risks 
as they are for females. Overall, it seems that males are more likely to benefit than be 
harmed by being castrated, but the balance of the evidence is close. The population 
benefits, of course, argue in favor of routine neutering of male dogs. For male cats, 
however, neutering is clearly more likely to benefit rather than harm them. As is the case 
with females, early or traditional age at neutering seem roughly equivalent for dogs, but 
there seem to be more benefits than risks for early neutering of cats.
The decision to neutering an individual animal, and when to do so, should take into 
account both the scientific evidence of the risks and benefits as well as the unique 
circumstances of the pet and the owners. Rather than a dogmatic, one-size-fits-all 
approach, owners and veterinarians should examine the benefits and risks given all the 
available information and make appropriate, rational decisions in each case. There is, 
unfortunately, a tendency for lay people and veterinarians to react to the complexity and 
uncertainty of the research data by making broad, unsupportable generalizations or by 
sticking to habit and tradition. Our pets are better served by our remaining open-minded 
and reasonable and evaluating the quality and meaning of the available data carefully in 
light of the individual circumstances and characteristics of each individual animal.

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