Benefits and Risks of Neutering–An Evidence-Based Approach

In 2010 I published an extensive narrative review of the research concerning the benefits and risks of neutering. Since then, there has been a proliferation of studies and reviews attempting to refine and strengthen the evidence and allow better individualized recommendations about neutering individual pets. I have collected my original review and links to articles updating the evidence on this subject here, and I will endeavor to keep this collection updated as more and better evidence is available on this complex and important subject.

As a general practice veterinarian, I think this subject provides a perfect example of the process of evidence-based medicine. Using the totality of the available evidence, with critical appraisal and an awareness of the limitations of each study and review, veterinarians can make informed recommendations to clients about the care of individual pets.

A one-size-fits-all policy about neutering can never be optimal as the particular risks and benefits of this procedure depend on many factors: the breed, age, size, health status, and purpose of the pet; the values, behaviors, and socioeconomic status of the owner; the independent risks for specific health problems that might also be associated with neuter status; the time and place in which the pet lives; and the availability and utilization of preventative and therapeutic veterinary care.  

Science makes the care we provide better, but it may not always make it simpler or easier. Individualization of care requires as comprehensive an understanding as possible about the real factors that influence health and disease. We cannot simply make up systems of diagnosis and therapy, such as those of homeopathy and so-called Traditional Chinese Veterinary Medicine, and then claim that we are providing individualized care if we have not validated the specific diagnostic and therapeutic methods we are using through controlled scientific research.

Conversely, we must make decisions based on the evidence we have, not the evidence we want. This means we have to be honest about what we know and don’t know, and we have to accept that there will always be significant uncertainty about the outcome in any individual case. Science can reduce this uncertainty but cannot eliminate it. And we must make the best effort possible to integrate all the complex and conflicting information available into as rational and evidence-based a recommendation as possible, rather than relying on simply, reflexive rules (such as always neuter before 6 months of age or never neuter until after 1 year of age) or the traditional but weak foundations of clinical experience and the advice of our teachers and mentors.

**In 2014, I updated my 2010 review, with the addition of scientific reports published in the intervening years. This led to a number of changes in my conclusions with regard to specific issues. The new version is available here, and while I will keep the link active to the previous version, this update supersedes the older version.

Benefits & Risks of Neutering Dogs and Cats

1) McKenzie, B. Evaluating the benefits and risks of neutering dogs and cats. CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources 2010 5, No. 045

2) Beauvais W, Cardwell JM, Brodbelt DC.The effect of neutering on the risk of urinary incontinence in bitches – a systematic review. J Small Anim Pract. 2012 Apr;53(4):198-204.

SkeptVet review of The effect of neutering on the risk of urinary incontinence in bitches- a systematic review

3) Beauvais W, Cardwell JM, Brodbelt DC. The effect of neutering on the risk of mammary tumours in dogs–a systematic review.J Small Anim Pract. 2012 Jun;53(6):314-22.

SkeptVet review of The effect of neutering on the risk of mammary tumours in dogs- a systematic review

4) Torres de la Riva G, Hart BL, Farver TB, Oberbauer AM, Messam LLM, et al. (2013) Neutering Dogs: Effects on Joint Disorders and Cancers in Golden Retrievers. PLoS ONE 8(2): e55937. doi:10.1371/journal.pone.0055937

SkeptVet review of Neutering dogs: Effects on joint disorders and cancers in golden retreivers

5) Hoffman JM, Creevy KE, Promislow DEL (2013) Reproductive Capability Is Associated with Lifespan and Cause of Death in Companion Dogs. PLoS ONE 8(4): e61082. doi:10.1371/journal.pone.0061082

SkeptVet review of Reproductive capability is associated with lifespan and cause of death in companion dogs

6) Lefebvre, SL. Yang, M. Wang, M. Elliott, DA. Buff, PR. Lund, EM. Effect of age at gonadectomy on the probability of dogs becoming overweight. Journal of the American Veterinary Medical Association. 2013;243(2):236-43.

SkeptVet review of Effects of age at gonadectomy on the probability of dogs becoming overweight.

7) Zink, MC. Farhoodly, P. Elser, SE. Ruffini, LD. Gibbons, TA. Riegr, RH. Evaluation of the risk and age of onset of cancer and behavioral disorders in gonadectomized Vizslas. J. Amer Vet Med Assoc. 2014;244(3):309-319.

SkeptVet Review of Evaluation of the risk and age of onset of cancer and behavioral disorders in gonadectomized Vizslas.

8) Hart BL, Hart LA, Thigpen AP, Willits NH. Long-Term Health Effects of Neutering Dogs: Comparison of Labrador Retrievers with Golden Retrievers. PLoS ONE 2014;9(7): e102241. doi:10.1371/journal.pone.0102241

Sketpvet Review of Long-Term Health Effects of Neutering Dogs: Comparison of Labrador Retrievers with Golden Retrievers.

9) O’Neill DG, Church DB, McGreevy PD, Thomson PC, Brodbelt DC. Longevity and mortality of cats attending primary care veterinary practices in England. J Feline Med Surg. 2015 Feb;17(2):125-33. doi: 10.1177/1098612X14536176. Epub 2014 Jun 12.

SkeptVet Review of Longevity and mortality of cats attending primary care veterinary practices in England.

10) M.M.E. Larsen, B. Børresen, A.T. Kristensen.  Neuter status and risk of cancer in a Danish dog population. 

SkeptVet Review of Neuter status and risk of cancer in a Danish dog population.

What is a Spay: Ovariectomy Versus Ovariohysterectomy forFemale Dogs

11) Salas Y, Márquez A, Diaz D, Romero L (2015) Epidemiological Study of Mammary Tumors in Female Dogs Diagnosed during the Period 2002-2012: A Growing Animal Health Problem. PLoS ONE 10(5): e0127381. doi:10.1371/journal.pone.0127381

SkeptVet Review of  Epidemiological Study of Mammary Tumors in Female Dogs Diagnosed during the Period 2002-2012: A Growing Animal Health Problem.

12) Graf R.  et al. Swiss Feline Cancer Registry 1965-2008: the Influence of Sex, Breed and Age on Tumour Types and Tumour Locations. J Compar Pathol. 2016.

Neutering and Cancer Risk in Cats

13) Hart, B. L., Hart, L. A., Thigpen, A. P. and Willits, N. H. (2016). Neutering of German Shepherd Dogs: associated joint disorders, cancers and urinary incontinence. Veterinary Medicine and Scienc. doi: 10.1002/vms3.34

SkeptVet review of Neutering of German Shepherd Dogs

14) Sundberg CR. Belanger JM. Bannasch DL. et al. Gonadectomy effects on the risk of immune disorders in the dog: a retrospective study. BMC Veterinary research. 2016;12:278.

SkeptVet review of Gonadectomy and Immune Disorders

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16 Responses to Benefits and Risks of Neutering–An Evidence-Based Approach

  1. Pingback: Benefits & Risks of Neutering, an Evidence Update–Cancer and Behavioral Problems in Vizslas | The SkeptVet Blog

  2. Pingback: Potential Risks of Neutering and Age at Neutering for Godlen Retrievers and Labrador Retrievers | The SkeptVet

  3. Massimo D'Accordi says:

    Hi,
    just reading through your updated review on the subject I find the following sentence possibly confusing:
    “Neutering female dogs at the time of surgical removal of mammary tumours or within 2 years before diagnosis is associated with longer survival and reduced risk of tumour recurrence”
    After only reading the abstract of the 2 references (the host website is down) I reach the following conclusion:
    ” Neutering female dogs at the time of surgical removal of NON MALIGNANT and HYPERPLASTIC mammary lesions is associated with a lower risk of recurrence (risk reduction by 50%) AND neutering within 2 years prior to diagnosis is associated with longer survival (45%) in cases of CARCINOMA”.
    You can still argue that neutering is beneficial but the evidence appears more circumstantial (bening versus malignant neoplasia)
    Does your practice recommend neutering at the time of tumour removal?
    Mine does, while I personally have a chat with the owner and most of the time find that they are unwilling to get the spey done ( concerned about longer anesthesia and more pain to cope with).

  4. skeptvet says:

    Here is a bit more from the text on Ref. #21 (Sorenmo 2000). What it suggests is that spaying at the time of surgery or within 2 years before surgery for mammary carcinoma improves survival.

    To assess both the effect and timing of spaying, the dogs were classified
    into 3 groups: (1) dogs with mammary tumors that were not
    spayed before or after the tumor surgery (intact), (2) dogs with mammary
    tumors spayed within 2 years before tumor development or concomitant
    with the tumor surgery (SPAY 1), and (3) dogs with mammary
    tumors spayed more than 2 years before tumor development
    (SPAY 2).

    The dogs in SPAY 1 were initially analyzed as 2 separate groups:
    dogs spayed less than 2 years before tumor diagnosis and those spayed
    concomitant with tumor surgery. There were no statistically significant
    differences between these groups (median survivals: 629 days versus
    755 days, P  .7), and it was therefore decided to combine them into
    1 group, ie, SPAY 1. The SPAY 1 group included 23 dogs that were
    spayed less than 2 years before tumor surgery and 22 dogs spayed
    concomitant with tumor surgery.

    When timing of spaying was considered, product–limit survival estimates
    indicated that dogs in the SPAY 1 group survived significantly
    longer than dogs in the intact group (P  .03) and
    the SPAY 2 group (P  .02), with median survival of 755
    days, compared to a median survival of 286 days in intact
    dogs, and 301 days in the SPAY 2 group (Fig 1). To adjust
    for differences in age, histologic vascular invasion, and histologic
    differentiation between the spay groups, Cox’s proportional
    hazards model was used. Dogs that were in the
    SPAY 1 group survived 45% longer compared with dogs
    that were either intact or in the SPAY 2 group (RR  .55;
    95% CI .32–.93; P  .03).

    The other paper (Kristiansen 2013) deals with benign mammary tumors. This one found no difference in survival with or without spayoing at the time of tumor removal (which is not surprising since benign tumors should lead to death), but it did find a significant reduction in the recurrence of tumors in the spayed group.

    The hazard ratio of 0.47 (OHE/nonOHE) for new
    tumor development re?ects that bitches spayed at the
    time of tumor removal, on average run approximately
    half the risk of developing new tumors at any given
    time compared with the bitches remaining intact.

    So for malignant tumors, neutering at the time of surgery likely improves survival. For benign tumors, it likely reduces the risk of new tumors in the future. It may or may not affect survival, but there were too few deaths in the second study to show this. So in both situations there is a likely benefit to neutering at the time of mammary tumor removal, and I do recommend it. Obviously, the evidence is not strong, but it is what we have.

    And, for what it’s worth, there is strong evidence in humans that ovariectomy improves survival in breast cancer patients, though patient age, the use of hormone antagonists, and other therapies used are also factors. So overall, I think the evidence best supports a recommendation to neuter at the time of surgical treatment of mammary tumors.

  5. skeptvet says:

    Here is a bit more from the text on Ref. #21 (Sorenmo 2000). What it suggests is that spaying at the time of surgery or within 2 years before surgery for mammary carcinoma improves survival.

    To assess both the effect and timing of spaying, the dogs were classified
    into 3 groups: (1) dogs with mammary tumors that were not
    spayed before or after the tumor surgery (intact), (2) dogs with mammary
    tumors spayed within 2 years before tumor development or concomitant
    with the tumor surgery (SPAY 1), and (3) dogs with mammary
    tumors spayed more than 2 years before tumor development
    (SPAY 2).

    The dogs in SPAY 1 were initially analyzed as 2 separate groups:
    dogs spayed less than 2 years before tumor diagnosis and those spayed
    concomitant with tumor surgery. There were no statistically significant
    differences between these groups (median survivals: 629 days versus
    755 days, P  .7), and it was therefore decided to combine them into
    1 group, ie, SPAY 1. The SPAY 1 group included 23 dogs that were
    spayed less than 2 years before tumor surgery and 22 dogs spayed
    concomitant with tumor surgery.

    When timing of spaying was considered, product–limit survival estimates
    indicated that dogs in the SPAY 1 group survived significantly
    longer than dogs in the intact group (P  .03) and
    the SPAY 2 group (P  .02), with median survival of 755
    days, compared to a median survival of 286 days in intact
    dogs, and 301 days in the SPAY 2 group (Fig 1). To adjust
    for differences in age, histologic vascular invasion, and histologic
    differentiation between the spay groups, Cox’s proportional
    hazards model was used. Dogs that were in the
    SPAY 1 group survived 45% longer compared with dogs
    that were either intact or in the SPAY 2 group (RR  .55;
    95% CI .32–.93; P  .03).

    The other paper (Kristiansen 2013) deals with benign mammary tumors. This one found no difference in survival with or without spayoing at the time of tumor removal (which is not surprising since benign tumors should lead to death), but it did find a significant reduction in the recurrence of tumors in the spayed group.

    The hazard ratio of 0.47 (OHE/nonOHE) for new
    tumor development re?ects that bitches spayed at the
    time of tumor removal, on average run approximately
    half the risk of developing new tumors at any given
    time compared with the bitches remaining intact.

    So for malignant tumors, neutering at the time of surgery likely improves survival. For benign tumors, it likely reduces the risk of new tumors in the future. It may or may not affect survival, but there were too few deaths in the second study to show this. So in both situations there is a likely benefit to neutering at the time of mammary tumor removal, and I do recommend it. Obviously, the evidence is not strong, but it is what we have.

    And, for what it’s worth, there is strong evidence in humans that ovariectomy improves survival in breast cancer patients, though patient age, the use of hormone antagonists, and other therapies used are also factors. So overall, I think the evidence best supports a recommendation to neuter at the time of surgical treatment of mammary tumors.

  6. Pingback: New evidence on the outcome of neutering dogs - Pippa Mattinson

  7. lorac says:

    Thank you for the information on desexing and disease correlations. It’s helpful in deciding when to desex. Another consideration on timing is the effect on growth and growth plate closures, as well as bone injuries. Do you know of any information about this topic?

  8. skeptvet says:

    Neutering slightly delays growth plate closure, which leads to animals neutered before puberty having slightly longer long bones and taller stature. It is not clear if there is any health benefit or harm associated with this, though one can hypothesize about possible relationships between this and some of the orthopedic diseases that are more common in neutered animals (such as cruciate ligament rupture).

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  15. Meagan says:

    But isn’t neutering ASAP better for most pet owners on the whole? Most pet owners are not very responsible and/or knowledgeable (they should be, but they generally aren’t) and the risks of unwanted litters are high, and I think more of a danger to pets as a community. Also animals that go into heat or seek mates cause more trouble for the pet owner, which can cause friction and unhappiness for all. I just think there is a whole other social/psychological side to it that trumps any increased health risks to individual pets, due to neutering. What about increased mental health of a neutered pet who isn’t constantly driven to seek sex? Curious about your thoughts…

  16. skeptvet says:

    Population control is a valid reason to recommend routine neutering, it is simply a separate issue from the medical risks and benefits. When I talk to owners, I do emphasize that neutering is an important part of preventing pet overpopulation and the suffering that goes with that. Anyone choosing not to neuter or to delay neutering has to accept a greater responsibility for preventing unplanned reproduction. But I am also obliged to be clear about the risks an benefits to the individual patient since their welfare is my primary responsibility as a clinician in companion animal private practice (as opposed to the responsibility of a vet practicing shelter medicine, or food animal herd health, for example).

    As for a “mental health” benefit from diminished sex drive, that’s a bit of a reach. As a behavioral biologist working in enrichment for captive animals (before I went to vet school), I studied and thought a lot about what healthy, “normal” life was for an animal. A big part of that is the ability to express species-typical behaviors, and I think an ethical case can be made that we reduce welfare in individuals to some extent by preventing reproduction. I think there are obviously also benefits in terms of the larger welfare picture, of course, but I don’t think it is reasonable to suggest individuals are happier because we have reduced their reproductive drives and behaviors.

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