The Spay/Neuter Decision: Studies Offer Conflicting Advice


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Spay/Neuter puppies according to your vet’s recommendations. Image by biewoef

Despite mandatory spay/neuter policies, and the availability of low cost clinics for the procedures, less than 60% of pet adopters  complied with the spay/neuter agreements they signed, according to a 2004 paper published in the Journal of  American Veterinary Medical Association. In light of this, most shelters and humane organizations have made the logical decision to spay and neuter rescue dogs before placement.

If you’re adopting a dog from other sources, however, be aware of the pros and cons of the procedure, and the ways in which age affects the decision; the extra information will help as you choose whether and when to spay or neuter your pet.

Retrospective Studies Catalog Secondary Health Benefits and Risks of Spay/Neuter

A retrospective study by Margaret Root Kustritz, DVM, Ph.D., DACT gives some insight into the societal, behavioral and health effects of spaying and neutering. A second, non-peer reviewed, retrospective study by Laura Sanborn, MS, provides further data and comes to somewhat different conclusions, although both authors support prompt neutering of unowned dogs.

Behavioral Effects of Spay/Neuter of Companion Dogs

  • Behavioral Effects According to Dr. Kustritz: Positive behavioral effects of spay/neuter include a decrease in separation anxiety and submissive urination, both common concerns for pet owners.  Breed predilection plays a role in both heightened noise phobia and increased aggression in spayed dogs. Further research is necessary, as there are no studies separating breed–specific behavior from the effects of lower hormone levels.
  • Behavioral Effects According to Laura Sanborn: The Sanborn retrospective paid close attention to the increased level of geriatric cognitive impairment in neutered male dogs vs. intact males, but with the development of new canine cognitive impairment medications, this may become less important overtime. There were too few unspayed females in the study to draw conclusions for female dogs, but a 2009 Medscape review of the literature regarding the neuro-protective effects of estrogen in human females reveals a mixed picture, dependent on a variety of other factors in addition to estrogen levels. This suggests that significant differences in impairment are also not likely for female canines.

Health Effects of Spay/Neuter of Companion Dogs

Tibetan Terriers are prone to hip dysplasia. Image by sue r b

The clearest positive health effect of spaying cited by both authors is the significant reduction in the risk of canine mammary tumors, which are generally malignant and highly metastatic. This risk is minimized when dogs are spayed prior to their first estrus but  increases with each successive cycle. In addition to mammary tumors, pyometra (uterine infection) is an additional risk factor for intact female dogs, with mortality rates as high as 17% according to Dr Kustritz.

Both authors note the increased risk of osteosarcoma (bone cancer) and hemangiosarcoma (splenic or cardiac tumors) when spay/neuter is performed before one year of age. The low hemangiosarcoma risk of  2% in the general dog population must, however,  be balanced against that of mammary cancer, which generally has higher incidence in dogs of all ages and breeds.

  • Health Effects According to Laura Sanborn: Sanborn’s report downplays the breed predilection for both types of tumors, and focuses on simply comparing spay/neuter vs intact dog disease incidence, rather than comparing by age.
  • Health Effects According to Dr. Kustritz: Dr. Kustritz makes it clear that increased risk of hip dysplasia and cruciate ligament rupture must also be considered in the light of breed predilection and health status of the individual dog. Dr. Kustritz also points out that the statistics often cited for osteosarcoma are based largely on breeds with a hereditary predilection for the disease. In addition, Kustritz  noted either no significant difference in life span, or an increased life span, for spayed and neutered dogs of these breeds despite the prevalence of these bone tumors.

Reducing Risk

There are a number of ways in which you can lower health risks for your pet, or reduce the chances of adopting an at-risk dog:

  • Choose breeds with low incidence of disease.
  • Research bloodlines of a preferred breed for lower risk of acquiring these cancers.
  • In the case of osteosarcoma, if you must have an at-risk breed,  delaying the spay/neuter  until the dog is one year of age may help.
  • Ensure that your pet maintains a healthy weight.
  • Provide consistent, structured exercise, to greatly reduce the risk of torn cruciates and the level of disability that arises from hip displaysia.

Deciding When and If to Spay or Neuter

There is another aspect of the spay/neuter question to consider: the individual owner or family’s ability to manage an intact dog. With appropriate preparation and awareness, a responsible owner can safely house an intact dog. But busy households in urban or suburban areas, particularly where many nearby households have one or several dogs, generally find this difficult. In the author’s experience, owners are rarely prepared for the intensity of the mating drive, particularly that of females at the height of estrus. Escapes from ‘secure’ yards and homes are common, often resulting in unplanned litters of puppies.

Talk to your Vet when deciding to spay or neuter your dog. Image by ruxy2009

Given these concerns, most households are likely to find spay/neuter the most appropriate option. If this is your choice, talk with a trusted veterinarian regarding the relevant health issues and their impact on timing of the surgery, as each situation is unique. If you choose a breed with a high risk for some of the health conditions discussed above,  talk to your veterinarian about your situation before trying to make a decision on your own.

Your veterinarian will understand the breed-specific risks and will be aware of your personal situation. This will allow him to help you through the decision-making process in a personalized manner, so you can do what is best for your pet and your family.


Root Kustritz, MV. 2007. Determining the optimum age for gonadectomy of dogs and cats. J Am Vet Med Assoc. 2007 Dec 231(11):1665-1675.

Sanborn, LJ. 2007. Long-Term Health Risks and Benefits Associated With Spay/Neuter in Dogs. Accessed online 7.11

Spain, CV., Scarlett, JM. And Houpt, KA. 2004. Long-term risks and benefits of early-age gonadectomy in dogs. J Am Vet Med Assoc. 2004.Feb 224(3):380-7.

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