The Most Asked Question


You don’t need to be in the dog world for very long before you hear about desexing benefiting the health of dogs. These claims talk about reducing cancer (testicular, mammary, prostate, ovarian, uterine, cervix), reducing prostate disease (in boys), and preventing pyometra (in bitches).

What we don’t hear about is the undesirable side effects of desexing, and how desexing is linked to increased risks of some cancers, and an increased likelihood of joint disorders.

Torres de la Riva et al, in their research published just this month, decided to look into the health effects of desexing in golden retrievers; Neutering Dogs: Effects on Joint Disorders and Cancers in Golden Retrievers.

They, indeed, hit back at these spruced ‘health benefits’ in the introduction of their piece, saying:

“In contrast to the rather strong evidence for neutering males and/or females as a risk factor for osteosarcoma, hemangiosarcoma, lymphosarcoma, mast cell tumours and prostate cancer, evidence for neutering as protection against a dog acquiring one or more cancers is weak.”

This research set out to investigate spay and neuter in Golden Retrievers from 1-8 years. They chose goldens because they are commonly used as assistance animals, and so they hoped the implications of this study may have consequences for related assistance organisations (of course, dog science only happens when it helps people!). It makes sense: it’s ‘wasteful’ to invest in a dog becomes invalid for the work they were trained in, especially if that invalidity could’ve been prevented by more-appropriate timing of desexing.

While other research has pooled many breeds and health affects together, this is the first study to look at desexing in just one particular breed. Prior-analysis determined several conditions to look at: hip dysplasia, cranial cruciate ligament tear, lymphosarcoma, hemangiosarcoma, mast cell tumours, osterosarcoma, and elbow dysplasia.

Dogs were included in the study if they were between 1-8 years of age. They were put into categories of either ‘early neuter’ (before 1 year of age), ‘late neuter’ (after 1 year of age), and ‘intact’. Data regarding their health was retrospective, gained from veterinary records. Any dogs where a health diagnosis was ‘grey’ (non-conclusive), they were excluded from the study.

Results

Hip Dysplasia

Males who were desexed early had twice (10.3%) the incidence of hip dysplasia than intact males (5.1%). Males that were late-neutered had a hip dysplasia rate of approximately 3%.

In bitches, approximately 5% of early-spayed bitches got hip dysplasia. Intact and late-neutered bitches had hip dysplasia rates of approximately 3%. This difference was not statistically significant.

This is in keeping with research (quoted within this research) that indicated that desexing increases the likelihood of hip dysplasia by 17%.
Cranial Crucial Ligament Tear

No intact dogs, males or females, got cranial crucial ligament tears. Animals that were neutered early were more likely to get tears (5.1% of boys and 7.7% of bitches). No late-spayed bitches developed a tear, and only 1 dog developed a tear (1%).

This is consistent with research that showed a 3-fold increase in excessive tibial plateau angle in desexed dogs (of both sexes, across all breeds). This excessive tibial plateau angle is a risk factor for crucial ligament tears.

Lymphosarcoma

Approximately 3% of intact males got lymphosarcoma, compared to 9% of early-neutered males, and 0% of late-neutered males. In bitches, approximately 1.5% of intact bitches got lymphosarcoma, compared to 6% of early-spayed bitches, and 1% of late-spayed bitches. The research states, ”Although the rates of occurrence of this disease were lower in both male and female intact dogs, than in the early-neutered dogs, the difference was statistically significant only in males. Early-neutered males had nearly 3 times the occurence of [lymphosarcoma] as intact males and no cases of [lymphosarcoma] were observed in the late-neutered males”.

The results of this study are not dissimilar to others quoted, where intact females were at a significantly lower risk of developing lymphosarcoma than neutered females, or males (entire or desexed).

Hemangiosarcoma

Intact males and early-neutered males had an incidence of hemangiosarcoma of around 3%, while only 1% of late neutered dogs got hemangiosarcoma. This difference was not statistically significant.

In bitches, the difference was more significant (statistically and otherwise!): 7.4% of late-neutered bitches were diagnosed with hemangiosarcoma, which was over 4 times the frequency seen in intact (1.6%) and early-spayed (1.8%) bitches. Late neutered bitches also got hemangiosarcoma early (3.2 years old), while intact and late-spayed bitches got the cancer later (at 6.4 years and 7.6 years respectively).

This is in keeping with existing research that shows that hemangiosarcoma occurs 4 times more in spayed females than those intact, and spayed females had more than 2 times the risk of developing splenic hemangiosarcoma than intact bitches.

Mast Cell Tumours (MCT)

This research found that approximately 3% of the intact males got MCT, approximately 2% of neutered early males got MCT, and about 4% of late neutered males got MCT. Clearly, these results were not statistically significant.

However, while no intact bitches got MCT, 2.3% of early-desexed bitches did, and 5.7% of late-neutered bitches developed tumours.

This is consistent with other research that showed cutaneous mast cell tumours had a 4 times greater frequency in neutered females than intact females.
Osterosarcoma

The incidence of osterosarcoma was so small in this study that it was excluded from analysis. Other studies (quoted in this research) showed that osteosarcoma occurred 2-times as often in neutered relative to intact dogs, while another study showed that neutering prior to 1 year of age increased the occurrence of osteosarcoma 3-4 times.

Other health implications (elbow dyplasia and mammary cancer)

Body condition scores were used to control for extra weight in neutered dogs, but the scores were similar between groups (neutered and entire).

In regard to Mammary Cancer (MC), the study says, “No cases of MC were diagnosed in intact females in this study. This finding is partially explained by the relatively low frequency in which MC is diagnosed in Golden Retrievers. While this finding contrasts with the general concern expressed about the risk of MC in gonadally intact females, it is consistent with recent findings from a systematic meta-analysis finding a weak link, if any, between neutering and a reduce risk of MC.”

This study also quoted other research, that found that prostate cancer occurred four times as frequently in neutered males as intact males.


Why would desexed dogs get joint problems?

It is likely that the joint problems (hip dyplasia and cranial crucial ligament tears) investigated in this study are more likely to occur due to neutering’s affects on growth plate closure. Other factors include breed related vulnerabilities, and sex related closure speeds (i.e. the growth plate closes more quickly in males than females). This may be why the occurrence of hip dyplasia in early-neutered males is double that of hip dyplasia in intact males.

Why would desexed dogs get cancer?

This is a harder question to answer. It’s believed that estrogen acts on “microsatellite instability” – or the likelihood of parts of DNA to change, which is linked to cancer. Estrogen effectively stops microsatellites for being ‘so changing’. When sex organs of bitches are removed, estrogen is also reduced, and so it can no longer act upon microsatellites and so cancer is more likely. However, it is also hypothesised that bitches need to have a heat cycle for estrogen to have this protective affect. This is why late neutering is perhaps associated with higher rates of hemangiosarcoma and mass cell tumours in entire and early-neutered bitches. There is a lot to learn in this area, and a lot more research needs to take place to understand the affects of hormones on cancer.

Conclusions from this Research

This research concludes that:

• Early neutering is associated with an increased occurrence of hip dyplasia, of cranial crucial ligament tears, and of lymphosarcoma.
• Late neutering was associated with the subsequent occurence of mass cell tumours and hemangiosarcoma in bitches.
• Affects of neutering on elbow dyplasia, osteosarcoma, and mass cell tumours were excluded from the study due to low sample number.
• There was a statisical difference in the health of early desexed and intact animals, and early desexed and late desexed animals, but not late desexed and intact animals. (That is, early-desexed animals had different health implications to late-desexed and intact dogs, but late-desexed and intact dogs had similar health.)
• The researchers call for this research to not be generalised to other breeds.
• For all five diseases analysed in this study, when an intact dog had a disease, it only occurred at one fourth or one half the rate of dogs that were early- or late-desexed.
• Recommended that males not be neutered before puberty to avoid problems of hip dyplasia, cranial crucial ligament tear, and lymphosarcoma. (But noted that even late neutering male be associated with age-related cognitive decline, as seen in other research.)
• For bitches, there is no clear recommendations. Early-neutering significantly increases the incidence of cranial crucial ligament tears from near 0 to almost 8%, while late neutering inreaces the risk of hemangiosarcoma to 4 times that of the 1.6% rate seen in intact females. Late neutering also was associated with mass cell tumours (5.7%), compared to no occurances in intact females. (As a personal comment, it seems from this research that leaving bitches in tact may be the best for their health. I am curious why the researchers did not make this suggestion.)


Criticisms of Research

This researched used veterinary records for this study, which means they were primarily using ‘ill’ dogs for their research. Some have argued that this research should’ve been more population based (though I’m not sure how easy it is to get population data of this sort).

Some have also theorised that perhaps the dogs that are entire were left entire because of superior health and conformation, meaning they were less likely to suffer from these conditions. That is, mostly ‘pet’ quality dogs were desexed, and more ‘high quality’ or ‘show quality’ dogs were left entire.

Further Research

Further research needs to consider the other types of cancers and risks associated with leaving dogs entire. A similar study looking at rates of testicular, ovarian and cervix cancer is well needed. Research also needs to consider non-cancer problems, such as prostate issues and pyometra, which are also a risk for dogs that are kept entire.

The research indicates that this should not be generalised to other breeds, so it makes sense that this research should be replicated to other breeds to see if consistent themes emerge.

As noted, more attention needs to be given to how estrogen seems to be tied with cancer.

Reference:

Torres de la Riva, G., Hart, B., Farver, T., Oberbauer, A., Messam, L., Willits, N., & Hart, L. (2013). Neutering Dogs: Effects on Joint Disorders and Cancers in Golden Retrievers PLoS ONE, 8 (2) DOI: 10.1371/journal.pone.0055937


Further reading on this study:

Question, Question, Question: New insights into the health effects of spaying and neutering from Gayle Watkins.

New Study: Neutering affects dog health from Time 4 Dogs.

Golden retriever study suggests neutering affects dog health from UC Davis.

New study find early neuter doubles the risk of hip dysplasia in dogs from Dogs Naturally.

What have we been saying from Angry Vet.

Another research paper on health impact of spay/neuter from the KC Dog Blog.

Studying the study from Victoria Stilwell.





 


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Long-Term Health Risks and Benefits Associated with Spay / Neuter in Dogs
 
Laura J. Sanborn April 1, 2007

INTRODUCTION

Dog owners in America are frequently advised to spay/neuter their dogs for health reasons. A number of health benefits are cited, yet evidence is usually not cited to support the alleged health benefits. When discussing the health impacts of spay/neuter, health risks are often not mentioned. At times, some risks are mentioned, but the most severe risks usually are not. This article is an attempt to summarize the long-term health risks and benefits associated with spay/neuter in dogs that can be found in the veterinary medical literature. This article will not discuss the impact of spay/neuter on population control, or the impact of spay/neuter on behavior. Nearly all of the health risks and benefits summarized in this article are findings from retrospective epidemiological research studies of dogs, which examine potential associations by looking backwards in time. A few are from prospective research studies, which examine potential associations by looking forward in time.

SUMMARY An objective reading of the veterinary medical literature reveals a complex situation with respect to the long-term health risks and benefits associated with spay/neuter in dogs. The evidence shows that spay/neuter correlates with both positive AND adverse health effects in dogs. It also suggests how much we really do not yet understand about this subject. On balance, it appears that no compelling case can be made for neutering most male dogs, especially immature male dogs, in order to prevent future health problems. The number of health problems associated with neutering may exceed the associated health benefits in most cases.

On the positive side, neutering male dogs
· eliminates the small risk (probably <1%) of dying from testicular cancer
· reduces the risk of non-cancerous prostate disorders
· reduces the risk of perianal fistulas
· may possibly reduce the risk of diabetes (data inconclusive)

On the negative side, neutering male dogs
· if done before maturity, increases the risk of osteosarcoma (bone cancer) by a factor of 3.8; this is a common cancer in medium/large and larger breeds with a poor prognosis. ?F?nincreases the risk of cardiac hemangiosarcoma by a factor of 1.6; this is a common cancer and major cause of death in some breeds
· triples the risk of hypothyroidism
· increases the risk of geriatric cognitive impairment
· triples the risk of obesity, a common health problem in dogs with many-associated health problems
· quadruples the small risk (<0.6%) of prostate cancer
· doubles the small risk (<1%) of urinary tract cancers
· increases the risk of orthopedic disorders
· increases the risk of adverse reactions to vaccinations For female dogs, the situation is more complex. The number of health benefits associated with spaying may exceed the associated health problems in some (not all) cases.

On balance, whether spaying improves odds of overall good health or degrades them probably depends on the age of the female dog and the relative risk of various diseases in the different breeds.

On the positive side, spaying female dogs
if done before 2.5 years of age, greatly reduces the risk of mammary tumors, the most common malignant tumors in female dogs
nearly eliminates the risk of pyometra, which otherwise would affect about 23% of intact female dogs; pyometra kills about 1% of intact female dogs
reduces the risk of perianal fistulas
removes the very small risk (_0.5%) from uterine, cervical, and ovarian tumors

On the negative side, spaying female dogs
if done before maturity, increases the risk of osteosarcoma by a factor of 3.1; this is a common cancer in larger breeds with a poor prognosis
increases the risk of splenic hemangiosarcoma by a factor of 2.2 and cardiac hemangiosarcoma by a factor of >5; this is a common cancer and major cause of death in some breeds
triples the risk of hypothyroidism
increases the risk of obesity by a factor of 1.6-2, a common health problem in dogs with many associated health problems
causes urinary “spay incontinence” in 4-20% of female dogs
increases the risk of persistent or recurring urinary tract infections by a factor of 3-4 ?F?nincreases the risk of recessed vulva, vaginal dermatitis, and vaginitis, especially for female dogs spayed before puberty
doubles the small risk (<1%) of urinary tract tumors
increases the risk of orthopedic disorders
increases the risk of adverse reactions to vaccinations

One thing is clear – much of the spay/neuter information that is available to the public is unbalanced and contains claims that are exaggerated or unsupported by evidence. Rather than helping to educate pet owners, much of it has contributed to common misunderstandings about the health risks and benefits associated of spay/neuter in dogs. The traditional spay/neuter age of six months as well as the modern practice of pediatric spay/neuter appear to predispose dogs to health risks that could otherwise be avoided by waiting until the dog is physically mature, or (perhaps in the case of many male dogs) foregoing it altogether unless medically necessary. The balance of long-term health risks and benefits of spay/neuter will vary from one dog to the next. Across-the-board recommendations for all pet dogs do not appear to be supportable from findings in the veterinary medical literature.

FINDINGS FROM STUDIES 

This section summarizes the diseases or conditions that have been studied with respect to spay/neuter in dogs.

Complications from Spay/Neuter Surgery All surgery incurs some risk of complications, including adverse reactions to anesthesia, hemorrhage, inflammation, infection, etc. Complications include only immediate and near term impacts that are clearly linked to the surgery, not to longer term impacts that can only be assessed by research studies. At one veterinary teaching hospital where complications were tracked, the rates of intraoperative, postoperative and total complications were 6.3%, 14.1% and 20.6%, respectively as a result of spaying female dogs. Other studies found a rate of total complications from spaying of 17.7%2 and 23%.

A study of Canadian veterinary private practitioners found complication rates of 22% and 19% for spaying female dogs and neutering male dogs, respectively4. Serious complications such as infections, abscesses, rupture of the surgical wound, and chewed out sutures were reported at a 1- 4% frequency, with spay and castration surgeries accounting for 90% and 10% of these complications, respectively. The death rate due to complications from spay/neuter is low, at around 0.1%5.

Prostate Cancer Much of the spay/neuter information available to the public asserts that neutering will reduce or eliminate the risk that male dogs develop prostate cancer. This would not be an unreasonable assumption, given that prostate cancer in humans is linked to testosterone. But the evidence in dogs does not support this claim. In fact, the strongest evidence suggests just the opposite. There have been several conflicting epidemiological studies over the years that found either an increased risk or a decreased risk of prostate cancer in neutered dogs. These studies did not utilize control populations, rendering these results at best difficult to interpret. This may partially explain the conflicting results. More recently, two retrospective studies were conducted that did utilize control populations. One of these studies involved a dog population in Europe6 and the other involved a dog population in America.

Both studies found that neutered male dogs have a four times higher risk of prostate cancer than intact dogs. Based on their results, the researchers suggest a cause-and-effect relationship: “this suggests that castration does not initiate the development of prostates carcinoma in the dog, but does favor tumor progression”6 and also “Our study found that most canine prostate cancers are of ductal/urothelial origin….The relatively low incidence of prostate cancer in intact dogs may suggest that testicular hormones are in fact protective against ductal/urothelial prostatic carcinoma, or may have indirect effects on cancer development by changing the environment in the prostate.”7

This needs to be put in perspective. Unlike the situation in humans, prostate cancer is uncommon in dogs. Given an incidence of prostate cancer in dogs of less than 0.6% from necropsy studies8, it is difficult to see that the risk of prostate cancer should factor heavily into most neutering decisions. There is evidence for an increased risk of prostate cancer in at least one breed (Bouviers)6, though very little data so far to guide us in regards to other breeds.

Testicular Cancer Since the testicles are removed with neutering, castration removes any risk of testicular cancer (assuming the castration is done before cancer develops). This needs to be compared to the risk of testicular cancer in intact dogs. Testicular tumors are not uncommon in older intact dogs, with a reported incidence of 7%9. However, the prognosis for treating testicular tumors is very good owing to a low rate of metastasis10, so testicular cancer is an uncommon cause of death in intact dogs. For example, in a Purdue University breed health survey of Golden Retrievers11, deaths due to testicular cancer were sufficiently infrequent that they did not appear on list of significant causes of "Years of Potential Life Lost for Veterinary Confirmed Cause of Death” even though 40% of GR males were intact. Furthermore, the GRs who were treated for testicular tumors had a 90.9% cure rate. This agrees well with other work that found 6-14% rates of metastasis for testicular tumors in dogs12. The high cure rate of testicular tumors combined with their frequency suggests that fewer than 1% of intact male dogs will die of testicular cancer. In summary, though it may be the most common reason why many advocate neutering young male dogs, the risk from life threatening testicular cancer is sufficiently low that neutering most male dogs to prevent it is difficult to justify. An exception might be bilateral or unilateral cryptorchids, as testicles that are retained in the abdomen are 13.6 times more likely to develop tumors than descended testicles13 and it is also more difficult to detect retained tumors by routine physical examination.

Osteosarcoma (Bone Cancer) a multi-breed case-control study of the risk factors for osteosarcoma found that spay/neutered dogs (males or females) had twice the risk of developing osteosarcoma, as did intact dogs14. This risk was further studied in Rottweilers, a breed with a relatively high risk of osteosarcoma. This retrospective cohort study broke the risk down by age at spay/neuter, and found that the elevated risk of osteosarcoma is associated with spay/neuter of young dogs15. Rottweilers spayed/neutered before one year of age were 3.8 (males) or 3.1 (females) times more likely to develop osteosarcoma than intact dogs. Indeed, the combination of breed risk and early spay/neuter meant that Rottweilers spayed/neutered before one year of age had a 28.4% (males) and 25.1% (females) risk of developing osteosarcoma. These results are consistent with the earlier multi-breed study14 but have an advantage of assessing risk as a function of age at neuter.

The researchers suggest a cause-and-effect relationship, as sex hormones are known to influence the maintenance of skeletal structure and mass, and also because their findings showed an inverse relationship between time of exposure to sex hormones and risk of osteosarcoma.15 The risk of osteosarcoma increases with increasing breed size and especially height14. It is a common cause of death in medium/large, large, and giant breeds. Osteosarcoma is the third most common cause of death in Golden Retrievers11 and is even more common in larger breeds14. Given the poor prognosis of osteosarcoma and its frequency in many breeds, spay/neuter of immature dogs in the medium/large, large, and giant breeds is apparently associated with a significant and elevated risk of death due to osteosarcoma.

Mammary Cancer (Breast Cancer) Mammary tumors are by far the most common tumors in intact female dogs, constituting some 53% of all malignant tumors in female dogs in a study of dogs in Norway16 where spaying is much less common than in the USA. 50-60% of mammary tumors are malignant, for which there is a significant risk of metastasis17. Mammary tumors in dogs have been found to have estrogen receptors18, and the published research19 shows that the relative risk (odds ratio) that a female will develop mammary cancer compared to the risk in intact females is dependent on how many estrus cycles she experiences: # of estrus cycles before spay Odds Ratio None 0.005 1 0.08 2 or more 0.26 Intact 1.00 The same data when categorized differently showed that the relative risk (odds ratio) that females will develop mammary cancer compared to the risk in intact females is indicated that: Age at Spaying Odds Ratio _ 29 months 0.06 _ 30 months 0.40 (not statistically significant at the P<0.05 level) Intact 1.00

Please note that these are RELATIVE risks. This study has been referenced elsewhere many times but the results have often been misrepresented as absolute risks. A similar reduction in breast cancer risk was found for women under the age of 40 who lost their estrogen production due to “artificial menopause”20 and breast cancer in humans is known to be estrogen activated. Mammary cancer was found to be the 10th most common cause of years of lost life in Golden Retrievers, even though 86% of female GRs were spayed, at a median age of 3.4 yrs11. Considering that the female subset accounts for almost all mammary cancer cases, it probably would rank at about the 5th most common cause of years of lost life in female GRs. It would rank higher still if more female GRs had been kept intact up to 30 months of age. Boxers, cocker spaniels, English springer spaniels, and dachshunds are breeds at high risk of mammary tumors16. A population of mostly intact female Boxers was found to have a 40% chance of developing mammary cancer between the ages of 6-12 years of age16. There are some indications that purebred dogs may be at higher risk than mixed breed dogs, and purebred dogs with high inbreeding coefficients may be at higher risk than those with low inbreeding coefficients21. More investigation is required to determine if these are significant. In summary, spaying female dogs significantly reduces the risk of mammary cancer (a common cancer), and the fewer estrus cycles experienced at least up to 30 months of age, the lower the risk will be.

Female Reproductive Tract Cancer (Uterine, Cervical, and Ovarian Cancers) Uterine/cervical tumors are rare in dogs, constituting just 0.3% of tumors in dogs22. Spaying will remove the risk of ovarian tumors, but the risk is only 0.5%23. While spaying will remove the risk of reproductive tract tumors, it is unlikely that surgery can be justified to prevent the risks of uterine, cervical, and ovarian cancers as the risks are so low.

Urinary Tract Cancer (Bladder and Urethra Cancers) An age-matched retrospective study found that spay/neuter dogs were two times more likely to develop lower urinary tract tumors (bladder or urethra) compared to intact dogs24. These tumors are nearly always malignant, but are infrequent, accounting for less than 1% of canine tumors. So this risk is unlikely to weigh heavily on spay/neuter decisions. Airedales, Beagles, and Scottish Terriers are at elevated risk for urinary tract cancer while German Shepherds have a lower than average risk24.

Hemangiosarcoma Hemangiosarcoma is a common cancer in dogs. It is a major cause of death in some breeds, such as Salukis, French Bulldogs, Irish Water Spaniels, Flat Coated Retrievers, Golden Retrievers, Boxers, Afghan Hounds, English Setters, Scottish Terriesr, Boston Terriers, Bulldogs, and German Shepherd Dogs25. In an aged-matched case controlled study, spayed females were found to have a 2.2 times higher risk of splenic hemangiosarcoma compared to intact females25. A retrospective study of cardiac hemangiosarcoma risk factors found a >5 times greater risk in spayed female dogs compared to intact female dogs and a 1.6 times higher risk in neutered male dogs compared to intact male dogs.26 The authors suggest a protective effect of sex hormones against hemangiosarcoma, especially in females. In breeds where hermangiosarcoma is an important cause of death, the increased risk associated with spay/neuter is likely one that should factor into decisions on whether or when to sterilize a dog.

Hypothyroidism Spay/neuter in dogs was found to be correlated with a three fold increased risk of hypothyroidism compared to intact dogs. The researchers suggest a cause-and-effect relationship27. They wrote: “More important [than the mild direct impact on thyroid function] in the association between [spaying and] neutering and hypothyroidism may be the effect of sex hormones on the immune system. Castration increases the severity of autoimmune thyroiditis in mice” which may explain the link between spay/neuter and hypothyroidism in dogs. Hypothyroidism in dogs causes obesity, lethargy, hair loss, and reproductive abnormalities.28

Obesity Owing to changes in metabolism, spay/neuter dogs are more likely to be overweight or obese than intact dogs. One study found a two fold increased risk of obesity in spayed females compared to intact females29. Another study found that spay/neuter dogs were 1.6 (females) or 3.0 (males) times more likely to be obese than intact dogs, and 1.2 (females) or 1.5 (males) times more likely to be overweight than intact dogs30. A survey study of veterinary practices in the UK found that 21% of dogs were obese.29 Being obese and/or overweight is associated with a host of health problems in dogs. Overweight dogs are more likely to be diagnosed with hyperadrenocorticism, ruptured cruciate ligament, hypothyroidism, lower urinary tract disease, and oral disease31. Obese dogs are more likely to be diagnosed with hypothyroidism, diabetes mellitus, pancreatitis, ruptured cruciate ligament, and neoplasia (tumors) 31.

Diabetes Some data indicate that neutering doubles the risk of diabetes in male dogs, but other data showed no significant change in diabetes risk with neutering32. In the same studies, no association was found between spaying and the risk of diabetes.

Adverse Vaccine Reactions A retrospective cohort study of adverse vaccine reactions in dogs was conducted, which included allergic reactions, hives, anaphylaxis, cardiac arrest, cardiovascular shock, and sudden death. Adverse reactions were 30% more likely in spayed females than intact females, and 27% more likely in neutered males than intact males33. The investigators discuss possible cause-and-effect mechanisms for this finding, including the roles that sex hormones play in body’s ability to mount an immune response to vaccination.33 Toy breeds and smaller breeds are at elevated risk of adverse vaccine reactions, as are Boxers, English Bulldogs, Lhasa Apsos, Weimaraners, American Eskimo Dogs, Golden Retrievers, Basset Hounds, Welsh Corgis, Siberian Huskies, Great Danes, Labrador Retrievers, Doberman Pinschers, American Pit Bull Terriers, and Akitas.33 Mixed breed dogs were found to be at lower risk, and the authors suggest genetic hetereogeneity (hybrid vigor) as the cause.

Urogenital Disorders Urinary incontinence is common in spayed female dogs, which can occur soon after spay surgery or after a delay of up to several years. The incidence rate in various studies is 4-20% 34,35,36 for spayed females compared to only 0.3% in intact females37. Urinary incontinence is so strongly linked to spaying that it is commonly called “spay incontinence” and is caused by urethral sphincter incompetence38, though the biological mechanism is unknown. Most (but not all) cases of urinary incontinence respond to medical treatment, and in many cases this treatment needs to be continued for the duration of the dog’s life.39 A retrospective study found that persistent or recurring urinary tract (bladder) infections (UTIs) were 3-4 times more likely in spayed females dogs than in intact females40. Another retrospective study found that female dogs spayed before 5 ½ months of age were 2.76 times more likely to develop UTIs compared to those spayed after 5 ½ months of age.41 Depending on the age of surgery, spaying causes abnormal development of the external genitalia. Spayed females were found to have an increased risk of recessed vulva, vaginal dermatitis, vaginitis, and UTIs.42 The risk is higher still for female dogs spayed before puberty.42

Pyometra (Infection of the Uterus) Pet insurance data in Sweden (where spaying is very uncommon) found that 23% of all female dogs developed pyometra before 10 years of age43. Bernese Mountain dogs, Rottweilers, rough-haired Collies, Cavalier King Charles Spaniels and Golden Retrievers were found to be high-risk breeds43. Female dogs that have not whelped puppies are at elevated risk for pyometra44. Rarely, spayed female dogs can develop “stump pyrometer” related to incomplete removal of the uterus. Pyometra can usually be treated surgically or medically, but 4% of pyometra cases led to death43. Combined with the incidence of pyometra, this suggests that about 1% of intact female dogs will die from pyometra.

Perianal Fistulas Male dogs are twice as likely to develop perianal fistulas as females, and spay/neutered dogs have a decreased risk compared to intact dogs45. German Shepherd Dogs and Irish Setters are more likely to develop perianal fistulas than are other breeds.45

Non-cancerous Disorders of the Prostate Gland The incidence of benign prostatic hypertrophy (BPH, enlarged prostate) increases with age in intact male dogs, and occurs in more than 80% of intact male dogs older than the age of 5 years. Most cases of BPH cause no problems, but in some cases the dog will have difficulty defecating or urinating. Neutering will prevent BPH. If neutering is done after the prostate has become enlarged, the enlarged prostate will shrink relatively quickly. BPH is linked to other problems of the prostate gland, including infections, abscesses, and cysts, which can sometimes have serious consequences.

Orthopedic Disorders In a study of beagles, surgical removal of the ovaries (as happens in spaying) caused an increase in the rate of remodeling of the ilium (pelvic bone)47, suggesting an increased risk of hip dysplasia with spaying. Spaying was also found to cause a net loss of bone mass in the spine 48. Spay/neuter of immature dogs delays the closure of the growth plates in bones that are still growing, causing those bones to end up significantly longer than in intact dogs or those spay/neutered after maturity. Since the growth plates in various bones close at different times, spay/neuter that is done after some growth plates have closed but before other growth plates have closed can result in a dog with unnatural proportions, possibly impacting performance and long term durability of the joints. Spay/neuter is associated with a two fold increased risk of cranial cruciate ligament rupture50. Perhaps this is associated with the increased risk of obesity29 or to changes in body proportions in dogs spay/neutered before the growth plates in the bones have closed49. Spay/neuter before 5 ½ months of age is associated with a 70% increased aged-adjusted risk of hip dysplasia compared to dogs spayed/neutered after 5 ½ months of age41. The researchers suggest “it is possible that the increase in bone length that results from early-age gonadectomy results in changes in joint conformation, which could lead to a diagnosis of hip dysplasia”. In a breed health survey study of Airedales, spay/neuter dogs were significantly more likely to suffer hip dysplasia as well as “any musculoskeletal disorder”, compared to intact dogs51, however possible confounding factors were not controlled for, such as the possibility that some dogs might have been spayed/neutered because they had hip dysplasia or other musculoskeletal disorders. Compared to intact dogs, another study found that dogs neutered six months prior to a diagnosis of hip dysplasia were 1.5 times as likely to develop clinical hip dysplasia.

Geriatric Cognitive Impairment Neutered male dogs and spayed female dogs are at increased risk of geriatric cognitive impairment compared to intact male dogs53. There weren’t enough intact geriatric females available for the study to determine their risk. Geriatric cognitive impairment includes disorientation in the house or outdoors, changes in social interactions with human family members, loss of house training, and changes in the sleep-wake cycle53. The investigators state “This finding is in line with current research on the neuro-protective roles of testosterone and estrogen at the cellular level and the role of estrogen in preventing Alzheimer’s disease in human females. One would predict that estrogens would have a similar protective role in the sexually intact female dogs; unfortunately too few sexually intact female dogs were available for inclusion in the present study to test the hypothesis”

CONCLUSIONS 

An objective reading of the veterinary medical literature reveals a complex situation with respect to the longterm health risks and benefits associated with spay/neuter in dogs. The evidence shows that spay/neuter correlates with both positive AND adverse health effects in dogs. It also suggests how much we really do not yet understand about this subject. On balance, it appears that no compelling case can be made for neutering most male dogs to prevent future health problems, especially immature male dogs. The number of health problems associated with neutering may exceed the associated health benefits in most cases. For female dogs, the situation is more complex. The number of health benefits associated with spaying may exceed the associated health problems in many (not all) cases. On balance, whether spaying improves the odds of overall good health or degrades them probably depends on the age of the dog and the relative risk of various diseases in the different breeds. The traditional spay/neuter age of six months as well as the modern practice of pediatric spay/neuter appear to predispose dogs to health risks that could otherwise be avoided by waiting until the dog is physically mature, or (perhaps in the case of many male dogs) foregoing it altogether unless medically necessary. The balance of long-term health risks and benefits of spay/neuter will vary from one dog to the next. Across-the-board recommendations for all dogs do not appear to be supportable from findings in the veterinary medical literature.

 

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