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Pain, cancer, obesity: Disturbing links uncovered in a decade's worth of research


We owe it to dogs and their owners to give them all the information about the optimal age for spaying and neutering their pet.Benjamin Hart, DVM, PhD, DACVBJanuary 2022
Pain, cancer, obesity: Disturbing links uncovered in a decade's worth of research

Benjamin Hart, DVM, PhD, DACVB

Dr. Benjamin Hart is distinguished professor emeritus at the School of Veterinary Medicine at University of California, Davis, where he developed the first U.S. veterinary medical teaching and research program in companion animal behavior. In addition to studying early spay-neuter effects on diseases in various breeds of dogs, he and his wife, Lynette Hart, PhD, have researched elephant and antelope behavior in Africa. He is a fellow of the Animal Behavior Society and the International Society of Anthrozoology, and in 2013 he received the Bustad Companion Animal Veterinarian of the Year Award from the American Veterinary Medical Association.


Not long after my wife and I started researching spay-neuter risks in dogs about 10 years ago, veterinarians began to ask us what, exactly, we were up to. After all, they had been telling clients for decades to spay or neuter their pets before adulthood to prevent unwanted behaviors and unexpected litters, and it was all very simple and straightforward. And here we were upsetting the apple cart. Shelters were not happy with us either.

But we had to make the truth known — because the news wasn't good. In golden retrievers, Labrador retrievers and German shepherds, we found a number of joint problems and cancers associated with spaying and neutering at an early age.1-3 And we felt we owed it to the profession and to dog owners to put that information out there.

How it started

Before embarking on this undertaking, I had done some research on the behavioral effects of neutering on dogs.4-6 Knowing this was an area of interest for me, the director of the Center for Companion Animal Health at the University of California, Davis, called me into his office and said, "We're hearing about some signs of trouble with current spay-neuter protocols for dogs. Would you look into it?"

I had been hearing the same things, so I agreed. I submitted a proposal to the American Kennel Club (AKC) Canine Health Foundation, and they saw the value of the research and agreed to support it, with additional support from our Center for Companion Animal Health.

It's not easy to do a prospective study on this topic. For example, if you had 50 dogs, then spayed or neutered some and left some intact, you would have to follow them for the next 10 years. So we turned to our computerized hospital records. At UC Davis we have the largest veterinary caseload in the world, consisting of 60,000-plus cases per year. And these records all reflected consistent diagnostic criteria for identifying disease. For most of the neutered dogs, the exact age of neutering was not included in our clinical records, so our team needed to call referring veterinary clinics to acquire this information on each patient.

We looked for breeds that were well-represented in that database: golden retrievers, Labs, German shepherds, Chihuahuas — we stuck with breeds we had the most data on so we could do statistically valid comparisons between dogs that had been spayed or neutered and those left gonadally intact. And that's when we started to see the association between early spay-neuter and certain health problems.

When we analyzed the records, we examined each one individually to see how the joint disorder or cancer had been confirmed. For example, a cancer had to be diagnosed by biopsy; the clinician couldn't just guess. It was quite labor-intensive. We hired students and trained them to do the analysis. One student would take one breed and stay with it for about three or four months before finishing. We met weekly so the team of six or seven students could report to the committee.

Once we completed 35 breeds (including the varieties of poodle), we decided to wrap it up. By this point, in addition to joint problems and certain cancers, we had validated a link with urinary incontinence as well.7 However, my coauthor (who also happens to be my wife) said we had to look at mixed breeds too, because that's what most people actually have. But how could we do that when our findings were so breed-specific?

We decided to divide the mixed breeds by weight, and we came up with some guidelines for owners: With dogs over 40 pounds, be on the lookout for joint disorders.8 We did not find an association with cancers, because cancer is too individualized, with certain breeds more susceptible to certain types.

The obesity link — or not

We have often been asked about a link with obesity. Specifically, if spayed and neutered dogs weigh more, could that be the cause of their joint disorders? So we looked at the average weight of the dogs, and the spayed and neutered dogs did indeed weigh more.

But then we looked for a correlation between dogs that developed joint disorders and those that were obese. We didn't find one. A higher body condition score certainly makes a joint disorder worse, and obesity may predispose dogs to other health problems, but neutered or spayed dogs of normal weight got joint disorders at the same rate as those that were obese. So after the first couple of breeds, we dropped that analysis because it wasn't going anywhere.

A paradigm shift

When I present our research at veterinary meetings, I propose that we undergo a paradigm shift in the profession. We're not saying dogs should not be spayed or neutered. What we're proposing is that veterinarians discuss the age of spaying or neutering with the client according to what the data suggest is safest. Then the client gets the last say.

For example, with Labrador retrievers, the most popular breed, females spayed before the first year of age have about double the risk of intact females for one or more joint disorders.5 This suggests that owners should wait until after the first year to spay their female Lab.

"What we're proposing is that veterinarians discuss the age of spaying or neutering with the client according to what the data suggest is safest. Then the client gets the last say."

With some dogs, age doesn't make a difference. Small breeds such as terriers, Chihuahuas, Yorkies and so on show no correlation between early spay-neuter and joint disorders or cancers. So if you have a Chihuahua, it's different than if you have a golden retriever, Dalmatian or standard poodle.

The paradigm we're suggesting is that when the subject of spaying or neutering comes up with a puppy owner, the veterinarian say something like this: "We know there are some breed differences in terms of optimal timing. Let me look up your breed and we'll see what the evidence shows, then I'll suggest the appropriate time to do the spay or neuter procedure."

The client ultimately makes the decision. If clients say, "No, we have to do it at 6 months," they get their wish. It's like human medicine, where we see much more patient-driven decision-making today than we used to. We're asking veterinarians to give dog owners the dignity of making their own decision once they have the information in front of them. There are some breeds where the data suggest delaying the spay or neuter for two years to avoid increasing the risk of a joint disorder or cancer. We have found that most people will choose to wait, watch their dog carefully until about 2 years of age, and do the spay or neuter procedure then.

If going through estrus twice a year becomes a problem, the client can bring the dog in at a year and a half to do the spay. It becomes an individual decision that depends on the dog and the owner. The days of carte blanche "Bring your dog in at 6 months and we'll line it up for a spay or neuter" should be over.

By taking this approach, veterinarians increase their level of client service. This is an area of practice that has been neglected, with clients of late simply taking their dog to a spay-neuter clinic and getting it out of the way. With a more individualized approach, spay-neuter becomes something you talk about with clients, something where you show personal interest in this specific client and this specific dog. And a personalized approach improves your client relationships.

"With a more individualized approach, spay-neuter becomes something you talk about with clients, something where you show personal interest in this specific client and this specific dog."

What's best for the dog?

Even more important, we have to look at what's comfortable for the dog. Joint disorders, especially canine cruciate ligament disorders, can be quite painful, not to mention expensive to treat with surgery. By recommending delayed spaying or neutering, you may be saving the client a major expense and saving the dog from going through a painful procedure.

Sometimes welfare groups or shelters will ask about unwanted pets resulting from delayed spaying and neutering. However, owners have become much more responsible, and many parts of this country are seeing a shortage of dogs available for shelter adoption. So we don't anticipate that a careful, individualized strategy of safe spaying and neutering will result in stray dogs roaming around.

In conclusion

Again, I want to emphasize that we're proposing a paradigm shift: Rather than automatically spaying or neutering at 6 months or earlier because that's what responsible pet owners do, look at the breed and the gender and consider the optimal age for sterilization. Give that information to clients so they have the option of making their own decision for the welfare of the dog.

After all, this is a long-term relationship. If an owner will be with a dog for the next 15 years, that's a serious commitment. It's worth taking the time for nuanced discussion. Clients will think more highly of veterinarians who offer options and point out the intricacies of the decision rather than proposing the same solution to everybody. If veterinarians pay attention to each client's dog, the dog's breed and the owner's ability to wait for sterilization, they'll gain respect. It will improve their reputation among their clients, and it will be better for the veterinary profession as a whole.

  1. Hopkins SG, Schubert TA, and Hart BL. Castration of adult male dogs: Effects on roaming aggression, urine marking, and mounting. Jour Am Vet Med Assoc. 1976;168:1108-1110.
  2. Hart BL. Behavioral effects of castration. Canine Pract. 1976;3(3):10-21.
  3. Neilson JC, Eckstein RA, Hart BL. Effects of castration on behavior of male dogs with reference to the role of age and experience. Jour Am Vet Med Assoc. 1997;211:180-182.
  4. Torres de la Riva G, Hart BL, Farver TB, et al. Neutering dogs: Effects on joint disorders and cancers in golden retrievers. PLoS One 2013;8(2):e55937.
  5. Hart BL, Hart LA, Thigpen AP, et al. Long-term health effects of neutering dogs: Comparison of Labrador Retrievers with Golden Retrievers. PLoS One 2014;9(7):e102241.
  6. Hart BL, Hart LA, Thigpen AP, et al. Neutering of German Shepherd Dogs: Associated joint disorders, cancers and urinary incontinence. Vet Med Sci. 2016;2(3):191-199.
  7. Hart BL, Hart LA, Thigpen AP, et al. Assisting decision-making on age of neutering for 35 breeds of dogs: Associated joint disorders, cancers, and urinary incontinence. Front Vet Sci. 2020;7:388.
  8. Hart BL, Hart LA, Thigpen AP, et al. Assisting decision-making on age of neutering for mixed breed dogs of five weight categories: Associated joint disorders and cancers. Front Vet Sci. 2020;7:472.

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