A breakthrough interview on how sterilization in dogs impacts health with expert biologist and researcher Linda Brent, PhD
For years now, I have felt uneasy about spaying and neutering dogs. My questioning of this practice started with spending more time in Europe, where spaying and neutering your dog is much less common. In contrast, in Canada, it was very rare for me to see a dog that wasn’t neutered, and I remember that society looked at those who didn’t spay or neuter their dogs as irresponsible.
I have always been against the early spaying and neutering that many major shelters and humane societies practice. It made no sense to me that young animals who needed hormones for their overall health and muscular, skeletal and brain development were being sterilized.
But I also started to see that even when we spay and neuter dogs later in life, they start losing muscle mass, their tendons and ligaments are more injury-prone, and there are significant changes in their behaviour — such as increased fearfulness, anxiety, and sometimes aggression.
When I decided to neuter my dog Pax two years ago, I didn’t have a full awareness of spaying and neutering. One of the requirements for Pax to become certified as a service dog for my sleepwalking was for him to be neutered.
Before he got neutered, he was solid, strong, playful, and ran super fast. It was about six months after his surgery when he injured his psoas muscle while running on the beach. I also noticed him becoming less interested in playing with other dogs. The fur around his neck got shorter and less thick, and I could see a change in his body shape and tone. Something that many people probably didn’t even notice.
This all prompted me to embark on the search for an expert who has researched the health effects of dog sterilization. That’s how I’ve found Linda Brent, PhD, MBA, an expert biologist who has dedicated the past ten years to research in this area through the Parsemus Foundation, a non-profit organization focusing on health-related research in animals and people, especially in areas neglected by conventional medical research entities.
The following blog presents the key insights from my interview with Dr. Linda Brent. If you want to listen to our full discussion on spaying and neutering and its impact on dogs’ health and wellness, you can watch the interview on my YouTube channel.
Conventional spaying and neutering disturbs the natural hormone feedback mechanisms and can lead to life-altering health and behavioural issues and shortened lifespans in dogs
Dr. Peter Dobias: What challenges have you identified in conventional spaying and neutering methods? How do they impact pet health, specifically cats' and dogs’ health?
Dr. Linda Brent: The biggest issue is that people don't think about spaying and neutering as affecting their pet's hormones. You're removing the sex organs from the animal so it can no longer produce the kinds of hormones that any normal animal would have throughout most of its life.
However, in North America in general, it's become such a standard that people haven't come to grips with the idea that there might be health implications to doing this. If you were talking about humans, for example, a two-year-old kid who, for some reason, lost the ability to have a normal endocrine system or normal hormones, everyone would be alarmed and acknowledge the impact of such a deficit.
Every animal, including humans, needs the right kind of hormones to grow and develop and not have diseases. So it's funny when we think about our pets that we don't also consider that the same issues come into play.
If you take the natural hormones out of the dog, there are going to be some long-term health implications. The research is interesting because, in the US, we didn't start massive wide-scale spaying and neutering until the 60s to early 70s, when pet overpopulation became an issue. So it hasn't been all that long.
It was years before anybody even started doing a lot of research on this topic. We're honing in on some of the most problematic issues, like cancers and joint issues. These are debilitating, life-ending problems that happen much more commonly in dogs that are spayed and neutered than in intact dogs.
There's a significant impact depending on how large the dog is, what breed it is, and the overall genetic background.
Dr. Peter Dobias: Can you explain how the hormonal axis works, from the hypothalamus to the pituitary gland to the testes or ovaries and why we see these challenges when we remove the testicles/ovaries?
Dr. Linda Brent: While the testicles or ovaries are the organs that produce the hormones, there's a whole cascade of chemical reactions that have to happen to have that work — starting with the hypothalamus and that influences the pituitary gland and then that puts chemicals into the blood system that would tell the ovaries or the testes to produce estrogen or testosterone or not.
What happens when you take those organs away is that the system keeps going because there's no feedback from the hormones in the body to tell it to stop. One of the products of that is the luteinizing hormone (LH), which is one of the signalling chemicals that tells the organs what to do. This is called a feedback loop. When that feedback loop is disrupted because you've stopped the feedback of the normal hormones, it just grows and grows and grows.
One of the collaborators I've worked with very closely, Dr. Michelle Kutzler, has been at the forefront of understanding the mechanism of why spaying and neutering would have a negative effect (1). She has found that luteinizing hormones aren't just for sexual function. There's also a luteinizing hormone receptor. These receptors are in many tissues in the body. When you have something so out of balance, it can drive tissues to act abnormally.
Dr. Peter Dobias: Are early spaying and neutering in dogs more damaging?
Dr. Linda Brent: There's a recent study that Dr. Chris Zink published in the Journal of American Veterinary Medical Association in 2023 that looked at intact dogs, spayed and neutered dogs, and the outcome was that dogs who had access to normal hormones for longer had fewer health complications and lived longer (2).
Dr. Peter Dobias: You said there is a difference between small and large dogs in the effects of neutering and spaying. Have you seen larger dogs suffering more than smaller dogs?
Dr. Linda Brent: The data is pretty clear that larger dogs generally suffer more from spaying or neutering as they have a longer trajectory of growth. You end up with all sorts of joint disorders when you've put their whole hormone system out of balance.
Many breeders and veterinarians now say if you have a large dog breed, or especially a giant breed, you need to wait at least 18 months, if not two years, until their growth plates have closed on their bones. This way, dogs don't get this extreme elongation of bone structure, which is the cause of some of the joint disorders that they end up with later on.
The best advice I could give somebody who has a puppy and is wondering if they should spay or neuter their dog is to look at their dog and consider its genetic background and disease susceptibility. For example, if it’s a breed like golden retrievers that are more susceptible to cancer.
Also, you should look at the risks and benefits of the different methods that you might be able to choose. Some people probably wouldn't do any kind of sterilization. If you have a small dog and it stays in an apartment most of the time, or it's on a leash the rest of the time, maybe that's the easiest thing.
Dr. Peter Dobias: I'm originally from the Czech Republic, and I spend quite a bit of time there now. Most dogs are not neutered and get along in the park off-leash. I see such a massive difference in how these dogs behave and how they interact with each other. It breaks my heart to see how many dogs in North America have behavioural issues, such as fearfulness, anxiety, and aggression. Is this related to hormonal imbalances?
Dr. Linda Brent: Yes, many behaviour problems are more common in dogs that have been spayed or neutered.
I've had purebred dogs that I didn't spay or neuter until they were quite old. I’ve also had rescue dogs and one of my rescue dogs was probably neutered when he was just a couple of months old. He didn’t have health problems throughout his life, but he developed severe dementia when he was older, which is a behavioural symptom of being spayed or neutered, and we had to euthanize him because of that.
Hormome-preserving sterilization can provide an alternative that removes dogs’ ability to reproduce without disrupting hormone production
Dr. Peter Dobias: Could you explain the concept of hormone-preserving sterilization and its benefits over traditional methods? How can it be done and what do you recommend?
Dr. Linda Brent: Instead of removing the ovaries and the testicles, you can remove the ability to reproduce without messing up the hormones. We call it hormone-sparing sterilization. Some people, for female dogs, call it ovary-sparing spay or hysterectomy.
Without a uterus, a dog cannot reproduce, but you leave the ovaries intact. The entire uterus is removed, including the cervix, so there's no chance of pyometra, which can be a life-threatening disease. This is one of the reasons that veterinarians often say you should get your female spayed, but there is a way to sterilize your pet without removing the hormones and still protecting them from pyometra.
Dr. Peter Dobias: When it comes to the two sterilization options, whether you do tubal ligation or remove the uterus, has anyone done studies on the frequency rate of pyometra?
Dr. Linda Brent: I don't know that it would be any less with tubal ligation than with an intact dog. Pyometra can be life-threatening, and a lot of times, people don't realize that it doesn't have a lot of symptoms until it's serious.
We prefer to recommend the removal of the uterus or a hysterectomy rather than tubal ligation. In all of our conversations with veterinarians, it would be tough to find a veterinarian who would recommend tubal ligation instead.
Dr. Peter Dobias: When it comes to blood spotting in female dogs, does it happen when the uterus is removed?
Dr. Linda Brent: This is why it's important that the veterinarian has experience and understands that all of that tissue has to be removed. But if you're still having the problem later, and especially if the dog is showing spotting at regular intervals, as it would be in heat, then it would probably make sense that the dog would need to be looked at again to be sure that there are no tissues left there.
We have a lot of information on our website, including videos, discussions, and papers specifically for veterinarians if they're interested in learning the techniques if they haven't been familiar with them in the past.
We also have a number of veterinarians, through the Parsemus Foundation, willing to advise specifically to new vets who are interested in it. We give people a lot of support so they can move into this new area.
Hormone replacement therapy may help improve health and quality of life in spayed and neutered dogs
Dr. Peter Dobias: I know you've done a hormone therapy study. What were the key health improvements observed in the neutered male dogs? Are there any side effects?
Dr. Linda Brent: Just to be clear, for male dogs, prostate cancer is actually higher if they're neutered. But for male dogs that have already been castrated and have had hormone therapy, and their hormones have been restored to a more normal level, prostate cancer is still the main thing that you need to watch for.
My dog Toby has now been on testosterone therapy (3) for four years since he was four and a half years old. He’s eight and a half now and when I take him for his annual exam with the veterinarian, I make sure that he has at least a manual prostate exam.
As he gets older, when you would expect the prostate to grow as it does in humans and potentially cause problems, you can have an ultrasound or other diagnostics done just to double-check that there are no issues going on.
Dr. Peter Dobias: How has your dog aged after being on hormone replacement therapy? What changes are you seeing?
Dr. Linda Brent: He's perfectly fine. There have been no side effects from the testosterone therapy. He's on a weekly injection with an insulin-sized needle. So it's an incredibly tiny amount. He has also been on a GnRH agonist because even with supplementing testosterone, the LH was still very high.
Originally, his LH was in the 80s, while the normal range for a male dog is zero to three for an intact dog. Even when we put him on testosterone therapy, the LH level was still outside the normal range. It went down to 30, but it was still high. With the GnRH agonist, the gonadotrophin-releasing hormone agonist, which overstimulates the pituitary and stops the flow of LH, his LH is now down to a normal level.
I get Toby’s blood work done regularly, but I don't check his hormones any longer because he's been on a standard testosterone dose all this time and hasn’t had any issues at all.
Before the hormone replacement therapy, he couldn’t jump up on you like a healthy dog would do, he was in constant pain and became horribly anxious. He was petrified and we couldn't take him anywhere. He was so scared of everything.
We spent the next three years, like many people do, trying one therapy after another. He's been on all sorts of medications and training, diets and everything else without any results.
I was fortunate because of my job. I was familiar with this background and knew some veterinarians who could counsel me and help my local veterinarian start this testosterone therapy regimen on him.
Within a couple of months, his entire body composition changed. Before we started, he was emaciated in his hips because he just couldn't use his legs very much. Within about three months, it was entirely noticeable how his ability to move became better, and he started running around.
It was interesting because, at about the same time, we had gotten a new dog that was younger than him and wanted to play. What really broke our hearts was that we had this dog who wasn’t old, but he couldn't run. He could hardly play. He was limping, and we had him on pain medicine all the time. It was awful.
Then, after a couple of months, he was totally changed, and you wouldn't even know now that he had any kind of hip issues. His weight issue also stabilized as he was overweight before. His anxiety took longer to come around, I would say several years. Overall, he is in better shape today than he was when he was a year and a half old.
Dr. Peter Dobias: Are there any behavioural changes if you start replacing testosterone in male dogs? Do dogs become more aggressive or more territorial?
Dr. Linda Brent: The only behavioural change we noticed in my dog, because we have two dogs, was that he started mounting the other dog, but it was transient. It didn't last.
Dr. Peter Dobias: Is there currently an established testosterone dose per pound or per dog? How do you dose it currently?
Dr. Linda Brent: You know, it's tricky. We did a lot of research on both animal and human dosage and what works. Toby is on 0.5 mg/kg weekly.
We tried it monthly, but his LH or his testosterone went really high and then really low. We wanted it to be levelled off. So, a weekly dose works great.
Dr. Peter Dobias: Do you do hormone replacement therapy in female dogs as well?
Dr. Linda Brent: Our foundation hasn't done any studies yet because female dogs get incontinent quite frequently when they're spayed. There is at least a small body of literature on using various types of estrogens and sometimes GnRH agonists to reduce LH, which helps.
For most dogs, you can pretty much get things under control by replacing what they lost. It doesn't seem to work quite well in males, at least with the little bit of literature there is for incontinence in male dogs. But, because of that, at least veterinarians are usually familiar with the concept.
We hope that if a female dog is having similar problems and you go to a veterinarian, you'll have an easier time potentially finding a vet that would offer hormone therapy. There's just such a lack of knowledge on hormone replacement therapy in male dogs, which is why we've kind of concentrated in that area.
Better health outcomes for neutered and spayed dogs start with educating dog parents and veterinarians about alternative sterilization methods
Dr. Peter Dobias: How do you plan to raise or how have you been raising awareness among pet owners and veterinarians about the alternatives to traditional?
Dr. Linda Brent: We've gone to a lot of veterinary conferences. I've given a presentation about this with other veterinarians talking about hormone-sparing sterilization. Our website, parsemus.org, has tons of information. We're one of the main repositories for research on this topic.
The other thing we do is fund studies, and we try to encourage people and veterinarians to try new sterilization methods and get the word out. We have a whole veterinary directory of people who offer alternatives to traditional spaying and neutering.
Dr. Peter Dobias: How do you envision the future of pet care, especially in terms of reproductive health?
Dr. Linda Brent: It's a matter of time before we all think about our pets in a different way. We're already on that path where we think of their lifetime health and nutrition
People are starting to realize they can't just give them this stuff in a can anymore. They are questioning, and that trickles down to all the different areas of health, just like we think of our own health in many different ways. I'm very optimistic that we're on the right path and that it is about educating people.
We're in an age where it's easy to find information and organizations like ours. Parsemus Foundation is a non-profit and we are here to help people and support the professionals in the area so that they can learn and grow and offer new treatments to their patients.
Dr. Peter Dobias: What advice would you give veterinarians regarding discussing sterilization options with pet owners?
Dr. Linda Brent: Being open-minded and giving individual care is critical. When you're talking about something as important as sterilization, it’s not a one-size-fits-all. The quicker people come around to that concept and take the time to understand each individual client they have, and what may work best for them, and look at the risks and benefits just like they might with any other type of procedure they do, that's going to work the best.
In our opinion, the demand from the public far outweighs the number of veterinarians offering this. So, as a veterinarian or even as a shelter or rescue, if you embrace the newer methods that consider the individual lifetime health of the animal, you're going to do well. There's an enormous amount of demand from the public.
We are contacted every day by people looking for a veterinarian in their town or in their state to do a certain procedure. That's why we have the directory, but it will separate veterinarians in terms of the ones that are in the most demand and those that aren't.
Dr. Peter Dobias: Reflecting on your career, what has been the most significant learning and insight regarding pet health?
Dr. Linda Brent: Don't accept the status quo. Just because it's something that has been done or people say is the only way to do it, that might not be true. Always question, always educate yourself. To me, that is the most important thing because that covers all the areas.
Many thanks to Dr. Linda Brent for her invaluable insights and I hope our discussion has provided you with a better understanding of how spaying or neutering affects our dogs’ long-term health and well-being and will help you make an informed decision about when or if you plan to get your dog neutered or spayed.
(1) Animals (Basel). 2020 Apr 1;10(4):599. doi: 10.3390/ani10040599. Possible Relationship between Long-Term Adverse Health Effects of Gonad-Removing Surgical Sterilization and Luteinizing Hormone in Dogs. Michelle A Kutzler
(2) Volume 261: Issue 3. doi.org/10.2460/javma.22.08.0382. Vasectomy and ovary-sparing spay in dogs: comparison of health and behavior outcomes with gonadectomized and sexually intact dogs. Chris Zink DVM, PhD, DACVSMR, Mikel M. Delgado PhD and Judith L. Stella PhD
(3) Top Companion Anim Med. 2021 Nov:45:100565. doi:10.1016/j.tcam.2021. 100565. Epub 2021 Jul 28. Restoration of Reproductive Hormone Concentrations in a Male Neutered Dog Improves Health: A Case Study
