New Evidence about When to Neuter Your Dog

I have followed the literature on the risks and benefits of neutering for many years, from writing my own literature review in 2010 to critically analyzing individual research studies on the subject here. The evidence is always growing and changing, and the attitudes of pet owners and veterinarians shift over time. When I started working as a veterinarian almost 20 years ago, routine neutering of all dogs and cats at about 6 months of age was still the dominant and recommended practice. There wasn’t much specific scientific evidence for or against this timing, and the focus of most debate about it was whether or not neutering earlier was better or worse (the evidence suggests there isn’t much difference).1–3

More recently, a series of studies have looked at health outcomes in dogs neutered at or before the traditional age and later than the traditional age.4–6 The initial papers focused on only a few breeds (golden retrievers, Labrador retrievers, and German shepherds), and they had a number of other limitations that made generalizing to neutering for all dogs or cats unreliable. I’ve discussed these individual papers in detail previously. 

A reasonable interpretation of these studies would be that there are some risks of neutering in some breeds, particularly in raising the incidence of diseases those breeds are already pre-disposed to. Earlier neutering might be a factor in this, though if you look at the studies in detail, this association doesn’t hold consistently. Some risks are seen, for example, inn golden retrievers but not Labradors, or in female dogs but not males, or in dogs neutered before 6 months or after 12 months of age but not between these ages. 

All-in-all, these studies should be viewed as evidence that the relationship between breed, size, sex, neutering and various health conditions is complex and hard to predict. No simple, one-size-fits all approach is likely to be optimal for everyone, whether it is traditional neutering at 6 months of age or alternative approaches.

Unfortunately, many people have gone well beyond such reasonable interpretations and used these studies to suggest no dogs should be neutered before 1-2 years of age, or that they should not be neutered at all. Some more extreme voices have even claimed these studies show neutering causes cancer or has other dire health effects. Such excessive claims risk causing harm when pet owners avoid the benefits of neutering out of fear of unlikely risks.

The group whose research has been most influential in changing attitudes about neutering has recently published a brief summary of a much more extensive research project that will hopefully be published in full this year. This more detailed study includes an additional 32 breeds, and the findings illustrate how complex and nuanced the issue is and how unreliable broad, rigid approaches are.

According to the authors, “Considering the occurrence of joint disorders…it is evident that vulnerability to early neutering is related to body size,” with both purebred and mixed small-breed dogs not showing the increased risk with early neutering previously reported in larger breeds.7 The issue of cruciate ruptures and hip dysplasia and other orthopedic diseases that may be influenced by neutering appear only to be a significant consideration in larger breeds which are already prone to these diseases.

Similarly, previous studies have suggested neutering, especially at an early age, may be associated with greater risk of some cancers. However, this risk varies dramatically by sex and breed, with differences seen between male and female golden retrievers, between golden retrievers and Labrador retrievers, and so on. The new evidence makes this variability even clearer. The researchers report no association between early neutering and cancer in mixed-breed dogs and, “in small-breed dogs, with the exception of the Shih Tzu, there was no association between cancer incidence and spaying at any age.”7 So much for the “neutering causes cancer” claim.

In making decisions about neutering, as with any other medical intervention, the key is to balance risks and benefits in the context of the best available evidence. As the evidence changes, we have to be willing to change our positions. I once recommended routine neutering at 6 months for all dogs and cats. I now tend to suggest that there are few medical benefits to neutering male dogs who don’t exhibit certain behavior problems, and these benefits may be offset by some risks in some breeds, especially larger breeds. For female dogs, the benefits of preventing mammary cancer and uterine infections still likely outweigh the risks in most dogs, but in breeds like golden retrievers who are at risk for some cancers that seem to be more common in neutered females, neutering later may have advantages. 

None of these are rigid, universal rules, and we must always consider the unique needs and circumstances of each animal. However, broad claims that we should never neuter or should always wait until some specific age are no more reasonable than broad claims that we should always neuter everybody at 6 months old, and our pets and patients will be better off if we move away from such simplistic thinking and consider the scientific evidence in all its complexity.

References

1.        Howe LM. Short-term results and complications of prepubertal gonadectomy in cats and dogs. J Am Vet Med Assoc. 1997;211(1):57-62. http://www.ncbi.nlm.nih.gov/pubmed/9215412. Accessed January 3, 2019.

2.        Olson PN, Kustritz M V, Johnston SD. Early-age neutering of dogs and cats in the United States (a review). J Reprod Fertil Suppl. 2001;57:223-232. http://www.ncbi.nlm.nih.gov/pubmed/11787153. Accessed January 5, 2020.

3.        Stubbs WP, Bloomberg MS, Scruggs SL, Shille VM, Lane TJ. Effects of prepubertal gonadectomy on physical and behavioral development in cats. J Am Vet Med Assoc. 1996;209(11):1864-1871.

4.        Torres de la Riva G, Hart BL, Farver TB, et al. Neutering Dogs: Effects on Joint Disorders and Cancers in Golden Retrievers. Williams BO, ed. PLoS One. 2013;8(2):e55937. doi:10.1371/journal.pone.0055937

5.        Hart BL, Hart LA, Thigpen AP, Willits NH. Long-term health effects of neutering dogs: comparison of Labrador Retrievers with Golden Retrievers. Coulombe RA, ed. PLoS One. 2014;9(7):e102241. doi:10.1371/journal.pone.0102241

6.        Hart BL, Hart LA, Thigpen AP, Willits NH. Neutering of German Shepherd Dogs: associated joint disorders, cancers and urinary incontinence. Vet Med Sci. 2016;2(3):191-199. doi:10.1002/vms3.34

7.        Hart B, Hart L, Thigpen A, Willits N. Best age for spay and neuter: A new paradigm. Clin Theriogenology. 2019;3(11):235-237.

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25 Responses to New Evidence about When to Neuter Your Dog

  1. Russell says:

    it seems crazy to cut off any organ unless absolutely necessary. why not push for Vasectomy or Ovary-sparing Spay to become standard procedure instead?

  2. skeptvet says:

    Well, of course “it seems crazy” isn’t really a very compelling argument. What I let the dentist do to me every 6 months seems crazy too, unless you are aware of the evidence that it improves my long-term health.

    The issue is what are the risks and benefits of neutering that we can identify and demonstrate through reliable scientific evidence, and these are complicated. Neutered animals, for example, often live longer than intact animals. Some diseases are prevented by neutering. Pushing for procedures that do not remove the gonads only makes sense if we can show that this makes animals healthier, and so far the evidence doesn’t really support that as a universal truth. There are specific circumstances in which the benefits of neutering outweigh the risks, circumstances in which the risks outweigh the benefits, and situations in which the net effect is neutral. We should be focused on understanding the best choice in the context of each individual rather than thinking always neutering or never neutering is going to be the best approach for everyone.

  3. Russell says:

    it really should be a compelling argument. medical practitioners really shouldn’t dick around cutting bits off unless they have absolutely compelling evidence that it is absolutely necessary to do so.

    by the by, what your dentist does will seem even more crazy if you read Weston A. Price’s book “Nutrition and Physical Degeneration” and see all the examples he catalogues comparing modern and primitive diets and the comparative state of their teeth. but i digress…

    if you asked medical professionals about the the risks and benefits of a Lobotomy 70-80 years ago i’m sure they could provide a very compelling argument for the continued use of the procedure. unfortunately the only true test of scientific evidence is time.

    so while we’re waiting for the final jury on this i’m personally going to avoid cutting any organs off my dog until there is compelling evidence that it is absolutely necessary.

  4. Russell says:

    i just noticed you didn’t actually answer my original question. do you offer the option of Vasectomy or Ovary-sparing Spay to your patients? and if not, why not?

  5. skeptvet says:

    You didn’t actually ask whether I offered the procedures. You asked “why not push for [them] to become standard procedure], and I did answer that question.

    I recommend either neutering or leaving the pet intact depending on the individual circumstances, and I discuss the risks and benefits of each option in detail with clients. I will do vasectomies if the owner wants that procedure, but it doesn’t really accomplish the goals of most owners who are considering neutering, so if they want to leave the dog intact I recommend leaving it intact and preventing access to females in heat. Only a few breeders who have intact females and males living together and who don’t feel confident they can prevent accidental breeding have found it worth doing.

    Hysterectomy without ovariectomy prevents pyometra, if done correctly, and pregnancy but not mammary cancer. I have yet to have any clients who want this procredure after understanding the options. Most either elect ovariectomy alone or leave the female intact for breeding.

  6. Liz Ford says:

    “I now tend to suggest that there are few medical benefits to neutering male dogs who don’t exhibit certain behavior problems, and these benefits may be offset by some risks in some breeds, especially larger breeds.”
    I’m interested in what you are referring to when you mention “certain behavior problems” and if there is evidence (research) you could share that speaks to that and your professional opinion on it. Thanks for the good article.

  7. skeptvet says:

    There is some evidence that neutering reduces the incidence of some behavioral problems in male dogs, particularly inter dog aggression, urine marking, and roaming in search of females in estrus. If a dog exhibits these behaviors, neutering may be beneficial along with behavioral interventions.

    From my review_

    Most studies have found intact male dogs to be disproportionately involved in aggressive behavior, particularly interdog aggression.[46-47,51] Others have reported marked reductions in aggression and other problem behaviors in male dogs as an effect of castration. In one study, roaming behavior decreased 90%, aggression between males decreased 62%, urine marking decreased 50%, and mounting decreased 80% following castration[48], and several other studies have found similar results.[49,50,54] Some studies have also reported intact dogs to be more likely to bite humans than neutered animals.[51] One study of dog-bite related fatalities found the vast majority of these incidents involved male dogs and intact dogs.[222]

    46. Borchelt PL. Aggressive behavior of dogs kept as companion animals: classification and influence of sex, reproductive status, and breed. Applied Animal Ethology 1983;10:45-61.

    47. Write JC, Nesselrote MS. Classification of behavioral problems in dogs: distributions of age, breed, sex, and reproductive status. Applied Animal Behavior Science 1987;19:169-78.

    48. Hopkins SG, Schubert TA, Hart BL. Castration of adult male dogs: effects on roaming, aggression urine spraying, and mounting. Journal of the American Veterinary Medical Association 1976;168:1108-10.

    49. Maarschalkerweerd RJ, Endenburg N, Kirpensteijn J, Knol BW. Influence of orchiectomy on canine behaviour. Veterinary Record 1997;140(24):617-69.

    50. Neilson JC, Eckstein RA, Hart BL. Effects of castration on problem behaviors in male dogs with reference to age and duration of behavior. Journal of the American Veterinary Medical Association 1997;211(2):180-82.

    51. Gershman KA, Sacks JJ, Wright JC. Which dogs bite? A case-control study of risk factors. Pediatrics 1994;93(6 Pt 1):913-7.

    54. Knol BW, Egberink-Alink ST. Treatment of problem behaviour in dogs and cats by castration and progestagen administration: a review. Veterinary Quarterly 1989;11(2):102-7.

    222. Patronek GJ, Sacks JJ, Delise KM, Cleary DV, Marder AR. Co-occurrence of potentially preventable factors in 256 dog bite-related fatalities in the United States (2000-2009). J Am Vet Med Assoc 2013 Dec 15;243(12):1726-36.

  8. Jo W says:

    Thanks for highlighting this issue again.
    I’ve been driving myself bonkers trying to work out the best time to get our young greyhound spayed, reading more and more papers, driving myself bonkers, again.
    It is reassuring that the scientists in the room reiterate it is a complex decision.

  9. John Stuckey says:

    Sorry, but I’m with Russell on this. Pardon my oversimplification, but it seems the takeaway from your article is that neutering “may” be beneficial for small breeds, “may” be detrimental for large breeds, “may” have an effect on certain behaviors. The lack of definitive evidence of predictable outcomes tells me not to surgically alter until such knowledge is available. I don’t go to the dentist and have all my teeth extracted to prevent caries. BTW-I was reading the rather detailed and professional surgeon’s log from the shelter paperwork on our most recent rescue and was surprised to see that a tubal ligation was performed instead of a hysterectomy. Is this common practice? I would assume in this context that the motive would be saving time rather than long term health of the animal.

  10. skeptvet says:

    I don’t believe you’ve interpreted the article correctly, and you appear to desire a level of certainty that will never be achieved in a domain as complex as biology. Neutering is absolutely beneficial in several clear and important ways, including preventing pyometra and mammary cancer in all females, reducing the risk of unintended reproduction, which not only leads to euthanasia of unwanted animals but to serious medical harm to brachycephalic breeds when they develop dystocia, and in many other ways. There are clear, unquestionable benefits. When the benefits outweigh the risks is a calculation that requires looking at the totality of the individual circumstances, including age, breed, procedure, and so on. There is always some level of uncertainty, but if your answer is never to neuter until you have absolute certainty, you will be rejecting all the benefits, but clearly established and potential, as well as any risks. That is not how the best medical decisions are made.

    Tubal ligation makes little sense. It prevents pregnancy, but it allows pyometra and mammary cancer to develop, which are common and serious diseases. Ovariectomy is the most common procedure, followed by ovariohysterectomy. Hysterectomy alone is done to prevent both breeding and pyometra, though it does. not prevent mammary cancer. As for time, an ovariectomy takes an experienced surgeon less than 10 minutes, and a tubal ligation would likely take about the same since the bulk of the time is spent closing the abdominal incision, which is the same for both, and a difference of a few minutes certainly has no benefit for the patient, so doing the procedure to save the doctor time is both unlikely to be worthwhile and, arguably, unethical.

  11. The issue with neutering/spaying is….
    1) people see you differently if you DONT do this operation
    2) vets pressure owners to do this operation for pups as young as 10 weeks old
    3) and the most important point is— if we neuter/spay all the good dogs then what are we left with?

  12. skeptvet says:

    1) This is true, but of course irrelevant to the risks and benefits for your pet.
    2) Sometimes true, but often not true. There are as many different opinions and approaches regarding neutering among vets as among pet owners. And vets will advocate for what they think is best for your pet because that is their responsibility.
    3) This is a bit of a reductio ad absurdum argument. No one issue talking about neutering “all the good dogs,” and despite the vast majority of pets in the U.S. being neutered for decades, we not only still have “good dogs,” we still have a problem of unwanted pets and overbreeding, so there is no reasonable argument against breeding on the basis of negative effects on the number or quality of available pets.

  13. Buddy says:

    Thank you for this, and for your thoughtful responses. We did not have a choice in the matter with our mixed breed rescue. The majority of interdog interaction issues we’ve had have been with intact animals, particularly with intact males. Our city now requires all dogs over 18 months visiting the dog park to be spayed or neutered.

    I generally don’t agree with the decision to leave a dog intact but also recognize the nuance and that the owner has a choice- I just don’t appreciate it when they continue to bring the dog to dog parks. It’s been my personal experience (not scientifically proven but I’d be curious if there’s any evidence) that almost every time we have an issue with an intact male, the owner is male and vigorously defends his decision.

  14. Katie McKay says:

    Thank for the update on the data being collected from this complex debate. I was wondering if you had recommendations for rescues on the age of spaying and neutering. We like to assure that this is completed before they leave the control of the rescue, but often get puppies who are <1 year. The original data seemed to suggest that 12 weeks was a good compromise and we tend to wait longer if there are other medical issues with the puppy, but in general I was wondering if you had a recommendation. We are a golden retriever rescue but take in many mixes as well.

  15. skeptvet says:

    Some of the answer depends on the goals. Early neutering is common in rescue situations because the priority is reducing unintended reproduction and unwanted animals. Neutering before adoption serves this goal, but there is some evidence it may lead to an increased risk of some health problems in larger breed dogs. Balancing those competing priorities is a judgment call. If owners can be trusted to neuter at the appropriate time after adoption, waiting may be reasonable. If there is real concern that dogs adopted intact won’t get neutered at all, though, it may be better to neuter before adoption even if that is earlier than otherwise optimal.

  16. Vince says:

    Most of the research on sex hormones (i.e. testosterone, progesterone, and estrogen) concentrates on its role is reproduction. This makes sense as they play a crucial role in reproduction. However, these hormones are also important signals in other important systems in the body such as the system of energy metabolism. Progesterone and testosterone are protective hormones that enable cells to function optimally.

    With this perspective, the newer studies showing benefits of delaying the removal of the organs that produce the bulk of these protective hormones should not be a surprise.

  17. skeptvet says:

    It is an oversimplification to call these hormones “protective” or to suggest they promote “optimal” functioning. Longer exposure to them may protect agains some diseases (e.g. prostate cancer, urinary incontinence, some cancers in predisposed dogs), but this also increases the risk of other diseases (mammary cancer in females, prostatitis in males, etc.). The idea that the intact state is somehow optimal is a misconception that interferes with evidence-based decisions about when to neuter.

  18. Tamera Cole Stenson says:

    Hi,
    Prostate cancer is increased in neutered males.

  19. skeptvet says:

    Yes, as is clearly discussed in my literature review on the subject. However, prostatitis is more common in intact males.

  20. Rick says:

    Thank you for the article – it was very informative. We have recently rehomed a nine year old female lab. She is in very good health but has not been desexed.

    I understand that there are some health benefits associated with neutering the dog. However I am worried about the impact on having such an intrusive operation on an older dog.

    Your advice on this matter would be appreciated.

  21. skeptvet says:

    Hello,

    If your dog is otherwise healthy, I would strongly encourage neutering her. The risk of life-threatening uterine infections is quite high after 8 years of age. There is also some risk of mammary cancer, but it is likely too late for neutering to protect much against that.

  22. Maddie says:

    Can you speak at all to the effects of coat growth and quality in regards to neutering dogs earlier/later in life? I’m a new dog groomer and see it claimed often that undercoat is more profuse or hair texture is less desirable in dogs who have been neutered early vs later or neutered at all.

  23. skeptvet says:

    There is evidence showing changes in hair growth and coat quality with neutering. Whether these changes make the coat “less desirable” is, of course, an aesthetic judgement. I am not aware of any studies looking specifically at age of neutering and the effect on coat.

  24. Vince says:

    “Over the last decade, it has become apparent that performing a gonadectomy (spaying or neutering) has unanticipated health implications. Specifically, gonadectomies and the age at which they are performed have been linked to the incidence of many cancers, orthopedic conditions, other medical conditions, and behavioural issues.”
    https://fall2018.iaabcjournal.org/2018/10/31/spay-and-neuter-surgery-effects-on-dogs/

  25. skeptvet says:

    To be fair, the issue is very complex, and while neutering raises the risk of some diseases, it lowers the risk of others and has a negligible effect on many. The most recent study, for example, finds little to no effect of neutering on cancer and orthopedic disease for small breed dogs, and unpredictable differences in the effects between breeds and sexes. No simple universal answers are likely to be coming.

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