Non Breed-Specific Research

The long-held belief that neutering is benign and universally beneficial to dogs has been based on surprisingly limited scientific study.
— Dr. Nancy Kay

Presented below is a bibliography of spay/neuter research that is not breed-specific. The articles are presented in reverse chronological order.


Aggression Toward Familiar People, Strangers, and Conspecifics in Gonadectomized and Intact Dogs

Aggression dog neuter castration

The goal of this study was to verify the accuracy of the long-standing prevailing dogma that neutering is a means of preventing/treating aggressive behaviors of dogs. To date, this is the largest study investigating the relationship between neutering and aggression.

Surveys of more than 13,000 dogs were analyzed assessing the correlation between neutering and the age of the dog at the time of neutering with aggressive behavior towards familiar people, strangers, and other dogs. The results showed that dogs neutered between 7-12 months of age were more likely to demonstrate aggression towards strangers. Otherwise there were no clear correlations found. In other words, there is no evidence that neutering at any age alters aggressive behavior towards familiar people or dogs.

Conclusions: These results dispute the long-standing dogma that neutering is a consistently effective means for preventing and/or treating aggressive behavior in dogs. Based on their results, the researchers concluded, "Given the increasing evidence of significant negative health effects of gonadectomy, there is an urgent need to systematically examine other means of preventing unwanted procreation, such as vasectomy or hysterectomy."

Castration spay neuter autoimmune immune mediated disease

The relationship between neuter status and development of a variety of autoimmune diseases was explored. Autoimmune diseases included atopic dermatitis (allergic skin disease), autoimmune hemolytic anemia, myasthenia gravis, colitis, hypoadrenocorticism (Addison's disease), hypothyroidism, immune mediated polyarthritis, immune mediated thrombocytopenia, inflammatory bowel disease, lupus erythematosus, and pemphigus complex. Patient records of 90,090 dogs were evaluated, and neutered dogs made up 75.93% of this population.

Results indicated that, compared to intact dogs, neutered dogs had a significantly greater risk for developing atopic dermatitis, autoimmune hemolytic anemia, Addison's disease, hypothyroidism, immune mediated thrombocytopenia, and inflammatory bowel disease. Neutered females, but not males, had a significantly greater risk of lupus compared to intact female dogs.

Conclusions: Neutering is associated with increased risk of several autoimmune disorders. Sex hormones appear to play a critical role in immune function and tissue self-recognition.

Lifespan dog spay neuter

The time and cause of death of 70,574 dogs (43.6% intact and 56.4% neutered) was evaluated. The age at the time of neuter was not assessed and 185 breeds were represented. The mean age of death of intact dogs was 7.9 years and of neutered dogs was 9.4 years. Neutering increased life expectancy of the male population by 13.8% and the female population by 26.3%

The incidence of various forms of cancer (transitional cell carcinoma, osteosarcoma, lymphosarcoma, mast cell tumor) was increased in the neutered population. The incidence of mammary cancer was increased within the intact population of female dogs. Intact dogs were more likely to die from infectious diseases, trauma, degenerative diseases, and vascular disease. Neutered dogs were more likely to die from cancer and immune mediated diseases.

Conclusion: Neutered dogs live longer than intact dogs.

Pyometra dogs

The overall prevalence of pyometra amongst the entire hospital population of female dogs was 2.2% (1,728 cases within a female dog population of 78,469). As the incidence of elective neutering decreased, the prevalence of pyometra increased. The mean age at the time of diagnosis was 7.7 years. The mortality rate associated with surgery for this disease was 3.2%.

Conclusion: Pyometra is a preventable disease (through neutering) and veterinary treatment is required if an affected animal is to survive.

Urinary incontinence dog canine neuter

The authors of this study reviewed seven prior studies examining the effect of neutering or age at the time of neutering on the risk of development of urinary incontinence in female dogs. They found three of the studies to be the most credible (the least amount of bias), and these three studies demonstrated weak evidence that neutering, particularly before the age of three months, increases the risk of urinary incontinence.

Conclusion: There is weak evidence linking neutering to urinary incontinence in female dogs. 

* Dr. Nancy Kay wants you to know that she and the majority of veterinarians she knows have the clinical impression that spaying, particularly at a young age, is more likely to result in adult-onset urinary incontinence than this study conveys.

The Effect of Neutering on the Risk of Mammary Tumours in Dogs- A Systematic Review

Mammary tumor mammary cancer dog neuter

The objectives of this study were to evaluate the evidence from previous studies of the impact of neutering, or age of neutering, on the risk of mammary tumors (breast cancer) in female dogs. Of the 13 studies they reviewed, 9 were judged to have a high risk of bias. The remaining 4 were classified as having a moderate risk of bias. One study found an association between neutering and a reduced risk of mammary tumors. Two studies found no evidence of an association. One reported “some protective effect” of neutering on the risk of mammary tumors, but no numbers were presented.

Conclusions: Due to the limited evidence available and the risk of bias in the published results, the evidence that neutering reduces the risk of mammary cancer, and the evidence that age at neutering has an effect, are judged to be weak and are not a sound basis for firm recommendations.

* Dr. Nancy Kay wants you to know that she and the majority of veterinarians she knows have the clinical impression that, neutering at a young age (particularly before the onset of the first heat cycle), is markedly protective against the development of mammary cancer.

canine testicular tumor

The purpose of this study was to provide information on the incidence of canine testicular tumors. Testicular tumors represented 16.8% of tumors diagnosed in male dogs. A total of 96 tumors from 80 dogs were evaluated and 34.4% were seminomas, 26% were interstitial cell tumors, 22.9% were mixed germ cell-stromal cell tumors, and 16.6% were Sertoli cell tumors. Of the 96 testicular tumors, 54.2% developed within cryptorchid testes and 45.8% developed within scrotal testes. in 35 dogs. Interstitial cell tumors were the only tumors not represented within the cryptorchid group.

The detection rate of testicular tumors in dogs younger than 10-years-old was significantly associated with cryptorchidism.

Conclusion: Cryptorchidism and age significantly increase the incidence of testicular tumors.

Prostate gland cancer dog castration neuter

A study of 2,219 male dogs with urinary bladder transitional cell carcinoma (TCC) prostate gland TCC, or prostate gland adenocarcinoma (ACA), prostate gland carcinoma (CA), and prostate gland tumor without a clearcut diagnosis were evaluated in terms of neutering status. The breakdown was as follows:

Urinary bladder TCC:  213 intact, 421 neutered 

Prostate gland TCC:   37 intact, 164 neutered 

Prostate gland ACA:   272 intact, 318 neutered

Prostate gland CA:   128 intact, 208 neutered

Unclassified prostate gland tumor:   182 intact, 208 neutered

Conclusion: Neutered males had a significantly increased risk for each form of cancer.

* Dr. Nancy Kay wants to point out that a commonly held misbelief is that neutering is protective against prostate gland cancer in dogs. This study confirms that this is not the case.


early spay neuter

Long-Term Risks and Benefits of Early-Age Gonadectomy in Dogs
This study compared the long-term risks and benefits of early-age neutering (at less than 5.5 months of age) compared with traditional-age neutering (between 5.5 and 12 months of age) among dogs adopted from a large animal shelter. The earliest dogs were neutered was at 6 weeks of age. A total of 1,482 dogs were evaluated and followed for up to 11 years of age. A total of 56 health and behavior outcomes were evaluated. Within the female population, early-age neutering was associated with with an increased rate of urinary tract infections and urinary incontinence. Amongst males and females, early-age neutering resulted in more dogs with hip dysplasia, noise phobias, and sexual behaviors. Also noted were decreased frequencies of obesity, separation anxiety, escaping behaviors, and submissive urination.

Conclusions: Early-age neutering appears to offer more benefits than risks for male dogs. For female dogs, the increased risk for urinary incontinence suggests that delaying neutering until at least 3 months of age may be beneficial.

* Dr. Nancy Kay wants to point out that cancer was not one of the outcomes evaluated.


Effect of Gonadectomy on Subsequent Development of Age-Related Cognitive Impairment in Dogs

Dogs who between the ages of 11 and 14 years of age (139 dogs in total) and were diagnosed with canine cognitive disorder were included in this study. They were assigned to four categories of cognitive impairment based on their clinical evaluations.

Canine cognitive disorder

Followup interviews were conducted 12 to 18 months later to learn how the dogs had progressed. The researchers learned that within the group of intact dogs symptoms were less likely to progress compared to the neutered group of dogs. This was true regardless of the severity of the canine cognitive disorder.

Conclusion: Reproductive status may impact the progression of canine cognitive disorder.

canine hypothyroidism

This study included 3,206 dogs diagnosed with hypothyroidism. A number of potential causal factors were evaluated. Results revealed that spayed females were at higher risk for developing this disease compared to intact females. Though not statistically significant, neutered males had 30% more hypothyroidism compared to their intact counterparts.

Conclusion: Neutering predisposes to hypothyroidism in female dogs.


Castration of Adult Male Dogs: Effects on Roaming, Aggression, Urine Marking, and Mounting

This study evaluated the effect of castration on undesirable behaviors in 42 dogs. The behaviors included roaming, fighting with other male dogs, urine marking in the house, and mounting other dogs or people. All of the dogs were neutered between 6 and 12 months of age.

castration effect on behavior

Results of this study were as follows: Roaming was reduced in 90% of the dogs, mounting behavior in 67%, aggression with other male dogs in 62%, and urine marking in 50%. There were no effects on territorial aggression, food aggression, and fear induced aggression.

Conclusions: Castration is a good deterrent of roaming and just a so-so deterrent of urine marking, mounting behavior, and aggression towards other male dogs. Castration has no impact on other forms of aggression.

* It appears that neutering has the potential to deter some behaviors and, based on the current body of research evidence, promote others. This impact on behavior may be gender and-breed specific.