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How Spaying and Neutering Influences Cancer and Disease Risk

Analysis by Dr. Karen Shaw Becker

dog neutering

Story at-a-glance -

  • Studies show that spaying/neutering large and giant breed dogs increases the risk for many diseases, including several types of canine cancer
  • While animal health groups are reconsidering their position on automatic spays/neuters, animal welfare groups understandably remain firmly on the side of early spays or neuters for all shelter pets
  • A reasonable compromise may exist in broader use of sterilization procedures that do not remove the ovaries or testes
  • If you’re considering, for example, an ovary-sparing spay for a female dog or a vasectomy for a male, resources exist but aren’t widely available
  • If you decide to spay/neuter your large or giant breed dog, consider waiting until your pet reaches musculoskeletal maturity, and in the case of a female dog, wait until she’s completed her second estrus cycle before scheduling the surgery

As I discuss often here at Mercola Healthy Pets, there’s a growing body of research that indicates 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:

Condition Effect of Spay on Female Large/Giant Breeds Effect of Neuter on Male Large/Giant Breeds
Overall longevity Mild increase Mild increase
Obesity Moderate increase Moderate increase
Cranial cruciate ligament disease Moderate increase* Moderate increase*
Hip dysplasia Mild increase* Mild increase*
Mammary tumors Marked decrease* --
Uterine, ovarian, vaginal tumors Prevents --
Testicular tumors -- Prevents
Perianal gland tumors -- Marked decrease
Prostatic carcinoma -- Prevents
Lymphoma Mild increase Mild increase*
Mast cell tumors Mild increase --
Hemangiosarcoma Mild increase* Mild increase
Osteosarcoma Mild increase* Mild increase*
Transitional cell carcinoma Mild increase Mild increase
Urinary sphincter mechanism incompetence Moderate increase* --
Cystitis Mild increase* --
Benign prostatic hyperplasia -- Marked decrease
Perineal hernia -- Moderate decrease
*Age at time of surgery may be important
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Animal Health vs. Animal Welfare

Given the mounting evidence that spay/neuter may not be appropriate in every instance, animal health organizations such as the Morris Animal Foundation and the American Veterinary Medical Association (AVMA)1 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.2 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 only have to 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.

A Potential Compromise: Sterilization Without Desexing

The reason, in simple terms, that spays/neuters can cause long-term health problems is because these procedures remove the hormone-producing gonads (ovaries or testes). These hormones are used by the body in a multitude of important ways — especially in the case of young animals. Removing the gonads removes dogs’ ability to make these very important naturally occurring substances.

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 veterinarians aren’t taught in vet school, is that dogs can be sterilized without being desexed.

For example, 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 vasectomies that preserve the testes. Both procedures result in sterilization, but without removing the gonads and the hormones they produce. There is also a zinc injection that can render male dogs sterile but still allows for a small amount of hormones to be produced.

Ovary-Sparing Spay

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.


The Parsemus Foundation also provides information on vasectomies for male dogs:

“Although it is not widely performed, vasectomy is an accepted way to sterilize dogs without impacting hormones. A number of veterinarians are willing to offer vasectomy (for example, for clients who want to sterilize their dogs right away to prevent accidental litters, but wait for neuter until after the dogs’ growth plates close). We are gathering a list of veterinarians who offer vasectomy.

If you choose vasectomy for your dog, be sure to ask the vet to look into current recommendations for human vasectomy techniques, as some techniques have been shown to be more effective than others (such as cautery with fascial interposition) and are now considered best practice.”3

Unfortunately, only full spays and neuters are taught in U.S. veterinary schools, so there are relatively few veterinarians across the country who have learned alternative 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. The good news is that the Parsemus Foundation has compiled a state-by-state list of veterinarians who do provide such services at this link.

Be sure to read the information at the top of the page before searching for a vet in your area. Other potential resources include the Facebook Ovary Sparing Spay and Vasectomy Info Group, and the Society of Theriogenology.

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.