Groundbreaking Study Released on Sterilization Alternatives

By Dr. Karen Shaw Becker

In North America, spaying and neutering (aka desexing) of dogs is used as a method to help address the pet overpopulation problem. In dogs, desexing of males is also thought to prevent aggression and other undesirable behaviors, and in fact, many apartment complexes, boarding kennels, and dog parks require that dogs entering their facilities be desexed.

However, given the mounting evidence that desexing may not be appropriate in every instance, animal health organizations such as the Morris Animal Foundation and the American Veterinary Medical Association (AVMA) have begun to back away from the 1970s-era orthodoxy that called for early spay/neuter for every pet in the U.S.

Back then, both veterinarians and responsible pet owners signed on wholeheartedly to the spay/neuter movement out of a genuine desire to help solve the terrible problem of unwanted pets, tens of millions of which were euthanized in shelters each year.

Today, 31 states and the District of Columbia require that pets adopted from shelters be spayed or neutered before they leave the facility, or that adopters contractually agree to have the procedure performed within a certain timeframe. The ASPCA (American Society for the Prevention of Cruelty to Animals) and other animal welfare organizations also advocate early desexing of all companion animals.

To animal welfare groups charged with managing unwanted pets, the current spay/neuter strategy makes sense. They are necessarily focused on the big picture, and the most effective way to reduce the homeless pet population is to prevent pregnancy.

Whereas individual pet owners need only make sterilization decisions for the dogs in their care, animal welfare organizations must make those decisions on a much larger scale, for the good of the many rather than the few.

Fact: Spays and Neuters Aren’t Harmless Procedures

Spaying (which is a hysterectomy plus ovariectomy) in female dogs removes the uterus and the ovaries, which obviously prevents pregnancy, but which also halts the production of important female hormones that influence the chemical balance of the body and the brain and may even affect memory.

Neutering of male dogs, aka castration, removes the testes, which again prevents puppy-making, but also halts the production of essential male hormones such as testosterone. Lack of testosterone can cause significant physical and behavioral changes and may also alter cognition.

Once an advocate for the desexing of every dog, I’ve since gone very public about my concerns for the health of my own spayed/neutered patients, as well as the emerging science that suggests desexing may increase the risk of both physical and behavioral problems in some dogs.

A growing body of scientific evidence indicates that spaying and neutering dogs, especially large and giant breeds, and especially at an early age, increases the risk for a wide range of long-term health problems, as outlined in the following table:

I routinely discuss sterilization alternatives to desexing that preserve the gonads (ovaries and testes) and the hormones they produce, including ovary-sparing spays and vasectomies. One of the main barriers to putting these alternative sterilization techniques to use is they aren’t taught in veterinary schools; another obstacle is a lack of scientific research to determine if these hormone-preserving procedures in fact result in better outcomes than spaying and neutering.

That’s why I’m very happy today to share a study, published in January 2023, that provides much-needed information about this topic. As reported by the study authors:

To our knowledge, this study provides the first data on health and behavior outcomes of vasectomy and ovary-sparing spay in dogs and is the first to compare these outcomes to sexually intact and gonadectomized dogs.

It adds to accumulating data on the mixed benefits and risks of removing the gonads to prevent reproduction and emphasizes the importance of developing an informed, case-by-case assessment of each patient, taking into consideration the potential risks and benefits of spaying or neutering and alternative reproductive surgeries.”

Intact Dogs, Dogs With Gonad-Sparing Procedures, Fare Better

The study involved over 6,000 dog owners from the U.S. and Canada who completed an extensive 36-page online survey on the health status and other characteristics of their pets. Respondents also answered questions about problematic behaviors (including anxiety, fearfulness, and aggression) and “nuisance” behaviors such as urine marking and mounting.

The researchers, led by Christine Zink of Integrative Sports Medicine in Ellicott City, Maryland, formerly of Johns Hopkins University, discovered that the single most important predictor of behavioral and health outcomes in dogs is the length of time their bodies are exposed to gonadal hormones.

Spayed or neutered puppies obviously have only brief exposure to sex hormones; dogs desexed as young adults have the benefit of longer exposure; and dogs who are intact or underwent a hormone-sparing sterilization procedure have lifelong exposure.

The study results suggest that the bodies and brains of dogs benefit from maximum exposure to the sex hormones. In general, dogs who were spayed or neutered wound up with more health problems — including orthopedic disorders, increased cancer risk, endocrine system abnormalities, increased likelihood of obesity, and shorter lifespans — than intact dogs and dogs who received hysterectomies or vasectomies.

In terms of behavior, the study results confirmed earlier findings that spaying and neutering resulted in a significant increase in problematic behaviors (aggression, fearfulness) compared to intact dogs. Nuisance behaviors were also more prevalent in spayed and neutered dogs.

Behavioral changes in general were also much less evident in dogs with vasectomies and hysterectomies, presumably because they, like the intact dogs, retained the benefit of functioning gonads and normal levels of sex hormones.

Zink and colleagues concluded that when it comes to decisions about surgery to prevent reproduction, "dogs might benefit from these alternative surgeries, with respect to general health and experience better behavior outcomes, compared to undergoing traditional spay-neuter surgery." They also suggest that "Delaying traditional spay-neuter surgery could offer similar benefits."

Sterilization Without Desexing

Spays and neuters are desexing procedures (i.e., they remove the animal’s ability to produce sex hormones) used primarily for purposes of sterilization to prevent pregnancy. What many pet owners don’t realize, and as I noted earlier, veterinarians aren’t taught in vet school, is that dogs can be sterilized without being desexed.

As mentioned above, female dogs can undergo a modified spay, also called an ovary-sparing spay or hysterectomy (vs. ovariohysterectomy) that removes the uterus but leaves the ovaries in place, and male dogs can have a vasectomy that preserves the testes. Both procedures result in sterilization, but without removing the gonads and the hormones they produce.

Because the ovaries are preserved in modified spays, female dogs continue to have estrous cycles (go into heat), but since the uterus has been removed, there’s no bloody discharge. However, the vulva does enlarge. In addition, females continue to secrete pheromones that are attractive to male dogs, and they are receptive to males during their cycles.

It’s recommended that female dogs who’ve undergone ovary-sparing spays not be allowed to mate while in heat, for post-surgery anatomical reasons that may increase the risk of vaginal trauma.

You can read more about ovary-sparing spays at the Parsemus Foundation, which also provides information on vasectomies for male dogs:

“Vasectomy in dogs is similar to the procedure for men. Each vas deferens (a tube that carries sperm from the testes and epididymis to the urethra during ejaculation) is cut or clamped so that sperm cannot move through. The procedure is completed under anesthesia but is relatively quick and simple. Technical details can be found here. This method of sterilization is accepted by the American Veterinary Medical Association.

There are few health concerns when completing a hormone-sparing sterilization on a male dog, since the only health conditions prevented by neuter are benign prostatic hyperplasia in older dogs (which is treatable by neuter or noninvasive electromagnetic therapy), and testicular cancer (which is also a disease of old age and is treated by castration, which is usually curative).

The dog will be sterile but will still have hormones and be attracted to females in heat. Thus, owners must be willing to keep their dogs from roaming in search of females.”

Unfortunately, there are relatively few veterinarians across the country who have learned these techniques. Please take a minute to email your state veterinary teaching hospital (if you have one) or the AVMA and ask that students be taught alternative techniques while in vet school.

My Sterilization Recommendations

My approach is to work with each individual pet parent to make decisions that will provide the most health benefits for the dog. Whenever possible, I prefer to leave dogs intact. However, this approach requires a highly responsible pet guardian who is fully committed to and capable of preventing the dog from mating (unless the owner is a responsible breeder and that's the goal).

It’s important to note that I'm not advocating the adoption of intact shelter animals to people who may or may not be responsible pet owners. Shelter veterinarians don't have the time or resources to build a relationship with every adoptive family, so the animals in their care must be sterilized prior to adoption to prevent more litters of unwanted pets.

My second choice is to sterilize without desexing so the testes or ovaries can continue to produce hormones essential for the dog's health and well-being.

Rarely, older, intact male dogs develop moderate to severe benign prostatic hyperplasia (an enlarged prostate) that may be improved with conventional neutering. Generally speaking, mature intact dogs have had the benefit of a lifetime of sex hormone production, so the endocrine imbalances we see with spayed or neutered puppies don't occur when dogs are desexed in their later years.

Unfortunately, as I mentioned earlier, veterinary schools in the U.S. only teach full spays and neuters, so unless your own vet has obtained additional training in sterilization techniques that spare the ovaries or testicles (which is unlikely), you may have only one option available to sterilize your pet.

In that case, my suggestion would be to wait until your dog has reached full musculoskeletal maturity, and if you have a female, I’d also wait until she’s completed her second estrus cycle before scheduling the surgery.