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long-term health risks, benefits of sPAY/NEUTERing

this article was writtern by laura j. sanborn, m.s. may 14, 2007.

we agree whole heartedly with this article.  we believe that if yOU aren't having any behavioral problems with your dog that they shouldn't be spayed before the age of 2 years for the female,  & neutered if at all for the male.

precis

at some point, most of us with an interest in dogs will have to consider whether or not to spay / neuter our pet.  tradition holds that the benefits of doing so at an early age outweigh the risks.  often, tradition holds sway in the decision- making process even after countervailing evidence has accumulated.  ms sanborn has reviewed the veterinary medical literature in an exhaustive & scholarly treatise, attempting to unravel the complexities of the subject.  more than 50 peer-reviewed papers were examined to assess the health impacts of spay/neuter in female & male dogs, respectively.  one cannot ignore the findings of increased risk from osteosarcoma, hemangiosarcoma, hypothyroidism, & other less frequently occurring diseases associated with neutering male dogs.  it would be irresponsible of the veterinary profession & the pet owning community to fail to weigh the relative costs & benefits of neutering on the  animal's health & well-being.  the decision for females may be more complex, further emphasizing the need for individualized veterinary medical decisions, not standard operating procedures for all patients.  no sweeping generalizations are implied in this review.  rather, the author asks us to consider all the health & disease information available as individual animals are evaluated.  then, the best decisions should be made accounting for gender, age, breed, & even the specific conditions under which the long-term care, housing & training of the animal will occur.  this important review will help veterinary medical care providers as well as pet owners make informed decisions.  who could ask for more??

LARRY  S.  KATZ, PHD

Associate Professor and Chair

Animal Sciences

Rutgers University

New Brunswick, NJ 08901

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 S/N, health risks are often not mentioned.  Some risks are mentioned, but the most severe risks usually are not.

This article is an attempt to summarize the long-term health risks & benefits associated with S/N in dogs that can be found in the Veterinary Medical Literature.  This article will NOT discuss the impact of S/N on population control, or the impact on behavior.  Nearly all of the health risks & 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. 

An objective reading of the Veterinary Medical Literature reveals a complex situation with respect to the long-term health risks & benefits associated with S/N in dogs.  The evidence shows that S/N has positive & negative health effects in dogs.  It also suggests how much we really do NOT yet understand about this subject.

For male dogs it appears that no compelling case can be made for Neutering most males especially immature males, 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 1 year of age, significantly increases the risk of Osteosarcoma (bone cancer); this is a common Cancer in medium & large breeds with a poor prognosis. Increases the risk of Cardiac Hemangiosarcoma by a factor of 1.6. Triples the risk of Hypothyroidism.  Increases the risk of Progressive 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.  Whether Spaying improves the odds of overall good health or degrades them depends on the AGE of the female dog & 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 (<or=0.5%) from Uterine, Cervical, & Ovarian Tumors.

On the negative side, Spaying female dogs. If done before 1 year of age, significantly increases the risk of Osteosarcoma (bone cancer); this is a common Cancer in Larger breeds with a poor prognosis. Increases the risk of Splenic Hemangiosarcoma by a factor of 2.2 & Cardiac Hemangiosarcoma by a factor of >5; this is a common Cancer & 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 problem 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. Increases the risk of Recessed Vulva, Vaginal Dermatitis, & Vaginitis, especially if done 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 & 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 & benefits associated of S/N in dogs.

The traditional S/N age of six months as well as the modern practice of pediatric S/N 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 & benefits of S/N will vary from one dog to the next.  Breed, age, & gender are variables that must be taken into consideration in conjunction with non-medical factors for each individual dog.  Across-the-board recommendations for all pet dogs DO NOT appear to be supportable from findings in the Veterinary Medical Literature.

All surgery incurs some risk of complications, including adverse reactions to Anesthesia, Hemorrhage, Inflammation, Infection, ETC.  Complications include only immediate & 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 & Total Complications were 6.3%, 14.1% & 20.6%, respectively as a result of Spaying female dogs.  Other studies found a rate of Total Complications from Spaying of 17.7% & 23% for Neutering.  A study of Canadian Veterinary private practitioners found complication rates of 22% & 19% for Spaying & Neutering, respectively.  The death rate due to Complications from S/N is low, at around 0.1%.

Much of the S/N 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 Europe & one in America.  Both studies found that Neutered male dogs have a 4 times HIGHER risk of Prostate Cancer than intact dogs.  Based on their results, the researchers suggest a cause-&-effect relationship: “this suggests that castration does NOT initiate the development of Prostatic Carcinoma in the dog, but does favor tumor progression” & also Our study found that most canine Prostate Cancers are of Ductal/Urothelial in origin!  The relatively low incidence of Prostate Cancer in intact dogs may suggest that Testicular Hormones are in fact protective against Ductal/Urothelial Prostatic Carcinma, or may have indirect effects on Cancer development by changing the environment in the Prostate.  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 studies, it’s hard to see that the risk of Prostate Cancer should factor heavily into most Neutering decisions.

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%.  However, the prognosis for treating Testicular Tumors is very good owing to a low rate of Metastasis, so Testicular Cancer is an uncommon cause of death in intact dogs.  For example, in a Purdue University breed health survey of Golden Retrievers, deaths due to Testicular Cancer were sufficiently infrequent that they did NOT appear on the list of significant causes of “Years of Potential Life Lost for Veterinary Confirmed Cause of Death” even though 40% of G.R. males were intact.  Furthermore, the G.R.s 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 dogs.  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 Testicles & it is also more difficult to detect Tumors in Undescended Testicles by routine physical examination.

A multi-breed case-control study of the risk factors for Osteosarcoma (Bone Cancer) found that spay/neutered dogs had twice the risk of developing Osteosarcoma as did intact dogs.  This risk was further studied in Rottweilers, a breed with a restively high risk of Osteosarcoma.  This retrospective cohort study broke the risk down by age at S/N, & found that the elevated risk of Osteosarcoma is associated with S/N of young dogs.  Rotts. S/N before one year of age were 3.8 (males) 3.1 (females) times more likely to develop Osteosarcoma than intact dogs.  Indeed, the combination of breed risk & early S/N meant that Rotts. S/N before one year of age had a 28.4% (males) 25.1% (FEMALES) RISK OF DEVELOPING Osteosarcoma.  These results are consistent with the earlier multi-breed study but have an advantage of assessing risk as a function of age at time of S/N.  A logical conclusion derived from combining the findings of these two studies is that S/N of dogs before 1 year of age is associated with a significantly increased risk of Osteosarcoma.  The researchers suggest a cause-&-effect relationship, as sex hormones are known to influence the maintenance of skeletal structure & mass, & also because their findings showed an inverse relationship between time of exposure to sex hormones & risk of Osteosarcoma.  The risk of Osteosarcoma increases with increasing breed size & especially height.  It’s a common cause of death in medium/large, large, & giant breeds.  Osteosarcoma is the third most common cause of death in Golden Retrievers & is even more common in larger breeds.  Given the poor prognosis of Osteosarcoma & its frequency in many breeds, S/N of immature dogs in the medium/large, large, & giant breeds is apparently associated with a significant & elevated risk of death due to Osteosarcoma.  

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 Norway 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 Metastasis.  Mammary Tumors in dogs have been found to have estrogen receptors, & the published research 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 indicated that:

Age at Spaying                                                      Odds Ratio

<or= 29 months                                                           0.06

>or= 30months                                                            0.40 (not statistically significant at the P<0.05 level)

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” & 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 G.R.s were spayed, at a median age of 3.4 years.  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 G.R.s.  It would rank higher still if more female G.R.s had been kept intact up to 30 months of age. 

Boxers, Cocker Spaniels, English Springer Spaniels, & Dachshunds are breeds at high risk of Mammary Tumors.  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 age.  There are some indications that purebred dogs may be at higher risk than mixed breed dogs, & purebred dogs with high inbreeding coefficients may be at higher risk than those with low inbreeding coefficients.  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), & the fewer estrus cycles experienced at least up to 30 months of age, which  lowered the risk.

Uterine/Cervical Tumors are rare in dogs, constituting just 0.3% of tumors in dogs.  Spaying will remove the risk of Ovarian Tumors, but the risk is only 0.5%.  While Spaying will remove the risk of Reproductive Tract Tumors, it’s unlikely that surgery can be justified to prevent the risks of Uterine, Cervical, & Ovarian Cancers as the risks are so low.

An age-matched retrospective study found that S/N dogs were two times more likely to develop Lower Urinary Tract Tumors (Bladder or Urethra) compared to intact dogs.  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 S/N decisions. Airedales, Beagles, & Scottish Terriers are at elevated risk for Urinary Tract Cancer while German Shepherds have a lower than average risk. 

Hemangiosarcoma is a Common Cancer in dogs.  It’s 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 Terriers, Boston Terriers, Bulldogs, & German Shepherd Dogs.  In an aged-matched case controlled study, Spayed females were found to have a 2.2 times higher risk of Splenic Hemanquiosarcoma compared to intact females.  A retrospective study of Cardiac Hemangiosarcoma risk factors found a >5 times greater risk in Spayed female dogs compared to intact females & a 1.6 times higher risk in Neutered male dogs compared to intact males.  The authors suggest a protective effect of sex hormones against Hemanqiosarcoma, especially in females.  In breeds where Hermangiosarcoma is a cause of death, the increased risk associated with S/N is likely one that should factor into decisions on whether or when to sterilize a dog.

S/N in dogs was found to be correlated with a three fold increased risk of Hypothyroidism compared to intact dogs.  The researchers suggest a cause-&-effect relationship: They wrote: “More important [than the mild impact on Thyroid Function] in the association between S/N & Hypothyroidism may be the sex hormones on the immune system.  Castration increases the severity of Autoimmune Thyroiditis in mice” which may explain the link between S/N & Hypothyroidism in dogs.  Hypothyroidism in dogs causes Obesity, Lethargy, Hair Loss, & Reproductive Abnormalities.  The risk of Hypothyroidism in breed health surveys was found to be 1 in 4 in Golden Retrievers, 1 in 3 in Akitas, & 1 in 13 in Great Danes.

Owing to changes in metabolism, S/N 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 females.  Another study found that S/N dogs were 1.6 (females) or 3 (males) times more likely to be Obese than intact dogs, & 1.2 (females) or 1.5 (males) times more likely to be overweight than intact dogs.  A survey of Veterinary Practices in the UK found that 21% of dogs were Obese.  Being Obese &/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, & Oral Disease.  Obese dogs are more likely to be diagnosed with Hypothyroidism, Diabetes Mellitus, Pancreatitis, Ruptured Cruciate Ligament, & Neoplasia (Tumors).

Some data indicate that Neutering doubles the risk of Diabetes in male dogs, but other data showed no significant change in Diabetes risk.  In the same studies, no association was found between Spaying & the risk of Diabetes.

A retrospective cohort study of adverse vaccine reactions in dogs was conducted, which included Allergic Reactions, Hives, Anaphylaxis, Cardiac Arrest, Cardiovascular Shock, & Sudden Death.  Adverse reactions were 30% more likely in Spayed females than intact females, & 27% more likely in Neutered males than intact males.

The investigators discuss possible cause-&-effect mechanisms for this finding, including the roles that sex hormones play in the body’s ability to mount an immune response to vaccination.  Toy breeds & smaller breeds are at elevated risk of adverse vaccine reactions, as are Boxers, English Bulldogs, Lhasa Apsos, Weimaraners, Amaerican Eskimo Dogs, Golden Retrievers, Basset Hounds, Welsh Corgis, Siberian Huskies, Great Danes, Labrador Retrievers, Doberman Pinschers, American Pit Bull Terriers, & Akitas.  Mixed breed dogs were found to be at lower risk, & the authors suggest genetic Hetereogeneity (Hybrid Vigor) as the cause.

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% for Spayed females compared to only 0.3%in intact females.  Urinary Incontinence is so strongly linked to Spaying that it’s commonly called “Spay Incontinence” & is caused by Urethral Sphincter Incometence, though the biological mechanism is unknown.  Most (but NOT all) cases of Urinary Incontinence respond to medical treatment, & in many cases this treatment needs to be continued for the duration of the dog’s life.  A retrospective study found that persistent or Recurring Urinary Tract (bladder) Infections (UTIs) were 3-4 times more likely in Spayed female dogs than in intact females.  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.  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, & UTIs.  The risk is higher for females spayed before puberty.

Pet insurance data in Sweden (where Spaying is very uncommon) found that 23% of all female dogs developed Pyometra before 10 years of age.  Bernese Mountain dogs, Rottweilers, Rough-Haired Collies, Cavalier King Charles Spaniels & Golden Retrievers were found to be high risk breeds.  Female dogs that have not whelped puppies are at elevated risk for Pyometra.  Rarely, Spayed female dogs can develop “Stump Pyometra” related to incomplete removal of the Uterus.  Pyometra can usually be treated surgically or medically, but 4% of Pyometra cases led to death.  Combined with the incidence of Pyometra, this suggests that about 1% of intact female dogs will die from Pyometra.

Male dogs are twice as likely to develop Perianal Fistulas as females, & Spay/Neutered dogs have a decreased risk compared to intact dogs.  German Shepherd Dogs & Irish Setters are more likely to develop Perianal Fistulas than other dogs.  The incidence of Benign Prostatic Hypertrophy (BPH, Enlarged Prostate) increases with age in intact male dogs, & occurs in more that 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, & Cysts, which can sometimes have serious consequences.

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), suggesting an increased risk of Hip Dysplasia.  Spaying was also found to cause a net loss of bone mass in the spine.  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 S/N that is done after some growth plates have closed but before other growth plates have closed might result in a dog with unnatural proportions, possibly impacting performance & long term durability of the joints.  Spay/Neuter is associated with a two fold increased risk of Cranial Cruciate Ligament Rupture.  Perhaps this is associated with the increased risk of Obesity.  Spay/Neuter before 5 ½ months of age is associated with a 70% increased aged-adjusted risk of Hip Dysplasia compared to dogs S/N after 5 ½ months of age, though there were some indications that the former may have had a lower severity manifestation of the disease.  The researchers suggest “it’s 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, S/N dogs were significantly more likely to suffer Hip Dysplasia as well as “any Musculoskeletal Disorder”, compared to intact dogs, however possible confounding factors were NOT controlled for, such as the possibility that some dogs might have been S/N 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.  Compared to intact dogs, S/N dogs were found to have a 3.1 times higher risk of Patellar Luxation.

Neutered male dogs & Spayed female dogs are at increased risk of progressing from mild to severe Geriatric Cognitive Impairment compared to intact male dogs.  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, & changes in the sleep-wake cycle.  The investigators state “This finding is in line with current research on the Neuro-protective roles of Testosterone & Estrogen at the cellular level & 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. 

An objective reading of the Veterinary Medical Literature reveals a complex situation with respect to the long-term health risks & benefits associated with S/N in dogs.  The evidence shows that S/N correlates with both positive & 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 god health or degrades them probably depends on the age of the dog & the relative risk of various diseases in the different breeds.  The traditional S/N age of six months as well as the modern practice of pediatric S/N 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 & benefits of S/N will vary from one dog to the next.  Breed, age & gender are variables that must be taken into consideration in conjunction with non-medical factors for each individual dog.  Across-the-board recommendations for all dogs do Not appear to be supportable from findings in the Veterinary Medical Literature.

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