Pros & Cons of Spaying/Neutering Dogs – Part I of Gia’s Research

First and foremost, I would like to start with this disclaimer: if your dog, whether male or female, is not neutered, you have the serious responsibility of making sure that your dog is properly supervised and unable to become pregnant or to impregnate another dog. This cannot be taken lightly. Unless you are a responsible professional breeder, you should not allow any chance whatsoever of your dog mating with another dog.

That being said, let’s get to the meat of the matter. I have been working to gather information on spaying and neutering (surgical operations that leave animals sterile, or in other words, unable to reproduce).  “Spay” refers to the operation for female dogs, and “neuter” is typically used in reference to male dogs, though my understanding is that it can also be used for females, too.

Since I tend to become overwhelmed with a ton of research at once and feel like I’m swimming in random facts, I am attempting to use this space to assemble the best parts of what I’ve gathered so far in a [hopefully] somewhat cohesive manner. I intend for my blog to be a learning platform for everyone (especially me!), so please feel free to ask questions and share differing views.

All in-text emphasis are mine. Here we go!

“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.” – Margaret Root-Kustritz, DVM, PhD

Why do veterinarians [in the United States] usually neuter dogs between 6-9 months?

In Margaret Root-Kustritz’s paper, “Determining the Best Age at Which to Spay or Neuter a Dog: An Evidence-Based Analysis,” she wrote that in the United States, most veterinarians “recommend bitches and dogs be spayed or castrated between 6 and 9 months of age.” I would have assumed that there are some sort of scientific beliefs, even if outdated, to back up these suggestions.

However, Root-Kustritz writes that “This is not based in science,” and that there has never been “a large-scale study in which bitches and dogs underwent gonadectomy at various ages and were tracked throughout life to determine what abnormalities developed relative to age at gonadectomy.” Wow! I’d think that it would be very important to conduct a large-scale study in order to explore the question of what would be the [scientifically based] ideal age. So if it’s not currently based on science, why 6-9 months, then?

Per Root-Kustritz,

“It is thought that the current age recommendation arose after the World War II, when increasing affluence of American families first permitted them to treat animals as household pets and were, therefore, more interested in controlling manifestations of reproductive hormone secretion and very interested in making sure the animal survived surgery. Anesthetic and surgical techniques available at that time necessitated the animal be at least 6 months of age.”

I find this [as the main reason why dogs are neutered at such ages] to be extremely interesting, but ultimately disturbing. We need some studies that look at neutered dogs’ development throughout their lives and how that was influenced by the age at which the dogs were neutered. Root-Kustritz’s paper was dated Spring 2008. If anyone has more recent/differing information to share, please contact me via the comment section below this post.

In Laura Sanborn’s paper, “Long-Term Health Risks and Benefits Associated with Spay/Neuter in Dogs” (dated May 14th, 2007), she wrote,

“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.”

E. Hardie of the Department of Clinical Sciences (North Carolina State University, Raleigh, NC, USA) wrote a short paper entitled “Pros and Cons of Neutering.” The paper was dated October 21, 2007. Hardie’s research explained the following:

“Orthopedic conditions associated with neutering (spaying or castration) include hip dysplasia, cruciate injury and slipped capital femoral epiphysis. Dogs that are spayed or castrated before bone growth is complete are taller than intact dogs or dogs that are spayed or castrated at a later age. Orthopedic diseases in neutered animals are thus likely to be conditions associated with late or incomplete closure of growth plates or altered joint anatomy due to changes in skeletal growth.”

I’ve always heard that neutered dogs (both male and female) live longer than dogs that are left intact. Is that true?

Margaret Root-Kustritz wrote that while several studies have demonstrated that dogs that are spayed and neutered live longer [than dogs who are not], “Cause-and-effect has not been described.” She speculates that “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.”

Therefore, while it is possible that dogs that are spayed and castrated may live longer [because of being spayed or castrated], I would say that there doesn’t seem to be any solid evidence to confirm this. I think that when vets (and random people) make this claim, we should definitely ask where they are getting their facts from. (And please, if you have more information on this subject, send it my way! I’ll add on to this and give you credit.)

How will my dog’s behavior be affected after being spayed or neutered?

According to Margaret Root-Kustritz:

“Behaviors that are most likely to be affected by gonadectomy are those that are sexually dimorphic (seen primarily in one gender). Examples of sexually dimorphic behaviors include flagging in bitches, and mounting and urine marking in dogs. Incidence of sexually dimorphic behaviors decreases after gonadectomy in bitches and dogs, with the decrease in incidence not correlated with length of time the animal has shown the behavior prior to gonadectomy.

Those behaviors that are not sexually dimorphic, including most forms of aggression, are not decreased in incidence by gonadectomy. One behavioral consequence of spaying that has been documented in several studies is an increase in reactivity towards humans with unfamiliar dogs and increased aggression toward family members. This may be hormonally related; there may also be a breed predisposition.”

E. Hardie wrote:

“The effect of spaying on behavior is controversial. Some studies have shown few effects, while others have demonstrated more reactivity and aggressive barking in spayed compared to intact bitches. In a study of 227 dogs that had bitten humans, neutered female dogs and male dogs were overrepresented. Castration has been shown to reduce urine marking, mounting and roaming, but is effective in reducing aggression in only about one third of dogs.”

What are some pros and cons of neutering male dogs?

Laura Sanborn’s paper detailed the following:

“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 and larger 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”

Laura Sanborn concluded:

“[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.”

Margaret Root-Kustritz’s paper showed that her feelings on the subject were similar to Sanborn’s. Root-Kustritz wrote:

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.”

What are some pros and cons of spaying female dogs?

Laura Sanborn’s research stated:

“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 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 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
  • increases 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”

Sanborn’s overall thoughts on spaying:

“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.”

Margaret Root-Kustritz’s overall thoughts on spaying:

“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.”

E. Hardie’s paper declared:

The mammary carcinoma data would suggest that performing a spay before the first heat would result in the lowest chance of mammary cancer. The pyometra data would suggest that as long as a spay was performed in the first 4 years of life, pyometra is not likely. One study of 809 bitches showed no difference in urinary incontinence in animals spayed before or after the first heat at the 5% significance level, but a difference at the 10% level. Other studies have documented urinary incontinence in 9.7% of bitches spayed before the first heat compared to a 20% incidence in bitches spayed after the first heat. Even though the incidence of incontinence was lower in bitches spayed before the first heat, severity of the incontinence in these dogs was much worse than in dogs spayed after the first heat.”

Gia’s note: I am understanding that in order to take the greatest prevention against mammary cancer, it is recommended that female dogs be spayed prior to their first heat. However, because some people–including me–have concerns about neutering before the dog’s body has finished developing, I wonder if one could make an argument for spaying after the first heat cycle but before the second. I really wish that I knew more about overall risks and benefits. This is definitely a topic to consult the vet(s) about.

Is there a way to sterilize a female dog in a less intrusive way than the standard spay procedure?

E. Hardie wrote about both ovariohysterectomy (the typical spay procedure, which removes the uterus and ovaries) and ovariectomy, where only the ovaries are removed.

“The ideal spay would result in positive effects from spaying with the least risk of developing negative effects. It would also result in minimal pain and immediate postoperative complications….

A major controversy is whether ovariohysterectomy or ovariectomy is the preferable surgery. The rationale for removing the uterus is that stump pyometra may develop if the uterus is not removed. The rationale for ovariectomy is that the surgery is less invasive and the risk of pyometra is minimal once the animal is no longer cycling. In several long term studies of dogs and cats under going ovariectomy, stump pyometra did not occur in any animal after surgery. Studies have also shown that there is no difference in the incidence of urinary incontinence after ovariectomy or [ovariohysterectomy] in the dog.

A second controversy surrounds whether or not a midline approach or a flank approach is preferable. The advantages of the midline approach are that it is technically easier to perform an ovariohysterectomy from this approach, the incision can be quickly opened if needed, and both sides of the reproductive tract are easily accessible from one approach. The advantage of the flank approach is that, in experienced hands, it allows ovariectomy or ovariohysterectomy to be performed through a very small lateral incision. The flank approach can be closed with a few buried sutures and has a minimal risk of dehiscence. Although the surgery is technically challenging, the advantage of performing a spay using minimally invasive surgery is that the dog experiences less postoperative pain and distress.”

Gia’s note: During an ovariohysterectomy, both the uterus and the ovaries are removed. During an ovariectomy, only the ovaries are removed. I have come across a couple mentions of a hysterectomy that would allow the female dog to keep her ovaries. I have also seen mentions of tubal ligations for female dogs (and vasectomies for males). I would like to learn more about all of these various options. If anyone has any resources or thoughts that they would like to share, please feel free to do so.

Final reminders/notes:

Consider the sex, age, and breed of your dog. Investigate which disorders are more likely to affect your dog [due to breed predispositions and genetics] if neutered. At the end of Margaret Root-Kustritz’s paper, there are some helpful charts that show which breeds may be predisposed to certain disorders. It would also be worth looking over Laura Sanborn’s paper, since she sometimes listed different breeds for the same disorder.

I believe that it is important to consult with multiple vets and trusted professionals. Always do your own research and ask a lot of questions. Please note that I am not [yet?] a professional! I’m just your friendly neighborhood Gia, trying to help out as best I can.


“Determining the Best Age at Which to Spay or Neuter: An Evidence-Based Analysis” by Margaret Root-Kustritz, DVM, PhD. Paper dated Spring 2008.

“Long-Term Health Risks and Benefits Associated with Spay/Neuter in Dogs” by Laura J. Sanborn, M.S. Paper dated May 14, 2007.

“Pros and Cons of Neutering” by E. Hardie of the Department of Clinical Sciences, North Carolina State University. Paper dated October 21, 2007.

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3 Responses to Pros & Cons of Spaying/Neutering Dogs – Part I of Gia’s Research

  1. I REALLY appreciate articles like this one. Thank you. I have been researching this issue for more than a few months now and, from what I’ve read, it made sense to me to consider neutering my male Shih Tzu TinkerToy only after his first birthday, but before 18 months. He was born in October 2014, so I am currently trying to come to a healthy decision we both can live with. I’m sharing some of the details of my own thinking in hopes it might be helpful to another responsible pet owner attempting to navigate the neuter/don’t neuter waters.

    Tinker’s a great little 10 pound dog, possibly my last, and I want to keep him happy and healthy for a long, long time. This is an extremely well behaved, well socialized dog who is in NO danger of adding to the puppy population, accidentally or on purpose, so unless I bring home a female puppy, that’s really not a factor. He sticks close to me and the seated chatting humans at the dog park, one of the few times he is off-lead, and seems to prefer all people of all ages to dogs!

    The only issues that *might or might not* abate upon castration are few, primarily territorial. Marking – but outside only, at least so far — and aggression toward other dogs (ONLY if they bark first and their owners don’t control them). Even then, Tink confines his aggression to barking, is not a leash puller, and socializes well on and off-lead with my friends’ two larger dogs (in their home and at a nearby park). He seems more to be barking to protect me, actually, which he also does inside our pre-war apartment as neighbors pass our door on their way up the central stairs (but only when I am home, so I am told!) We’re working on those behaviors, and they are slowly responding.

    In any case, castration for those reasons seems extreme to me, especially since I lost my 19 pound male Shih Tzu Tuxedo, who declined rapidly post-anesthesia, following a history of immune system health problems. I never again want to have to euthanize after watching a beloved pet waste away for far too long before coming to the decision that he was unlikely to ever recover, despite syringe feeding and medication. I will never fully get the memory out of my mind, so I am terrified of having to put Tink to sleep unless I can believe that it is absolutely necessarily, for even the shorter, safer, laser surgery.

    Due to my own allergy issues, my 35 years of pet ownership has been confined to the Shih Tzu breed, both large and small – and I have usually had two or three at once, so I have a great deal of history and experience. Past male Shih Tzus in my care have been neutered, as has 1 female, who was fixed after a problem pregnancy (she came to me on a breeder agreement). My last pup Tabitha, an 11-pound non-neutered female, bred once (3 pups, 2 placed with family, 1 stayed with me), was the healthiest and longest lived dog I’ve owned (19 years – easily 3 to 4 years longer than the longest lived neutered dogs).

    It’s difficult to separate correlation, causation, coincidence and confounding factors, even in much larger samplings than my own, but it seems there really isn’t much scientific research to back up either side of the decision. Most of the pet health field seems to be reduced to defending their own confirmation bias, effectively doing what they’ve always done. My mind spins recalling one of my favorite quotations, by Anatole France: “If fifty million people say a foolish thing, it’s still a foolish thing.”

    Is neutering one of those foolish things? I still have come to no conclusion. If anyone reading can point me to more recent studies than the ones referenced above, please leave links and/or references in the comments section here, and I will do the same.

    (Madelyn Griffith-Haynie – ADDandSoMuchMore dot com)
    – ADD Coach Training Field founder; ADD Coaching co-founder –
    “It takes a village to educate a world!”

    Liked by 1 person

    • Gia says:

      Thank you so much for your comment. It’s wonderful to hear that you’ve been researching for several months. It’s obvious that you’re very dedicated to your dog and making choices about his body in a thoughtful manner. I’d like to hear more about your decision to neuter him before 18 months, if you do so. (I’m assuming that you waiting until after his first birthday has to do with allowing his body to develop properly, which I think is the right choice.)

      I agree that we need a lot more scientific research to better guide us in making these important decisions. That being said, I do feel strongly that letting an animal’s body develop and mature before neutering is important; the sex organs produce hormones which affect the whole organism, and I think that we need to realize that removing them is going to affect the inner workings of the body and its overall development.

      I really enjoyed the article “Risks and Benefits to Spaying and Neutering Your Dog” from Whole Dog Journal, and I liked how the author said, “And for early spay/neuter, I asked myself: Does it make sense to think that you can remove a puppy’s major reproductive organs – and all the hormones that go with it – and not expect there to be some biological ramifications?” I would recommend checking out that article, and there’s an interesting part that mentions dog parks and how “neutered dogs tend to be aggressive toward intact dogs.” Have you found this to be true?

      Tinker sounds like an amazing little dog. I think that when people make the decision to neuter animals, they should focus on the actual result of the operation (leaving the animal sterile), and not the fact that the animal’s behavior may or may not change. I agree that castration for the reasons you mentioned seems extreme, and I think that continuing to implement behavior modification techniques is the way to go.

      I understand your fear of anesthesia after losing Tuxedo. (I am so, so sorry.) I hope that you can consult with multiple veterinarians about neutering Tink. I think that you should tell them what happened with Tuxedo and openly express your concerns. Since you have a great deal of experience owning Shih Tzus, I would encourage you to think about how neutering your past males affected them. Perhaps you could also consult with Shih Tzu breeders/participate in an online forum that is breed specific?

      You sound like a very responsible person, and I commend your excellent care standards.

      Liked by 1 person

      • We seem to agree on everything where taking excellent care of a pets is concerned. I found your site because of your article. RE: Risks and Benefits — I recently discovered Whole Dog Journal and am loving my subscription and their site. GREAT info, well written and researched, IMHO (wrote them a fan letter after my first issue)

        Good suggestion about checking out Shih Tzu forums for more information. I’m more inclined to trust responsible pet owners than pet professionals, actually, since our PRIMARY unconscious bias is keeping them with us, healthy and happy, for as long as possible, and there is so little scientific research to guide any of us clearly.

        Most pros tend to stand on the “let’s make SURE we don’t add to the unwanted pet population” side of the fence, in my experience. Breeders can’t help but want to protect their bloodlines and their businesses, even more so if they’ve conducted a long and careful breeding program and sell many of their dogs as pets vs. show dogs or for breeding. So I’m not sure they give the “don’t” side of the neutering discussion the same careful consideration I would. (PLEASE NOTE: this is NOT to say that they don’t care a great deal about the health of the animals in their care and do a great deal of careful research!)

        I DO KNOW that if I decide to do it, I want to give TinkerToy as long as possible to develop with all hormones intact, keeping in mind that older animals have a tougher time with surgery of any sort – and that I want to research the vasectomy stats as well. Pain and recovery statistics are also important criteria, in my mind anyway.

        I researched local vets before I brought him home, so Tink’s vet is excellent in every way. She knows about Tux (who was also given his booster vaccinations while he was under) – and she was appropriately sympathetic about /responsive toward my fears. She is already working with me about spacing out Tinker’s vaccinations where possible (vs. slugging him with combos, even tho’ that’s not the way she usually works) – and that I want tests before automatically adding immunity boosters that may not be necessary – for as long as there is any way I can afford the increased vet costs.

        She takes time to consider questions carefully and is the most informative vet I’ve had so far, with a very “moderation where possible” approach to most issues. GREAT office staff too. Tinker actually likes to visit, believe it or not (but then, anywhere his fans congregate seems to be fine with him, lol)

        I can’t say that I have noticed that neutered dogs are more aggressive toward Tink – but then, he prefers to hang with humans and is only 13 months old, so that may be part of the reason. In any case, I wish I had known what I know NOW from my very first little dog (and had the amount of time & cash I am able to spend with/on Tink to spend with each and every one of them through the years) – or that I could have them all back for do-overs!

        Alas, all I can do now is lavish the very BEST of everything I’ve learned on Tinker – including many *positive/loving* ways to play with him, teaching him tricks and words to keep his brain sharp. (Since the early Chinese Dynasty, Shih Tzus have been bred for palace companionship alone; they don’t respond to tough-love anyway — thus their undeserved reputation for being stubborn, IMHO — but they will practically do backflips for praise and attention, and are very smart, loving little dogs).

        btw – Please don’t feel like you must return length with length – or rush to respond. I know you are still grieving, and the LAST thing I want is for my visits/comments to add to your load.


        Liked by 1 person

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