Phil Bushby, DVM, MS, DACVS Dr. Phil Bushby holds the Marcia Lane Endowed Chair of Humane Ethics and Animal Welfare at the Mississippi State University College of Veterinary Medicine, where he has served on the faculty for 44 years. For more than 20 years, his primary focus has been taking veterinary students to animal shelters in north Mississippi to provide basic wellness care and spay-neuter services.

Those in the veterinary profession who have heard me lecture over the years know that I am a strong advocate of early spaying and neutering for one primary reason: It increases an animal's adoptability, helping it get out of the shelter and stay alive. In addition, of course, it helps control overpopulation, limiting the number of homeless pets coming into shelters in the first place.

Recent research out of the University of California, Davis, by Dr. Benjamin Hart and others has prompted many of us in shelter medicine to reexamine our recommendations in light of findings related to orthopedic risk and other potential health problems. And for the most part, I still hold the same opinion — early sterilization saves the lives of dogs and cats.

However, for certain animals in certain situations, a more nuanced approach may be necessary. Let's take a closer look, starting with large-breed male dogs.

Owned large-breed male dogs

There is enough evidence out there to consider that certain large-breed male dogs may be at increased risk of orthopedic problems if they are castrated prior to the cessation of growth,^1-5 which occurs at 12 to 15 months of age in large breeds. In light of this evidence, even the traditional standard of castration at 6 months may increase the risk of orthopedic injury.

However, there's another side to the coin, and that's behavioral issues associated with male hormones. Therefore, I think a veterinarian is obligated to talk to clients about the risks of castrating before an animal stops growing and the risks of not castrating before sexual maturity.

If the owner will be upset by the dog continually trying to hump somebody's leg, then castration before sexual maturity is called for, because it will solve or significantly reduce a behavioral issue that bothers the owner. However, the owner should be informed that this may increase the risk of certain orthopedic problems later in life.

Sheltered large-breed male dogs

I still support castration prior to adoption in large-breed male dogs in shelters. If we want to guarantee that a dog will never develop an orthopedic problem, then we kill the dog before it ever gets out of the shelter. After all, it will never have an orthopedic problem if it's dead.

Sarcasm aside, we know animals are much more adoptable if they're spayed or neutered. And a dog that's alive with a slightly higher risk of orthopedic problems is preferable to a dog that's dead.

Yes, we have made tremendous progress decreasing the euthanasia of shelter animals, having gone from 8 to 10 million euthanasias annually 30 years ago to 1 to 2 million now. But many of us fear those numbers could go back up. So, in large-breed sheltered male dogs, I still support castration prior to adoption, regardless of age. That means if a shelter adopts dogs out at 8 weeks of age, they should castrate at 6 or 7 weeks.

"If we want to guarantee that a dog will never develop an orthopedic problem, then we kill the dog before it ever gets out of the shelter. After all, it will never have an orthopedic problem if it's dead."

Owned large-breed female dogs

Now let's look at females. For large-breed female puppies in families, the owner needs to be involved in the decision — again, there is no one-size-fits-all recommendation. As with males, we're highly suspicious that sterilizing large-breed females prior to cessation of growth increases the risk of certain orthopedic problems. But we absolutely know that allowing the dog to have even one heat cycle significantly increases the chances of mammary neoplasia later in life.^6-7

The decision again is a balance between risk and benefit. The owner needs to be aware that if their large-breed female dog is spayed at 4 or 5 months of age, she has an increased chance of cruciate rupture or hip dysplasia down the road. And if she's not spayed at that age, she absolutely has an increased chance of mammary neoplasia.

Of course, there's an inevitable question every veterinarian will face in this scenario: "What would you do if it were your dog, Doc?"

For me, I spay early. I have two almost-14-year-old female dogs that are half-Doberman, half-Labrador retriever, and I spayed them when they were 6 weeks old. My personal recommendation still leans toward preventing mammary neoplasia and taking the risk of the orthopedic injury as opposed to reducing the risk of orthopedic injury and increasing the risk of mammary neoplasia.

Sheltered large-breed female dogs

With large-breed female dogs in shelters, I recommend spaying before they're adopted. As with males, the risk in this situation is between rupturing a cruciate ligament at 6 or 7 years of age and shelter euthanasia at 4 months because the dog hasn't been adopted.

Bottom line, in the shelter, we should spay and castrate everything — large-breed dogs, small-breed dogs and cats — prior to adoption.

Owned small-breed dogs

For small-breed dogs, we don't have the same level of suspicion about orthopedic risk. We do have the same level of suspicion about mammary neoplasia and the same behavioral challenges in intact animals. So, in owned small-breed dogs, I advise spaying and neutering prior to 5 months of age.

Why 4 or 5 months of age? Why not 6 or 7 weeks, as we do in shelters? Because it's best for the animal to be fully vaccinated before surgery. The two most dangerous places where a dog can catch an infectious disease are the animal shelter and the veterinary clinic. Do I want to hospitalize an 8-week-old puppy for an elective surgery and have it catch an infectious disease because it's not fully vaccinated? Absolutely not.

Have owners get their dogs vaccinated first. The series are generally over by 16 weeks of age. Then the spay or neuter procedure can be completed between 4 and 5 months of age.

Sheltered small-breed dogs

When a dog is already in the shelter, you want to get it out of the shelter as fast as possible. Spay or castration helps remove that animal from the risk of both infectious disease and euthanasia. If you sterilize at 6 or 7 weeks, the animal has the best chance of eluding both risks.

Cats

I support the spaying and neutering of sheltered cats at 6 or 7 weeks to speed adoption and of owned cats at 4 to 5 months so they receive their full vaccination series first. However, I often encounter concern from veterinarians who believe castration of young male cats predisposes them to urinary obstruction.

The theory is that the penis is smaller, so logically the urethra will be smaller as well. And if you want to create a plumbing problem, have a large pipe go into a teeny pipe. Many veterinarians still will not castrate a cat before 5 months of age because of this fear.

This "prepubertal castration leads to urinary obstruction" myth continues to circulate despite the fact that it has never been proven. It's a widespread belief, but there is nothing in the literature to document it. The few long-term studies that have looked at the health of cats castrated at an early age have not identified an increased incidence of urinary obstruction later in life.

So, castrate male cats before 5 months of age. They're sexually mature at that point and can impregnate a 5-month-old female cat. Why wait until you're making the overpopulation problem worse?

Another fear is whether the orthopedic issues reported in large-breed dogs also occur in cats. Again, there is no evidence that they do. In fact, an article published earlier this year looked at owner-reported mobility issues in cats and the factors associated with increased incidence.⁸ It's not a strong study, but here's what it found: Outdoor cats, cats with a history of trauma and cats that were obese all showed a tendency for increased mobility issues.

Cats sterilized prior to 5 months of age had a decreased incidence of mobility issues. Again, this was not a strong study, and what is a "mobility issue" anyway? But the fear that cats may have orthopedic issues is not documented in the literature.

We know that dogs and cats (and people) grow because of growth plates in the long bones: femur, tibia, humerus, radius and ulna. And when they reach sexual maturity, these bones stop growing. We know gonadal hormones play a role in stopping growth of the long bones and early removal of those hormones delays closure of the growth plates. So maybe the cat sterilized at 3 months of age grows a quarter-inch taller than if it had never been sterilized or was sterilized at a year and a half.

While the logic in large-breed dogs is that increased bone length changes the angles of the joints, potentially leading to increased risk of cruciate rupture, there's no documented clinical significance of the cat growing a quarter-inch taller. We know it is a physiological effect, but it's not a clinically important one.

In summary

The grand summary is that we must consider population issues and individual animal issues. As for individual animal issues, we must always consider a risk-benefit analysis: does early spay-neuter cause problems for the animal or prevent problems? Does delayed spay-neuter cause problems for the animal or prevent problems?

Based on what we know today, the only situation in which delaying sterilization may benefit the individual animal is in owned large-breed male dogs. And here the potential orthopedic issues must be balanced against potential behavior issues, so it has to be an individual decision. In every other situation, spaying and neutering prior to 5 months of age most likely supports both population health and individual animal health.

No longer black-and-white

For decades we in the veterinary profession spayed and neutered at 6 months of age or older. We didn't have a scientific basis for choosing 6 months; it may have just been an arbitrary number that made the process simple. Now we have more data. While that's a good thing, it has created confusion because we no longer have a one-size-fits-all, universally accepted approach.

Six or seven years ago, I recommended sterilizing everything before 5 months of age. I've toned it down with large-breed male dogs because of information mostly from Dr. Hart's articles. I've been critical of these studies because they're retrospective and based on small numbers from an atypical referral institution caseload. But they've added to the knowledge pool we have. And so we have to take the information for what it's worth and adjust our approaches appropriately.

Back in 2004, in the midst of the Iraq War, Defense Secretary Donald Rumsfeld said, “You go to war with the army you have." It's the same with spaying and neutering. We have to make decisions based on the knowledge we have, knowing that our information is incomplete, some of it may be inaccurate and we might change our minds tomorrow if new information arises. Today, we're doing our best for dogs and cats with the data we have.

1. 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.
2. 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.
3. 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.
4. 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.
5. 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.
6. Hayes HM, Milne KL, Mandel CP. Epidemiological features of feline mammary carcinoma. Vet Rec. 1981;108:476-479.
7. Dorn CR, Taylor DON, Schneider R, et al. Survey of animal neoplasms in Alameda and Contra Costa counties, California. II Cancer morbidity in dogs and cats from Alameda county. J Natl Cancer Inst. 1968:40:307-318.
8. Maniaki E, Murrell J, Langley-Hobbs SJ, et al. Associations between early neutering, obesity, outdoor access, trauma and feline degenerative joint disease. J Feline Med Surg 2021;23:965-975.