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Determining the Best Age at Which to Spay or Neuter a Dog


Effects of gonadectomy on health


Neoplasia, or cancer, is abnormal growth of tissue. Benign tumors tend to stay in one location and cause disease by altering the single tissue involved and compressing tissue around it. Malignant tumors tend to spread in the area from which they arise and to spread to distant tissues, causing widespread disease. Virtually all tumors are more common in aged than in young animals, with average reported age at time of diagnosis of about 10 years. For the tumor types described below, exact cause-and-effect relationship between gonadectomy and development of tumors is unknown.


Mammary neoplasia, or breast cancer, is a very common disorder of female dogs, with a reported incidence of 3.4%; this is most common tumor type in female dogs. Of female dogs with mammary tumors, 50.9% have malignant tumors. Risk factors for mammary neoplasia in female dogs include age, breed (Table 1), and sexually intact status. Multiple studies have documented that spaying bitches when young greatly decreases their risk of developing mammary neoplasia when aged. Compared with bitches left intact, those spayed before puberty have a 0.5% risk, those spayed after one estrous cycle have an 8.0% risk, and dogs spayed after two estrous cycles have a 26.0% risk of developing mammary neoplasia later in life. Overall, unspayed bitches have a seven times greater risk of developing mammary neoplasia than do those that are spayed. While the benefit of spaying decreases with each estrous cycle, some benefit has been demonstrated in bitches even up to 9 years of age. The exact cause-and-effect relationship between intact status and development of mammary neoplasia in female dogs has not been identified. The genetic and hormonal causes of breast cancer identified in women have not been consistently identified in female dogs despite extensive research.


Prostatic cancer in dogs is uncommon, with a reported incidence of 0.2 to 0.6%. Prostatic adenocarcinoma is a highly malignant tumor that cannot be cured medically or surgically. A 2.4 to 4.3 times increase in incidence in prostatic neoplasia with castration has been demonstrated, with that information verified in multiple studies.


Testicular neoplasia is a very common tumor in dogs, with a reported incidence of 0.9%. Unlike in humans, testicular tumors occur late in life in dogs, are readily diagnosed, and are rarely malignant. Ovarian and uterine tumors are very uncommon in bitches.


Several tumors of non-reproductive tissues have been reported to be increased in incidence after gonadectomy. Transitional cell carcinoma, a malignant tumor of the urinary tract, was reported in two studies to occur 2 to 4 times more frequently in spayed or castrated dogs than in intact female or male dogs. Exact incidence is not reported; estimated incidence is less than 1.0%. A breed predisposition exists (Table 1). Surgical removal of transitional cell carcinoma may or may not be possible, depending on site of the primary tumor.


Osteosarcoma is a low incidence (0.2%), highly malignant tumor of bone. It is reported to be more common in large breed dogs with some specific breeds predisposed (Table 1). Two studies have documented a 1.3 to 2.0 times increased incidence of osteosarcoma with gonadectomy. However, one study evaluated solely Rottweilers, a breed with a reported genetic predisposition. Treatment often includes limb amputation and radiation or chemotherapy.


Hemangiosarcoma is a malignant tumor of vascular tissue, including the heart, major blood vessels, and spleen. Large breeds in general are at increased risk with some breeds specifically predisposed (Table 1). Two studies have documented increased incidence, from 2.2 to 5 times, in gonadectomized males and females compared to intact animals. Overall incidence of hemangiosarcoma is low, at 0.2%. Surgical removal is the treatment of choice, if possible.


Orthopedic abnormalities
Long bones grow from growth plates on either end. The growth plates close after exposure to estrogen and testosterone, explaining why growth in height is largely completed after puberty. In bitches and dogs, removal of the gonads before puberty slows closure of the growth plates, leading to a statistically significant but not overtly obvious increase in height. There is no evidence that after gonadectomy some growth plates will close on time and some late, however most studies have only examined long bones of the forelimb. No studies have demonstrated increased incidence in fractures or other abnormalities of the growth plates associated with age at time of spay or castration.


Hip dysplasia is abnormal formation of the hip joint with associated development of arthritis. Genetic, hormonal, and environmental factors, including diet, are involved (Table 1). In the one study describing increased incidence of hip dysplasia in female or male dogs spayed or castrated before 5 months of age, it is not clear that the diagnosis of hip dysplasia was made by a veterinarian in all cases.


The paired cruciate ligaments form a cross within the knee (stifle) joint. The cranial cruciate ligament (CCL) undergoes tearing or complete rupture when the stifle is stressed from the side, especially if the animal twists while bearing weight on that limb. CCL injury is very common, with reported incidence of 1.8%. Large breed dogs are generally at risk, with some breeds predisposed (Table 1). Overweight female and male dogs also may be at increased risk. It has been demonstrated that CCL injury is more common in spayed or castrated animals than in intact animals. The basis may be hormonal, as it has been demonstrated that CCL injury in humans is more common in women than in men with incidence varying with stage of the menstrual cycle. A very recent study documented change in anatomy of the stifle joint of female and male dogs with CCL injury with gonadectomy prior to 6 months of age; further research is pending. CCL injury is treated with surgery and rehabilitation; treatment is costly and recovery protracted.


Obesity is very common in dogs, with reported incidence of 2.8% in the general dog population; incidences of 34% of castrated male dogs and 38% of spayed female dogs were reported in one study. Multiple risk factors exist, including breed (Table 1), age, and body condition and age of the owner. A very commonly reported risk factor for development of obesity is gonadectomy. In cats, it has been demonstrated that gonadectomy causes a decrease in metabolic rate. There are no reports documenting metabolic rate in female or male dogs relative to gonadectomy. Obesity is itself a risk factor for some forms of cancer, CCL injury, diabetes mellitus, and decreased life span. Obesity is controllable with appropriate diet and exercise.


Urinary incontinence
A very common form of urinary incontinence, formerly termed estrogen-responsive urinary incontinence and now more commonly called urethral sphincter mechanism incompetence, occurs in spayed female dogs. Urine leaks from the spayed female dogs when they are relaxed and so most often is seen by the owners as wet spots where the dog sleeps. Reported incidence ranges from 4.9 to 20.0%, with female dogs weighing more than 44 pounds and some specific breeds predisposed (Table 1). While multiple studies have documented correlation between gonadectomy and occurrence of this disorder, only one has demonstrated a correlation between incidence and age at gonadectomy. In that study, it was demonstrated that spaying before 3 months of age was significantly more likely to be associated with eventual occurrence of urinary incontinence in a given female dog than was spaying later. Urethral sphincter mechanism incompetence is easily controlled medically in most female dogs.


Pyometra is uterine infection overlying age-related change in the uterine lining. Incidence increases with age; 23 to 24% of dogs developed pyometra by 10 years of age in one Swedish study. Specific breeds are at increased risk (Table 1). This very common disorder of aged intact bitches is treated surgically.


Benign prostatic hypertrophy/prostatitis
Benign prostatic hypertrophy (BPH) is age-related change in prostate size. By 6 years of age, 75 to 80% of intact male dogs will have evidence of BPH; by 9 years of age, 95 to 100% of intact male dogs will have evidence of BPH. The increased size of the prostate is associated with increased blood supply. The most common clinical signs are dripping of bloody fluid from the prepuce and blood in the semen. Development of BPH predisposes the dog to prostate infection (prostatitis). Medical therapy for BPH can be used to control clinical signs but surgical therapy (castration) is curative.


Diabetes mellitus
Only one study has demonstrated a possible increased incidence of diabetes mellitus in dogs associated with gonadectomy. That study did not consider the effect of obesity, a known risk factor for diabetes mellitus.


Two studies have demonstrated increased incidence of hypothyroidism in female and male dogs after gonadectomy. Genetic factors also are involved (Table 1). Cause-and-effect has not been described, nor has a specific numerical factor for increased incidence been reported.


Life span
Several studies have demonstrated that spayed and castrated female and male dogs live longer than do intact bitches or dogs. Cause-and-effect has not been described. It is possible that gonadectomized dogs are less likely to show risky behaviors or that owners who have invested in animals by presenting them for spay or castration continue to present them for consistent veterinary care.




So how do you reconcile all this information in helping make decisions for individual animals? Considerations must include evaluation of incidence of various disorders, breed predisposition, and health significance of the various disorders (Table 2 and Table 3).


For female dogs, the high incidence and high percentage of malignancy of mammary neoplasia, and the significant effect of spaying on decreasing its incidence make ovariohysterectomy prior to the first heat the best recommendation for non-breeding animals. The demonstrated increased incidence of urinary incontinence in bitches spayed before 3 months of age and possible effect of CCL injury in bitches spayed before 6 months of age suggest that spaying bitches after 6 months of age but before their first heat is most beneficial. For bitches of breeds predisposed by ovariohysterectomy to highly malignant tumors and for breeding animals, spaying at a later age may be more beneficial.


For male dogs, castration decreases incidence of disorders with little health significance and may increase incidence of disorders of much greater health significance. For non-breeding animals, evaluation of breed and subsequent predispositions to disorders by gonadectomy should guide when and if castration is recommended.


As dog breeders, you are a source of information for people seeking a dog for companionship, to show or work as a hobby, or to grow up with their children. As veterinarians, we are one of the guardians of safety and good health for all animals in our society. It behooves all of us to thoughtfully consider why we recommend spay or castration for dogs, to ensure we are not putting our own convenience above their good health. For every individual bitch or dog, careful consideration of their breed, age, lifestyle, and suitability as a breeding animal must be a part of the decision as to when or if they should undergo gonadectomy.




Table 1. Breeds predisposed to various disorders


Mammary neoplasia Boxer, Brittany, Cocker Spaniel, Dachshund, English Setter, English Springer Spaniel, German Shepherd Dog, Maltese, Miniature Poodle, Pointer, Toy Poodle, Yorkshire Terrier
Transitional cell carcinoma Airedale Terrier, Beagle, Collie, Scottish Terrier, Shetland Sheepdog, West Highland White Terrier, and Wire Fox Terrier
Osteosarcoma Doberman Pinscher, Great Dane, Irish Setter, Irish Wolfhound, Rottweiler, Saint Bernard
Hemangiosarcoma Boxer, English Setter, German Shepherd Dog, Golden Retriever, Great Dane, Labrador Retriever, Pointer, Poodle, Siberian Husky
Hip dysplasia Chesapeake Bay Retriever, English Setter, German Shepherd Dog, Golden Retriever, Labrador Retriever, Samoyed, Saint Bernard
Cranial cruciate ligament injury Akita, American Staffordshire Terrier, Chesapeake Bay Retriever, German Shepherd Dog, Golden Retriever, Labrador Retriever, Mastiff, Neapolitan Mastiff, Newfoundland, Poodle, Rottweiler, Saint Bernard
Obesity Beagle, Cairn Terrier, Cavalier King Charles Spaniel, Cocker Spaniel, Dachshund, Labrador Retriever
Urinary incontinence Boxer, Doberman Pinscher, Giant Schnauzer, Irish Setter, Old English Sheepdog, Rottweiler, Springer Spaniel, Weimeraner
Pyometra Bernese Mountain Dog, Cavalier King Charles Spaniel, Chow Chow, Collie, English Cocker Spaniel, Golden Retriever, Rottweiler, Saint Bernard
Diabetes mellitus Miniature Poodle, Miniature Schnauzer, Pug, Samoyed, Toy Poodle
Hypothyroidism Airedale Terrier, Cocker Spaniel, Dachshund, Doberman Pinscher, Golden Retriever, Irish Setter, Miniature Schnauzer, Pomeranian, Shetland Sheepdog


Table 2. Conditions associated with ovariohysterectomy (spay)

Mammary neoplasia High High Decreased
Ovarian and uterine neoplasia Low Low Decreased
Pyometra High High Decreased
Transitional cell carcinoma Low High Increased
Osteosarcoma Low High Increased
Hemangiosarcoma Low High Increased
CCL injury High High Increased
Obesity High Moderate Increased
Urinary incontinence High Low Increased
Diabetes mellitus High Low Increased
Hypothyroidism High Low Increased


Table 3. Conditions associated with castration

Testicular neoplasia High Low Decreased
Benign prostatic hypertrophy High Low Decreased
Prostatic neoplasia Low High Increased
Transitional cell carcinoma Low High Increased
Osteosarcoma Low High Increased
Hemangiosarcoma Low High Increased
CCL injury High High Increased
Obesity High Moderate Increased
Diabetes mellitus High Low Increased
Hypothyroidism High Low Increased



Used with permission from the AKC Canine Health Foundation, a nonprofit organization dedicated to advancing the health of all dogs and their owners by funding sound scientific research and supporting the dissemination of health information to prevent, treat, and cure canine disease. 


Image: Jeff / via Flickr







Comments  9

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  • Disputing a statement
    10/27/2013 03:04pm

    I simply HAVE to dispute a statement made in this article. Where in the world did you find data to support saying "However, in the United States, virtually all bitches and dogs are rendered sterile by surgery at some point in their life."??? That's preposterous!

  • 02/26/2014 01:05am

    Excellent observation. Here is the South, MOST dogs and cats are NOT EVER surgically altered; there are too many antiquated cultural beliefs and mistrust of real science and medicine as well as the fact a large or majority of pets live outdoors and the owners are not recognizing many or the behaviors, and some have a close identification with their pet's sexuality and their own. Regardless of the reason, I have no earthly idea where the author came up with the idea that most or virtually all pets in the USA are sterilized at some point. Were that the case, we would not be killing over 4 million adoptable pets each year for lack of available homes, and dog bites, HBC cases, dog on dog trauma, cat bites abscesses, FIV, and countless other daily avoidable negative interactions would be drastically reduced. And how many small and medium dogs can wait till 6 months of age to spay and STILL "beat" the first heat cycle?? Large breeds, maybe so, but most smaller ones will start a first estrus cycle at 5-6 months, and many veterinarians (and owners) hesitate to spay a female while she is in active estrus, anyway. This makes me wonder about the veracity of the rest of the article.

  • 02/27/2014 02:57pm

    Pat, when I received notice of your comment, it reminded me of this post. Since it was reprinted from another source, I figured the author likely never saw our comments and felt I just HAD to follow up. I tracked the doctor down at the U of MN and emailed her. She replied:
    "Thank you for bringing this to my attention. Research has been done since this was written showing variation in spay and castration rates across the country. It is less than I stated and more than was stated by those responding.
    Dr. Peggy Root"
    How's THAT for clearing things up statistically? hahaha

  • 12/23/2015 09:30pm

    Many dogs in the US are neutered due to misinformation from vets. The information on this site is untrue, old and scientifically ridiculous.

    All of he awful things that happen to humans if neutered happen to all mammals including dogs.

    There are many scientific articles which show how bad the effects of spay/neuter on dogs.

    As a breeder of 35 years and a Phd I recommend to my puppy buyers that they NOT spay or neuter their dogs but instead use low tech birth control:
    fences and leashes to kept their pets from conceiving unwanted littes.

    Dog population in the US is at 0%. .All the dogs in the shelters are those given up or let loose by people who acquired them without first considering how they were going to take of them. and they didn't get those dogs from careful and concerned breeders like me who are fanatic about where their puppies go and the conditions in the homes they go to. I never ship puppies.
    I do lengthy interviews with my owners and I have a condition on my contract that states if the family cannot keep the dog, it comes back to me. So if the unthinkable happens no one has to worry about who will take the dog.

    I started Poodle rescue in my area and it is now a flourishing organization.
    Most purebred breeds have rescue operations.

    All the shelters in the Northeast have to import dogs from the south or out of the country in order to have any for adoption.I haven't seen dogs running loose in my area for years. Most people here take care of their pets or are forced to give them up. While all is not perfect education seems to have made a big impact on the pet owning public.

    It is pet shops and puppy mills and the new scam rescue operations that are to blame for dogs that go to inappropriate owners. We, in poodle rescue.see these dogs all the time.Scam rescue operations which go south and pick up dogs from overloaded shelters and sell them up here out of trucks have grown in a multi million dollar business.

    As a well known dog breeder/trainer I get calls frequently from people who have paid $400 or more to get a sick dog or one who had developed behavioral problems due to poor training and/or socialization. These very expensive "free" dogs cause lots of problems to people who have been made to feel they need to "rescue" a dog rather than get one from a reputable breeder. These dogs, too wind up in our local shelters.

  • Where is evidence?
    10/01/2014 05:18pm

    Where is the evidence that spaying a dog reduces her risk of cancer? Everyone throws numbers around but no one provides a reference study. It's almost like an urban legend that gets perpetuated. I found this metastudy that looked at all other related studies and it says that when you remove the studies that had a bias, there is little if any correlation:


  • 12/24/2015 05:01am

    The National Golden Retriever Club did a huge study on cancer in their breed which the second largest in the US. Goldens get hemangiosarcoma at a rate much higher than any other breed so they did a survey which included
    thousands of dogs. It was published in a recognized scientific journal and was peer reviewed.

    I showed that spayed bitches get cancer at 5 times the rate of unspayed bitches and dogs get cancer at 2 1/2 times the rate. That is huge. There are also many small studies which demonstrate the same effect.

    Dogs also get hip dysplasia, blow out their ACLs and have other bone disorders when neutered. Lack of circulating hormones isa prime cause of
    bones degradation. Just think about what happens to us when we go thru menopause.
    Vet schools are centuries behind when it comes to birth control in animals.
    We know the answers because we use them on people. Micro surgery to tie tubes is routine in people because it is safer, less painful etc. and doesn't upset the natural balance of hormones in the body.
    Europe doesn't not spay dogs. And breeders like me don't either.

    Spaying takes away one third of your dog's life. Why do my dogs live to be
    on average 18 years old. I have had some live to 20. And my dogs almost never get sick. Even dogs who have very active lives in agility and hunting almost never get injured.

    There are literally hundreds of studies on this topic. I would recommend Dr Christine Zinc's article,"EARLY SPAY-NEUTER CONSIDERATIONS for theCANINE ATHLETE" to start. By the way she is the head of Pathology at John's Hopkins Hospital for 30 years and far more knowledgeable than most vets as she is a PhD as well as a DVM.

    AS PhD myself I did a lot of research on neutering dogs and the only conclusion I can come up with is that vets make statements that there is no proof for and that there is scientific proof for the opposite. After taking some of the classes that vets have to take to renew their licenses,
    I know that many do procedures just to make money. After reading many papers on on shots, I took a class given by Dr. Jean Dodds and Dr. Ron Schultz( the acknowledged authority on canine/feline immunity) They stated it is unnecessary to give repeated shots for canine viruses. Puppy shots don't need to be given again after 20 weeks. After the lecture many of the vets said that since shots were a substantial percentage of their profit, they would continue to give them. When was the last time your doctor advised you to have a polio or other childhood shot? Never. Because they last for life if it is a virus.
    Spaying and neutering dogs is not the best thing for your beloved pet. You wouldn't do it to your child. so why do it to your dog. There are new and better ways to keep your dogs (or cats) from irresponsible reproduction, some that you already do just to keep them safe.

  • Completely misleading!!
    04/11/2016 04:15am

    How in the world can you folks at this site state such rampant lies? It has been proven over and over again that desexing before growth plates close naturally is extremely detrimental to a dogs health and life span.

    Results of a study shared by the Golden Retriever Club of America and the Canine Health Foundation:
    1. A spay/neuter before puberity have a 2-5 times greater risk for hemangio sarcoma.
    2. Osteosarcoma is significantly more common is spayed/neutered dogs
    3. Mammory cancer is increases in females left intact. But, since most all mammory cancers are benign, I'll take my chances with mammory cancer to ward off hemangio or osteosarcoma any day of the week.
    4. Uninary incontinence is greatly increased with early spay
    5. Hypothyroidism in spay/neutered dogs increases 80% males and 60% in females
    6. Decreased lifespan in spayed and neutered dogs
    7. Over growth in male and females that have been desexed early.

  • Risks of castration
    10/03/2016 11:51pm

    The statement that castration can increase health risks for dogs is very misleading. The health risks are still very, very, small. Pet dogs should always by neutered or spayed to prevent backyard litters. Pet owners are, for the most part, not prepared for unplanned litters and the total responsibility that comes with them. Not to mention finding homes for up to 12 puppies.
    Male dogs that are not castrated will wander, fight, and be a danger to the community as they try to stake out a territory. As a dog trainer I advise all my clients to spay or neuter all dogs over 6 months old. The way this article is written could give owners an excuse to not spay or neuter as they may believe that it is dangerous for their dogs health.

  • 11/25/2016 05:27am

    I have had dogs for many years, and always get them spayed or neutered, Both my King Charles were long lived Bertie died of Heart Problems at 18 plus years and Indi a female died of Heart problems at 16 years, ( Heart decease is extremely common with King Charles Cavaliers) Sophie my small female Shitzu Maltese lived until 18 and a half, and died of old age.
    They were all healthy fit dogs.
    I now have 2 more King Charles Spaniels male and Female both have been Spade and neutered, and a 17 month Small Shitzu Maltese whom will be spade soon.
    I strongly believe my dogs were healthy and long lived because of it.
    I am English and have always had long lived Healthy Happy Dogs.

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