New Research That Raises Questions About Current Neutering Recommendations

Results from a hot-off-the-press study published by the Journal of the American Veterinary Medical Association, raise questions about traditional neutering recommendations within the United States where most veterinarians advise that dogs be neutered at a young age in order to induce sterility and eliminate behavioral issues before they have a chance to begin. This new information along with data from other recent studies are a prompt for all of us to reconsider current neutering dogma.

The title of the newest study is, “Evaluation of the risk and age of onset of cancer and behavioral disorders in gonadectomized Vizslas.” The word “gonadectomized” is medical jargon for “neutered”. The research included 2,505 dogs and was supported by the Vizsla Club of America Welfare Foundation.

Effect of neutering on the incidence of cancer

Here is what the researchers learned about the prevalence of cancer as it relates to neutering:

Mast cell cancer: 3.5 times higher incidence in neutered male and female dogs, independent of age at the time of neutering.

Hemangiosarcoma: 9.0 times higher incidence in neutered females compared to nonneutered females, independent of age at the time spaying was performed. No difference in incidence of this disease was found for neutered versus nonneutered males.

Lymphoma (lymphosarcoma): 4.3 times higher incidence in neutered male and female dogs, independent of age at the time of neutering.

Other types of cancer: 5.0 times higher incidence in neutered male and female dogs. The younger a dog was at the time of neutering the younger the age of the dog at the time the cancer was diagnosed.

All cancers combined: 6.5 times higher incidence of cancer in neutered females compared to nonneutered females; 3.6 times higher incidence of cancer in neutered males compared to nonneutered males.

Effect of neutering on the incidence of behavioral issues

The research documented that dogs neutered at or before 6 months of age were at greater risk for developing a variety of behavioral issues including: separation anxiety, fear of noises, fear of gunfire, timidity, excitability, submissive urination, aggression, hyperactivity, and fear biting. Neutering after 6 months of age did not create increased risk. Fear of storms was the behavioral exception. Regardless of age at the time of neutering, neutered Vizslas were at greater risk for developing fear of storms than their nonneutered cohorts.

What does all this mean?

Interesting stuff, eh? From my perspective, I think this is a good wakeup call for anyone still clinging to the notion that all dogs not used for breeding purposes should be neutered at a young age. The recent studies that challenge traditional neutering recommendations seemingly raise more questions than they answer. All have studied large breed dogs (Rottweilers, Golden Retrievers, and now Vizslas). Do these results translate to small and medium sized dog breeds as well? Would similar studies within every breed produce differing results? Should males and females be spayed at a different ages? Are the effects of neutering on behavior breed-specific?

Clearly, there is much more research to be done before determining exactly how current neutering recommendations should be altered (pun intended). For now, what makes the most sense is one-on-one discussion between family veterinarians and their clients to determine how factors such as current knowledge about the effects of neutering, intended use of the dog, breed, temperament, and the way in which the dog will be housed and cared for influence the decision of whether or not to neuter and, if so, at what age.

An important disclaimer from this author: Please do not interpret what I have written to mean that I am opposed to neutering. Nothing could be further from the truth. From a pet overpopulation point of view, I am strongly in favor of neutering. Unplanned/unwanted litters of puppies are profoundly more problematic for me than any of the conclusions recent studies have reported.

Would recent research results influence your decision of whether or not to neuter your dog?

Best wishes,

Nancy Kay, DVM

Diplomate, American College of Veterinary Internal Medicine
Author of Speaking for Spot: Be the Advocate Your Dog Needs to Live a Happy, Healthy, Longer Life
Author of Your Dog’s Best Health: A Dozen Reasonable Things to Expect From Your Vet
Recipient, Leo K. Bustad Companion Animal Veterinarian of the Year Award
Recipient, American Animal Hospital Association Animal Welfare and Humane Ethics Award
Recipient, Dog Writers Association of America Award for Best Blog
Recipient, Eukanuba Canine Health Award
Recipient, AKC Club Publication Excellence Award
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Please visit to read excerpts from Speaking for Spot and Your Dog’s Best Health.   There you will also find “Advocacy Aids”- helpful health forms you can download and use for your own dog, and a collection of published articles on advocating for your pet’s health. Speaking for Spot and Your Dog’s Best Health are available at,, local bookstores, and your favorite online book seller.



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30 Comments on “New Research That Raises Questions About Current Neutering Recommendations

  1. People! People!

    I think there is an obvious answer here! Change the surgery! For instance, a male dog can have a vasectomy rather than a neuter. Still have litter control but the dog still has the advantage of having his endocrine system intact. A female can have a tubal ligation instead of a full spaying with the same results. I think you can easily have both litter control and a dog that still has his/her natural endocrine system intact. It is what I plan to do for all my future pets.

  2. You must be joking. Shelters are nowadays flooded with dogs we can’t find homes for. The situation is even worse in Eastern European countries and Third World countries. I’m having trouble believing my eyes that you are in any way discouraging people from neutering any dog. I’d like to remind you of the Veterinarian’s Oath:

    Being admitted to the profession of veterinary medicine, I solemnly swear to use my scientific knowledge and skills for the benefit of society through the protection of animal health, the relief of animal suffering, the conservation of livestock resources, the promotion of public health and the advancement of medical knowledge. I will practice my profession conscientiously, with dignity and in keeping with the principles of
    veterinary medical ethics. I accept as a lifelong obligation the continual
    improvement of my professional knowledge and competence.

    You should have been much more specific that you are challenging the practice of infantile sterilization and no other. I would remind you that sterilizing a female before her first heat gives 98% protection against mammary tumors, after the first heat still 95% protection — and after that, almost none at all.

    What are you really up to here?

  3. I would also suggest consideration of the following –

    J Small Anim Pract. 2012 Jun;53(6):314-22. doi: 10.1111/j.1748-5827.2011.01220.x.

    The effect of neutering on the risk of mammary tumours in dogs–a systematic review.

    Beauvais W, Cardwell JM, Brodbelt DC.

    Author information


    A commonly-stated advantage of neutering bitches is a significant reduction in the risk of mammary tumours, however the evidence for this has not previously been assessed by systematic review. The objectives of this study were to estimate the magnitude and strength of evidence for any effect of neutering, or age of neutering, on the risk of mammary tumours in bitches. A systematic review was conducted based on Cochrane guidelines. Peer-reviewed analytic journal articles in English were eligible and were assessed for risk of bias by two reviewers independently. Of 11,149 search results, 13 reports in English-language peer-reviewed journals addressed the association between neutering/age at neutering and mammary tumours. Nine were judged to have a high risk of bias. The remaining four were classified as having a moderate risk of bias. One study found an association between neutering and a reduced risk of mammary tumours. Two studies found no evidence of an association. One reported “some protective effect” of neutering on the risk of mammary tumours, but no numbers were presented. Due to the limited evidence available and the risk of bias in the published results, the evidence that neutering reduces the risk of mammary neoplasia, and the evidence that age at neutering has an effect, are judged to be weak and are not a sound basis for firm recommendations.

    © 2012 British Small Animal Veterinary Association.

    PMID: 22647210 [PubMed – indexed for MEDLINE]

    Trying to find the full article, which is available in a number of places online.

    Neutering for reasons of population control is simply an ‘easy’ (and dare i say potentially lucrative) way of dealing with a social responsibility problem.

  4. I know you mostly talk about dogs, but we have a 1 1/2 year old kitten who is now recovering from FHO surgery. After rescuing him from the skimmer of our pool, I was not about to let him go. So, after the operation, we looked on the internet and read several items regarding the correlation between early neutering and the occurrence of femoral head breaks. Because the relationship is based on smaller studies, I would still have him neutered at about 6 months.

  5. I have had all my dogs, male and female neutered and spayed throughout my 45 adult years of dog ownership. All my dogs are rescued.

    I lost a Neapolitan Mastiff to liver cancer at the age of 11…a long life for a mastiff. He was neutered at 3 years of age.

    I had a border collie cross that developed lymphoma at the age of 11. Chemo-therapy gave us 4 more years with him and he died at 15 of old age, I guess. He was neutered at 7 months.

    I will continue to get future dogs from rescues. They come already spayed or neutered.

    I will not ask how old was this wonderful dog when it was neutered.

    I will take it home, give it good quality food and veterinary care, give it copious amounts of love, try to help it overcome any hardships or unkindness that it encountered before it found me and I will allow my dog to fill my heart with joy that cannot be gotten from ANY OTHER SOURCE.

    I hope that rescues continue to neuter and spay all animals before adopting them to good homes. The animals end up at the rescue because somebody didn’t want them or couldn’t keep them.

    5 million animals euthanized each year is unconscionable.


  6. Hi Dr. Kay,
    Another controversial subject! I can understand both sides here. I’d like to see more documented research on this matter. Like someone else pointed out: commercial food plays a big role.
    I know a Vet nearby who is advocating tubal ligations & vasectomies. There is a known correlation between neutering & one of the most common (and Expensive) injuries seen today namely torn ACLs. Having gone though this horrendous ordeal I’d opt for the lesser of the two —vasectomies, tubal ligation.
    As far as being a responsible owner/ breeder and guarding against an unwanted breeding—well that’s just crap. It can happen to anyone. It only takes one mistake. An open gate, door unlocked not to mention kids.
    In my opinion the average person will end up with an unwanted liter or a male gone missing.
    Unfortunately this is one time we need to mess with mother nature despite the risks. I ( if I could re do it) would choose vasectomies’ & ligation over neutering. You attain the same goal— NOPUPPIES— and cure a lot of behaveral issues . You are not taking the necessary hormones from your dog & the price is pretty much the same. And just maybe you will not have a cruciate ligament problem!

  7. I live in the UK, where, like so many countries, it is “standard” to spay bitches mid-way between 1st and 2nd heat. I have a 12 month old labrador retriever, Coco, who is at the heart of our family. I would hate to make an incorrect opinion. I had discussed “to spay or not to spay” with our Vet, and gave him some of the “evidence” (which, as so many respondents have suggested, is scant) and he readily countered with plenty of arguments to say there is as much evidence to suggest that spaying non-breeding bitches remains the best course of action. I am no vet, I love my dog, all I want to do is the best for her… so… what is the best course of action?


  8. Clare wrote:If you cannot control the reproductive life of your dog, then maybe you shouldn’t own one!

    Or maybe as a breeder who has families that are outstanding owners of dogs who neverless have had an accident in over 20 years of ownership, a breeder and a friend offers useful and supportive assistance and help. And their owners contact them immediately because they know that such support will be there and they and the love of their dogs will not be thrown under the bus with statements about their suitability as dog owners.

    Very few of those who have had multiple dogs over a long lifetime of dog ownership will never have had a mistake breeding..whether they say so or not. If they were wonderful dog owners before such a mistake, they remain in my books, wonderful dog owners.

  9. Why is there no, or so little discussion about alternatives such as vasectomies or Zeuterin for males and tubal ligation for females? These procedures allow the hormones to remain so the animal can go through normal growth and devekopment. And they cost no more that spay/neuter. One major problem, of course, for owners who do not want to go through the heat period for a female, is that activity still occurs with a tubal ligation. Not as good a substitution for females yet.

  10. “Claire says:
    January 27, 2014 at 9:46 am
    If you cannot control the reproductive life of your dog, then maybe you shouldn’t own one!”

    I couldn’t agree more! But until we can get around that pesky 4th amendment and make it a priviledge rather than a right to own dogs, these are issues that must be dealth with.

  11. Nancy, did you run out of good ideas for columns? I am surprised to see you promulgating drivel like this study based on hearsay!
    Personally, I think any actual scientific study of animal mortality MUST address all factors, especially food. Most commercial kibble is made with ingredients proven to cause cancer.
    I completely agree with many of the respondents here, especially Vet Changes World, Joye Turock, and Sherryanne: Anyone who thinks accidental litters don’t happen is living on a cloud. And this study had NO scientific rigor. What I AM SEEING as a result, is men at the dog park who now have a reason not to neuter their males, and citing the “new studies” showing it is not recommended :-( In America, our “shelters” kill an average of 5 MILLION unwanted pets every year. In the lovely world in the clouds, all owners are responsible (and intelligent!), and all animals are loved and cared for…but not here in the real world. I just took in a lovely malamute puppy, about a year old, who lived in the yard most of her life, and was frequently beaten for the cime of being a puppy. They bred her at 6 months “to calm her down”. After they sold off the pups, they decided to ditch her, and I was asked to take her on. When I went to pick her up, I was told she had just finished a heat 2 weeks earlier, and couldn’t have been bred by the intact toy poodle that also lived in the yard…could she? I almost fell over. Because if she was pregnant, then they wanted to keep her so they could sell the pups. This is what those of us in rescue deal with daily. So, for those rarified “responsible” owners and breeders, delayed altering may be fine; but for most dogs, the earlier they are altered, the better: notwithstanding “studies” based on hearsay like this one.
    When I was young and stupid, and before I knew about rescue, I bought a breathtaking husky puppy. As she grew in beauty and spirit, I thought there should be more of her in the world, and so bought a spectacular male. Luckily, my girl was smarter than I: she refused all males, even when in heat, although I did have the neighbor’s dogs escaping, breaking down their fences, and trying to get through my door into my house. Her heat cycles were an ordeal for both of us (not to mention local males), and even a few artificial inseminations did not take; but she almost died of a uterine infection, saved only by an emergency spay. And my lovely male…he was so well tempered, never chased girls, never fought with others, so I left him intact…until he got testicular cancer. After another emergency surgery, we followed up with frequent sonograms for years to make sure he was cancer free.
    Nor will I leave a female unspayed. My choice is at 6 months, before their first heat; but in rescue, I am all in favor of s/n at 8 weeks or any time before the dogs get adopted. No dog should ever leave any rescue, “shelter” or dog pound unless de-sexed. Because as awful as losing our pets is, it is even worse to kill 5 MILLION every year. If you gave me a choice between losing my beloved 5 years early, or killing 25 million pets, I’d have to let her go.
    And Nancy, PLEASE stick to actual scientific studies :-)

  12. These are studies of large breeds who already have a stronger predisposition to cancer. To take that data and suggest that ALL male dogs delay neutering is bad science at best. There is no doubt in my mind that a dog who is not neutered at around 6-8 months of age, will start making in the house and that is a dog who is in danger of ending up in a rescue organization or a shelter.

  13. If you cannot control the reproductive life of your dog, then maybe you shouldn’t own one!

  14. I noted that this paper indicates that spayed/neutered and intact dogs have the same expected life span.

    As to those who believe that accidental breedings are unusual or do not happen in households where owners are careful, my experience does not prove either of these to be correct. Regardless of the reasons, unwanted pregnancies can be devastating to the owners, and to the pups produced by those pregnancies. All you have to do is spend a few days in a vet clinic and you will see it. Spend some time in a rescue centre and see it. And don’t over look the stress of being sued if it is your male who scaled the fence..even if he had never done it before or none of your other dogs have ever done it..

  15. Over the past years we have owned 3 giant breed dogs, a Newfoundland, and two Leonbergers. When we had our Newfoundland ( 1992) the current belief then by most vets was to neuter a male as soon as possible. In 1999 when we had our first Leonberger the then accepted guideline for giant breeds was to wait until he was 12-14 months old. By 2011 the updated guidelines were it was best to wait until at least the male was 24 months old because until this time their bones were not fully layered and matured.
    I actually saw this first hand. Our first Leonberger was rather leggy and not as stout looking as his father or grandfather.
    With our current boy we waited as the newest guidelines suggested. He is much more solid looking with all the proper proportions listed for the breed’s characteristics.
    Thankfully to date we never experienced any type of cancers with our boys.

  16. As always, I think it is healthy to question what is commonly accepted as truth. I also think it is helpful to look at other countries. In Norway, for example, the individual is taken into account. According to the Norwegian Animal Welfare Act :
    “Medical and surgical treatment shall be carried out taking into account the animal’s welfare, and protect the animal’s ability to function and its quality of life.”
    These recent studies call into question whether we are truly taking into account the quality of our dog’s life.

  17. Zink is not the only author to have found linkage between desexing and increased cancer risk. UC Davis’ Golden Retriever study was very interesting. I don’t have the link handy, but it is open access on Plos One. Reviews of the literature seem to indicate that the risks are breed/mixture dependent and complex. Not at surprising given that the endocrine system is involved in many if not all bodily functions. Health studies (even in humans) are routinely cohort analyses where correlation is the best that can be done. If you are interested in the effect of desexing it has to correlational, because you can not put ovaries back in after they are remove. Given how many dogs are relinquished because of behavioral issues, I would like to see that part of the study verified by another researcher.

    At this point, for me the burden has shifted. If you want me to believe that spaying or castrating my dog has a real health benefit you are going to have to prove it and prove that it out weighs the risks. Your opinion may be different and that’s fine with me. It is, and should be, an owner’s decision made in consultation with a well-informed vet.

  18. I’m glad to see this study reaching the public eye. This study was began in Purdue University many years ago by Dr David Waters. I watched one of his early talks at the Medallion Rottweiler Club Specialty in Illinois back in the ’90’s.

    Whether I spay or neuter my dog has never had anything to do with getting them to the vet for necessary immunizations and care. I have a spayed female Rottweiler that was neutered at 5 after she got her Championship. We have a 2 year old Lab who will get neutered when he is 5 if it becomes necessary. He has no contact with unneutered females.

    It only takes one visit to get your animal neutered. I don’t see how this affects going in for regular and necessary Vet visits.

  19. Thank you “Vet Changes the World” as I was reading this article I’m thinking that along with early spay/neuter there are responsible pet owners that are guided to give their pets multiple and frequent vaccinations which there is very real evidence is causing much cancer among other ailments to our pets! And, Kate, if you think North American doesn’t have a dog population problem I’d love to enlighten you. Contact me through our website and I’ll forward the dozens of emails coming out of the mid-west and southern shelters that are pleading with us rescues (the Northeast has a lesser problem) to save the dogs in these areas that get 3-5 days and then are mass gassed or heart stick euthanized and tossed into our landfills!!!!!
    Also, getting your dogs sp/n prevents many other cancers such as testicular, uterine and ovarian. It lowers the risk of mammary and prostrate cancers and then there is the deadly, if not caught in time, pyometra which is a massive infection of the uterus and even a heat cycle can cause the prolapse of the uterus which is a sight I’ll never forget. I firmly believe this study is gong to be like so many others that prove to be a waste of time, money, and not accurate in lengthening our dog’s lives.

  20. Quick comment:
    Correlation does not equal causation.

    The method used (survey) shows the value of cooperation and good records. It neglects other factors (vaccination history, diet) as factors. And the author has an established bias against spay neuter.

    I think the study does point out the need for controlled research studies, as well as the need to re-investigate the behavior paradigm (imo altering does not cure behavior problems… training and maturity do). It also supports the need to make an informed decision about whether and when to spay neuter.

  21. As a breeder, I have changed my health guarantee for pups I sell to be void if the dog is neutered prior to two years of age for anything other than emergency circumstances supported by a letter from the veterinarian. The recommendation I make to buyers is to never neuter the male unless for health reasons and to spay the female at the age of 4 (prior to the highest risk age for pyometra).
    I don’t sell my dogs to irresponsible people so I am not at all worried about accidental breedings and that has not occurred in fact.
    When speaking to my behavior and training clients, I have to wear a somewhat different hat because I don’t want to dispense veterinary advice to clients who will go back to their vet and say, “but our trainer told us not to…” With clients who ask for my opinion, I email them links to the research and tell them to discuss it with their veterinarian.
    In all my years I have never felt that wholesale removal of a huge part of the endocrine system could be a healthy choice unless specific health reasons require it.

  22. What about different ways of sterilization? Have there been studies on those and their effects?

    I really wish I’d known earlier… we lost two goldens to cancer and now I wonder if I just hadn’t had them fixed….

  23. The accidental litter issue is a red herring. A pet owner weighing the spay/neuter pros and cons is already invested in the welfare of their dog – in other words, a responsible owner. They’re unlikely to have “accidental” litters. Spay/neuter for such a person is a lifestyle decision – heat cycles, male marking, etc.

    Most unwanted litters are from intentional breedings, bred by casual owners who hope to make a few dollars, and then find they can’t sell the puppies.

    The overpopulation issue is no longer relevant now that shelters across North America are actually importing dogs from other countries.

  24. I’ve had already made up my mind that I wouldn’t spay my dogs before AT LEAST a year of age. This makes me thing further.

    I guess there is a reason for the saying “If it ain’t broke, don’t fix it …”

    The problem is, that if one adopts, all of these dogs are already spayed/neutered and many of them at extremely young ages.

    Plus, as you know, there are places that are considering mandatory spay/neuter at young ages.

    So the question is, what are we going to do about this problem.

  25. I am astonished that a DVM does not observe the difference between *sterilization* and *gonadectomy.*
    surely it is even easier to perform a tubal ligation on females, and no inherent reason to eliminate the ovary hormones?
    males have possible behavioral considerations, but these could be evaluated later, say at six months, than current practice.
    the only real ‘harm’ in doing so lies in the overpopulation problem–some owners would not be responsible.
    puppy vasectomies?
    and please, why is no one doing more complete studies??

  26. All my dogs have been adopted from shelters, female, and spayed when I got them (~ 8 months, ~ 1.5 yrs old). With my current dog, I do not know if she was a very early spay (8 weeks) or perhaps a bit later (~ 6 months). When I am ready for my next dog, I think I may be taking a closer look at the 2-3 year old females that have had a litter or 2 of puppies and just recently spayed.

    I understand the need for shelters to spay/neuter puppies at an early age to prevent unwanted litters, and to not to rely on an adopters word and/or deposit that they will get the puppy spayed/neutered later.

    I would like to see alternative spay/neuter procedures that do not eliminate the sources of estrogen and testosterone. Yes, they are the main hormones in reproducing, but they also have roles in growth and development; roles we should consider more seriously when there is an option to delay the spay/neuter.

  27. No it will not change my requirements that most of my pups be spayed/neutered. For many reasons, some of which are the following:

    1. Association does not mean cause and affect relationship. Also many such associations gleamed from ‘personal’ reporting such as the survey that formed the bases for this analysis in the end prove to be non associations when other parameters are applied..such as common ancestory ofdogs who have cancer or enviromental influences.
    2. In the original survey, due to previous discussions, I feel many people who responded already had the idea that there was a causal relationship between spay/neuter and health issues. Breeder respondents were not required to enter data on all dogs in a litter or any other mechanism that might help to ensure that the data collected represented some percentage of dogs they have bred. It is easy to say that if I had two spay/neutered dogs who got some sort of cancer and only one intact dog that got some sort of cancer, the data would reflect a higher incidence in the spay/neuered dogs..but not if most of the dogs I had produced/owned were spayed/neutered as compared to those intact.
    3. If you extrapolate from their data, the best time to spay/neuter either a femal or male dog to decrease the chance of the cancers they looked at in the spay/neuter group would be 7-12 months – interesting.
    4. It seems from my experience that breeders and owners often have little idea of what actually caused their dog’s death or what cancer their dog had. Cancer often seems to be grouped in their minds as one thing, where even a benign tumor is cancer. Also when you ask many people about the cancer their dogs had, there was no necropsy done on the dog nor histo-pathology to make the actual diagnoses. This to me makes owner/breeder surveys suspect.
    I hope that is looked into further because out it all, what I would want to know is when is the best time to spay/neuter for the chances for the best overall cancer and other health outcomes. I think it will always be a risk management choice of both health and population control issues.

    What we need in my opinion, is more patho-physiology research and less survey data into these issues; less stabbing in the dark at looking at two or three parameters and then assigning importance to associations between them when often it seems more red herrings than real answers are being seen.
    Just my opinion.

  28. There is no doubt that this data is compelling and definitely going to be something that I speak with my clients about. There has been enough similar data coming out there is something here we can’t ignore.

    However, one of my main concerns about these kinds of studies is that there seems to be no data or understanding of how owners who choose to neuter or not neuter their dogs may approach veterinary care for their pets differently.

    What if owners who do not have their pets spayed and neutered just do not get as much veterinary care in general? If they come to the vet less often, decline more tests, euthanize pets before finding out the underlying cause – there will be a delayed and decreased discovery rate for all diseases, fundamentally and systematically biasing the data.

    The only ways of working around this would be to look at the difference between owners of neutered and unneutered dogs and see if their veterinary behavior does differ – OR to have a randomized and controlled study of dogs in a controlled environment to see if the same changes and incidence of disease occurs with controlled levels of veterinary care.

    I would just break my heart to start losing ground with accidental litters and reversing pet overpopulation based on biased conclusions.