Hormone-Sparing Sterilization

Two female dogs with hair bows

Project Summary

Spaying and neutering pets to reduce pet overpopulation is standard veterinary practice, and usually involves removal of the sex organs to sterilize the animal. More recent research on dogs has shown that the lack of natural hormones resulting from spay/neuter can lead to other serious health complications. Hormone-sparing methods — like hysterectomy (also called ovary-sparing spay) for females and vasectomy for males — are ways to sterilize pets without the negative impacts from hormone loss.

Use our Veterinarian Directory to find a veterinarian near you who offers hormone-sparing sterilization.

NEW: Infographic on the risks of spay and neuter, which you can use to raise awareness.

  • Dogs, cats and other pets reproduce very quickly, producing thousands of animals in only a few generations.
  • Pet sterilization is necessary to keep population levels under control. In the U.S. and many other countries, spay and neuter has become standard practice and is completed by removal of the sex organs – the ovaries and uterus in females (ovariohysterectomy) and the testes in males (castration).
  • After several decades of widespread spay/neuter practice, research in dogs has found that removal of the sex organs and associated hormones can result in significant health issues for some pets, including cancers.
  • Sterilizing dogs while retaining natural hormones is a way to “have one’s cake and eat it too”: a way to address population concerns without the negative health impacts from hormone loss.
  • The goal is to sterilize pets to prevent overpopulation without negatively impacting individual health and longevity.
  • The Parsemus Foundation is proposing that we all think more creatively about individualizing pet sterilization, as is recommended by the Society of Theriogenology.
  • Hormone-sparing sterilization options, including ovary-sparing spay (hysterectomy), vasectomy, and intra-epididymal calcium chloride injection, can enhance the lifelong well-being of pets while maintaining population control.
  • Educating pet owners on the potential health and behavior problems associated with spay and neuter, and encouraging the availability of hormone-sparing sterilization options, is being achieved through website resources, public presentations, research, and maintenance of a veterinary directory of providers.

Project Topics

Promoting choice for optimal animal health

Hormone-sparing sterilization methods ensure population control while guarding the lifetime well-being of dogs. Hysterectomy (also called ovary-sparing spay) for females and vasectomy for males are ways to sterilize pets without the negative impacts from hormone loss. For a recent overview on the health implications of traditional spay and neuter, and a review of hormone-preserving contraceptive options, see this article in the Innovative Veterinary Care journal. Pet owners can find veterinarians in your area offering ovary-sparing spay as an option.  Veterinarians can get training on canine hysterectomy and vasectomy and register their veterinary clinic in the free directory to connect with clients. Also, visit the Facebook Ovary Sparing Spay and Vasectomy Group for a list of veterinarians who offer alternatives to traditional spay and neuter and to share your experiences.

What’s wrong with spaying and neutering pets?

Have you had a beloved dog that suffered from cancer or a cruciate ligament tear? You might be surprised to know that a number of serious health conditions like these are more common in dogs that have been spayed or neutered.

Our understanding of the health impacts of spay and neuter started with research evaluating dogs that had their sex organs removed (called gonadectomy) at a very young age. In 2007, a respected veterinarian named Margaret Root Kustritz published a review of spaying and neutering dogs and cats at different ages. This generated a lot of discussion and further research reporting positive and negative health consequences of gonadectomy that vary by age, gender, and breed.

Today there is mounting evidence that the health benefits of keeping the sex organs in dogs may outweigh the health risks. A key 2013 publication from the University of California, Davis reported that the rates of two joint disorders and three cancers in Golden Retrievers were significantly higher in both males and females that were neutered early or late compared with intact (non-neutered) dogs. Many other studies have followed. For example, a 2014 study indicated that gonadectomized Vizslas had significantly increased odds of developing mast cell cancer, lymphoma, all other cancers, and fear of storms compared with the odds for sexually intact dogs. Brain tumors are also 11 times more likely in spayed than intact females. On the positive side, since dogs who have been spayed or neutered no longer have sex organs, they don’t get many diseases associated with sex organs or hormones. Many factors, including the breed, sex, age of sterilization, and body size impact the outcome for any particular dog. Follow-up studies from the University of California, Davis have reported risk profiles of 35 breeds and mixed breed dogs, providing important information for veterinarians and pet-owners when considering sterilization. The following table summarizes our current knowledge of potential positive and negative health impacts.

Summary of health impacts of spaying and neutering (gonadectomy):

Lower chance of:But higher chance of:
Diseases of the sex organs:
  • Mammary, ovarian, and testicular cancers
  • Pyometra – infection of the uterus
  • Perineal and inguinal hernias
  • Prostatitis, benign prostatic hyperplasia, prostatic cysts, and squamous metaplasia of the prostate
  • Obesity
  • Urinary incontinence and urinary calculi
  • Immune-mediated diseases: atopic dermatitis, autoimmune hemolytic anemia, hypoadrenocorticism, diabetes mellitus, hypothyroidism, immune-mediated thrombo-cytopenia, inflammatory bowel disease
  • Hip dysplasia and cranial cruciate ligament rupture
  • Aggressive and fearful behavior, cognitive dysfunction syndrome
  • Cancer: hemangiosarcoma, osteosarcoma, transitional cell carcinoma, prostate adenocarcinoma, lymphosarcoma

How does spaying and neutering impact health?

A significant contributor to the negative health impacts of removing the hormone-producing sex organs in dogs is that the natural feedback mechanisms become unregulated. Normally, the hypothalamus and pituitary glands in the brain stimulate the production of luteinizing hormone (LH), which then stimulates the production of steroid hormones from the gonads. Without the ovaries or testes, there is no feedback signal to reduce production, so LH continues to be produced. A study by Zwida and Kutzler in 2016 reported on the implications of high LH levels in dogs without gonads. A review article on the health problems associated with spay and neuter — and the connection with LH — was also published by Kutzler. As a result of the health problems that occur more frequently after gonadectomy, pet owners and veterinarians have sought alternatives. That is where hormone-sparing sterilization comes in. Ovary-sparing spay (or hysterectomy) for females and vasectomy for males are surgical methods that sterilize the pet while preserving the organs (ovaries and testes) that produce natural hormones. A nonsurgical hormone-sparing option for males is the use of calcium chloride dihydrate solution injected into the epididymis (see our calcium chloride page for general details). So it is possible to ensure that your pet does not reproduce while retaining natural hormones. These options are great for breeds known to have greater risk of certain problems after spay/neuter (for example, Boxers have a higher chance of brain tumors, and Rottweilers and giant breeds are prone to bone cancers).

Infographic about the health risks to dogs of traditional spay and neuter
Click image to download infographic

How is ovary-sparing spay (OSS) performed?

The best hormone-preserving sterilization for dogs is a hysterectomy, or removal of the uterus and cervix while leaving the ovaries, sometimes called ovary-sparing (OSS) or partial spay. The procedure is not new: Belfield published on the need for the technique in 1972. OSS removes the nuisance of bleeding during heats, along with the risk of infection of the uterus (pyometra), as long as ALL of the uterus is removed. If some uterine tissue is left, “stump pyometra” may occur. More details of the technique are included in this publication and see Additional Resources for videos and other training information. (Note: the reason that tubal ligation is not recommended in dogs is that pyometra can still occur).

The female dog will still have hormones, and may behave differently during the heat cycle. And while she cannot get pregnant, she may be attractive to males during heat — so owners may want to keep her indoors during this time.

What about health concerns of keeping the ovaries? Ovarian cancer is rare enough in dogs that the ovaries should not be removed just to try to prevent it. That makes mammary tumors the remaining concern. Although early studies reported a higher incidence of mammary tumors in intact dogs, a review article indicated that most of these studies were biased, making conclusions suspect (Beauvais et al., 2012). In one study, mammary tumors were reported in 1.25% of older female dogs, with higher incidence in poodles, English cocker spaniels, and dachshunds (Zatloukal et al., 2005). Luckily, mammary tumors are pretty easy to detect when rubbing your dog’s belly, and prognosis for the tumors that become cancerous is good when they are caught early and removed. Thus, many believe the low health risk associated with OSS is preferable to the more serious potential health risks of traditional spay. Ovary-sparing spay is approved by the AVMA.

See “Additional Resources” below for details and training materials on OSS.

How is vasectomy performed?

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.

Other sterilization options for males

Epididymal sterilization

Immature sperm leave the testes and travel to the epididymis, where they mature and are stored until needed. Methods aimed at the epididymis, rather than the testes, have the advantage of stopping sperm while preserving natural hormones (which are made in the Leydig cells of the testes). Several epididymal sterilization studies show promise for this as a hormone-preserving sterilization method.

Calcium chloride dihydrate solution is an effective chemosterilant that is normally injected into the testes to cause sterility and a significant drop in testosterone. But when injected into the epididymis, calcium chloride provides a nonsurgical hormone-preserving option for male dogs (see our Calcium Chloride Male Sterilization page).  A recent study by Leoci and colleagues confirms that sterility can be achieved with an intra-epididymal injection of calcium chloride in dogs, with no drop in serum testosterone. However, the procedure required ultrasound guidance and took as long as castration. Intra-epididymal use of calcium chloride may be best for dogs whose owners do not wish to have any change in behavior or anatomy.

Sotradecol is an inexpensive drug commonly used as a sclerosant to collapse spider veins in humans. Although it has not been tried, foaming sotradecol might be an optimal epididymal injection, sclerosing the epididymis and staying in place more than a liquid sterilant. This prior art description of epididymal sotradecol injection provides more information on the approach.

Another epididymal approach tested in rams is epididymal ligation. This is an interesting, extremely simple, low-cost technique. It might not work in dogs because of anatomical differences in epididymal size and position, but could be tried.


Ultrasound is a non-invasive sterilization treatment for males using equipment commonly available in medical offices. Researcher Raffaella Leoci is the primary investigator who published two studies in dogs; a 2009 publication can be found online, and the 2015 publication details the exact methods necessary for effectiveness in mid-size dogs. This method has not progressed because permanent sterilization depends on specific equipment settings.


Vasalgel is a contraceptive that is currently being developed for humans (see our Vasalgel page), but the concept is also applicable to dogs and other species. Vasalgel works by injecting a polymer gel into the vas deferens — the tube that carries the sperm — which blocks the sperm but allows fluids to pass through. The procedure does not affect hormones and is being developed as a reversible option.


Please note that as of early 2016, Zeuterin is no longer available but was approved by the FDA.

Zeuterin/Esterisol was an intra-testicular injection of zinc gluconate neutralized by arginine that reduced — but did not eliminate — testosterone in male dogs.

Don’t hormones cause undesirable dog behavior?

Many people believe that neutered male dogs make better pets because they are less aggressive. However, research does not support this conclusion. A recent large survey reported that spayed or neutered dogs did not differ from intact dogs in terms of aggression directed toward familiar people or pets, and they actually showed higher levels of aggression toward strangers. Dog aggression is a main reason for relinquishment to shelters, and neutered male dogs are more often surrendered for behavioral reasons. Gonadectomized dogs may also develop more anxiety and fear, and show more cognitive decline than intact dogs.

Behavior is complex and related to the dog’s environment, rearing, and training. Neutering male dogs may decrease reproductive-related behaviors (urine marking, mounting and roaming) but impacts on other behaviors are variable

Benefits to shelters and veterinarians

Now that we understand the potential negative health impacts of spay and neuter, why don’t all veterinarians and shelters offer hormone-sparing options? Veterinarians are not yet routinely trained to perform these alternative sterilization procedures. Shelters may worry that changing the current standard of spay/neuter may confuse potential pet adopters or that dogs with natural hormones will be more difficult for pet owners, leading to more dogs at the shelter.

The Parsemus Foundation encourages a consideration of all options and an individualized decision-making process regarding sterilization, as is recommended by the American College of Theriogenologists. Shelters offering hormone-sparing sterilization options will have an answer for potential adoptees who would otherwise be turned off from shelter adoption because of mandatory traditional spay. Veterinarians offering the option are already in great demand.

Loie, a perfect example of the dilemma. Abandoned at the rural shelter with her litter of puppies, Loie was a lab/rottweiler mix and just starting her second heat when a rescue organization saved her.  Remove her ovaries now? Spayed labs and rottweilers face particularly great health risks. The loss of hormones could quadruple her risk of bone cancer  and raise her risk of hemangiosarcoma to 10-20%. That’s not to mention the increased risk of CCL tears, incontinence, and weight gain common in these breeds. But what if she could have a hysterectomy and keep her ovaries (but still be sterilized) instead?

Rescued Dog Loie

Identification of dogs with hormone-sparing sterilization

Knowing whether a dog was sterilized is critical for preventing unnecessary repeat surgeries. However, lack of standardization has been a barrier. After a review of common identification methods for owned dogs and methods being used currently by veterinarians, a simple tattoo in the inguinal area is recommended as the standard for hormone-preserving sterilization methods:

  • Hysterectomy: Green “X” slightly lateral to midline near umbilicus
  • Vasectomy: Green “V” lateral and cranial to scrotal region

The simplicity of the marks does not require dedicated tattoo equipment or special skill, so that most veterinarians could provide the identification. Tattoos can be created using a needle or scalpel to apply the tattoo ink to the dermis. The procedure is completed while the dog is sedated for a sterilization procedure, and no additional input from the owner is required and no additional equipment is needed for detection. Although clarity of a tattoo may decrease over time, in this case it is not necessary that it is legible — just that it’s visible.

What about dogs who have already had a spay or neuter?

Many dogs have already received a traditional spay or neuter surgery, removing the ovaries or testes, especially if they were adopted from a shelter or rescue organization. For many, they will live a healthy life. But for some dogs, the loss of natural hormones may result in a variety of ailments. In humans, a premature loss of hormones would be treated by hormone therapy (supplementing with estrogen, testosterone, or others) to maintain health. Couldn’t we supplement the lost hormones in spayed or neutered dogs? This idea makes sense, but unfortunately restoring normal hormone levels is not a standard veterinary treatment. The only common use is in dogs with incontinence. 

A recent case report published in Topics in Companion Animal Medicine reviewed the use of hormone restoration therapy in a neutered male dog that had significant health issues shortly after gonadectomy: weight gain, loss of mobility due to pain in the right hip, and debilitating fear of strangers. After three years of standard medical treatments with no significant improvement, hormone restoration was attempted. Low testosterone was increased and extremely high luteinizing hormone was decreased so that both were within normal levels. Within months, this treatment resulted in improved mobility, stabilized weight, and  reduction in fear and anxiety. The dog could run, jump, and visit public parks  – things he could not do previously without pain or extreme anxiety.

If you think your dog suffers from loss of hormones, you might consider checking hormone levels (including LH) on his or her next checkup. Feel free to share this case study publication when you speak to your veterinarian about treatement. And send us an update on your experience if you and your veterinarian decide to try hormone restoration for your dog.

Take Action

Demand for hormone-sparing dog sterilization is growing as informed pet owners understand the health implications of spay/neuter. However, at this time, few veterinarians offer these procedures. To help connect pet owners with veterinarians who perform OSS, vasectomy, and other alternative procedures, the Parsemus Foundation maintains a veterinary directory.

For Veterinarians

  • Veterinarians who wish to be listed in the directory may do so by filling out the info on the clinic registration form.
  • Use this flyer as an informational piece for your website or as a handout for clients.
  • Email us at info@parsemusfoundation.org if you would like to consult with a veterinarian who is experienced with hormone-sparing sterilization.
  • Review the Additional Resources below. Take note of the  information about gonadectomy risks for specific breeds and mixed-breed dogs published by Hart and colleagues (2020) and stay abreast of recent updates in this rapidly evolving area of research.

For Pet Owners

  • Review this brief flyer to understand whether hormone-sparing sterilization is right for you and your dog. Keep in mind that impacts vary on breed, size and other factors. Check the recent publications about cancer and joint disorders by dog breed and mixed-breed size.
  • Consult with a veterinarian who is familiar with the health issues related to spay/neuter and familiar with hormone-sparing options. Each dog and family is different, and having an expert consultation on the best method of sterilizing your dog is important. Be courteous and be prepared to pay for a phone consultation — do not expect busy veterinarians to be able to spend a great deal of time answering questions from non-clients.
  • To find a provider you can search the Veterinary Directory or check the information at the Ovary Sparing Spay and Vasectomy Info Facebook Group. If you cannot find a provider nearby, you could pass along the information on this website to your own veterinarian and ask if he or she can provide the procedure. Direct him or her to the information on this website and email us at info@parsemusfoundation.org if additional assistance/advice is required.
  • Please let us know if you find a veterinarian who offers OSS or vasectomy and would like to join our directory!
  • If you choose to preserve your dog’s hormones, consider joining this Facebook group: Training and behavioral advice for Intact Dogs.

Summary documents

  • Download our flyer for a brief overview of hormone-sparing sterilization.
  • Download our infographic on the risks of spay-neuter.

Select publications on health impacts of gonadectomy

  • For a video overview of the topic: Michelle Kutzler 2021 Presentations at AMVAC Conference.
  • Belfield WO. 1972.For a more normal life for a pet: Partial spay (hysterectomy).Vet Med Small Anim Clin
    67(11):1223-4  Full text. First publication covering why and how to conduct a partial spay on dogs.
  • Brent L. 2019. Growing interest in hormone sparing dog sterilization and recommendations for standard identification methods. Clin Theriogenology 11(3):247-253. Full text.
  • Brent L, Kutzler M. 2018. Alternatives to traditional spay and neuter – evolving best practices in dog sterilization. Innovative Vet Care. Free full text.
  • de la Riva GT, Hart B, et al. 2013. Neutering dogs: Effects on joint disorders and cancers in Golden Retrievers. PLoS One. Free full text.
  • Farhoody P, Mallawaarachchi I, et al. 2018. Aggression toward familiar people, strangers, and conspecifics in gonadectomized and intact dogs. Front Vet Sci 5:1-13. Free full text.
  • Hart BL. 2001. Effect of gonadectomy on subsequent development of age-related cognitive impairment in dogs. J Am Vet Med Assoc 219:51-6. Abstract.
  • Hart BL, Hart A, et al. 2014. Long-term health effects of neutering dogs: comparison of Labrador Retrievers with Golden Retrievers. PLoS One 9(7). Free full text.
  • Hart BL, Hart A, et al., 2020. Assisting decision-making on age of neutering for 35 breeds of dogs: Associated joint disorders, cancers, and urinary incontinence. Front Vet Sci 07 July, 2020. Free full text. Supplementary data by breed.
  • Hart BL, Hart A, et al., 2020. Assisting decision-making on age of neutering for mixed-breed dogs in five weight categories: Associated joint disorders and cancers. Front Vet Sci 31 July, 2020. Free full text. Supplementary data by weight.
  • Kutzler M. 2020. Possible relationship between long-term adverse health effects of gonad-removing surgical sterilization and luteinizing hormone in dogs. Animals. Free full text.
  • Lissner E. 2013. “The pros of partial spay. Innovative Vet Care. Full text.
  • Mattravers M. 2017. Ovary sparing spay in canines: An alternative to traditional ovariohysterectomy. Full text. Student review.
  • Society for Theriogenology, Board of Directors, and the American College of Theriogenology. 2013. Basis for Position on Mandatory Spay-Neuter in the Canine and Feline. Free full text. A good review of the data and recommendations from veterinarians who specialize in reproduction.
  • Zink MC, Farhoody P, et al. 2014. Evaluation of the risk and age of onset of cancer and behavioral disorders in gonadectomized Vizslas. Abstract.
  • Zink, MC. 2020. Canine gonadectomy: Rethinking long held beliefs. YouTube video.
  • Zwida K, Kutzler M. 2016. Non-reproductive long-term health complications of gonad removal in dogs as well as possible causal relationships with post-gonadectomy elevated luteinizing hormone ( LH ) concentrations. J Etiol Anim Health 1(1):1-11. Full text.

OSS and vasectomy training materials

Epididymal sterilization references

  • Bowman TA, Senger PL, Koger LM et al. 1978. Blockage of Sperm Transport Using Intraepididymal Calcium Chloride Injections in Rams. J Anim Sci 46(4):1063-1065. Free full text.
  • Gur FM, Timurkaan S, Timurkaan N. 2011. The effects of prepubertal epididymal ligation upon androgen receptor distribution in the rat caput epididymis. Veterinarni Medicina 56(6): 286–293. Free full text.
  • Leoci R, Aiudi G, Cicerelli V, et al., 2019. Effects of intratesticular vs intraepididymal calcium chloride sterilant on testicular morphology and fertility in dogs. Theriogenology 127:153-160. Free full text.
  • Pineda MH, Reimers TJ, Faulkner LC, et al. 1977. Azoospermia in dogs induced by injection of sclerosing agents into the caudae of the epididymides. Am J Vet Res 38(6):831-8. Abstract.
  • Pineda MH, Hepler DI. 1981. Chemical vasectomy in dogs. Long-term study. Theriogenology 16(1):1-11.  Abstract.
  • Pineda MH, Dooley MP. 1984. Surgical and chemical vasectomy in the cat. Am J Vet Res 45(2):291-300. Abstract.
  • Tamadon A, Nikahval B, Sepehrimanesh M et al. 2010. Epididymis ligation: a minimally invasive technique for preparation of teaser rams. Veterinary Surgery 39:121-127. Free full text.

Media, interviews, books and websites

  • Check out this interview with Dr. Kutzler by Dr. Karen Becker of Healthy Pets
  • “Pukka’s Promise: The Quest for Longer-Lived Dogs”, A thought-provoking book by Ted Karasote with a chapter on spay-neuter and health.
  • In 2019, the New York Times published an essay by Dr. Alexandra Horowitz, a cognitive scientist who studies dogs, in which she argued that spaying and neutering puppies shouldn’t be standard policy — and isn’t automatically the “responsible” choice either.
  • In this 2019 article in the Washington Post, Karin Brulliard writes about how research is causing some owners and veterinarians to question the long-held tenet that fixing puppies — or fixing, period — is a necessary part of responsible pet ownership.
  • An August 2021 Washington Post piece by Alexandra Ellerbeck discusses the evidence that spay and neuter is no longer an automatic option for dogs.
  • For a well-researched and thoughtful review of ovary sparing spay from a pet owner’s perspective: Jessica Rhea “The Pros and Cons of Ovary Sparing Spay (OSS) for Your Dachshund” 2019.

Veterinarians Offering Alternative Methods of Contraception

Pet Owners

Looking for a veterinarian willing to perform procedures beyond surgical spay or neuter? Browse our directory of qualified veterinary professionals.


Do you offer alternative methods of contraception like ovary-sparing spay and vasectomy? Join our referral directory so new clients can find you.

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