Clients often ask my opinion on whether and when to spay or neuter their dog, and I can understand why they want help making this decision—there’s a lot of conflicting information out there!  The decision, however, is one best made by the client in consultation with their veterinarian.  Still, knowledge is a good thing.  Knowledge helps us evaluate for ourselves the risks and benefits of spaying or neutering our dogs, and then helps us form solid questions to ask when we meet with our vets.  This is why, when I recently read a lengthy literature review on sterilization and its effect on dogs’ health, lifespan, and behavior, I thought it might be helpful to summarize the information here (please note, I will use the term “sterilization” going forward when referring to both spay and neuter, or other sterilization procedures). 

For people with a serious interest in this topic and who would like access to the full article, its citation (along with a link) is as follows.  Urfer S. R. and Kaeberlein, M. (Dec., 2019) Desexing dogs: A review of current literature.  Animals, 9 (12) at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6940997/.

Sterilization as a population management technique

One thing that may surprise folks—it did me—is how much attitudes vary toward sterilization among various western countries.  In the U.S., around 64% of dogs are sterilized, and it is mandatory in some municipalities.  The UK and Ireland run a little lower, 54% and 47 % respectively; whereas, only 10% of dogs in Sweden are sterilized.  In Germany and some other Nordic countries sterilization is actually illegal (see, p. 2).

One reason for the high rate in the U.S. is our desire, stemming back to the 1970s, to decrease the number of dogs placed in shelters.  The interesting thing here is that, based on current research, sterilization does not reduce this number for companion and shelter-dog populations.  The reason seems to be that, in the U.S., around “two thirds of litters are intentionally bred” (p. 6).

In contrast, there is data supporting sterilization as a population control method for free-roaming-dog populations.  In this instance, however, there has not been any emphasis on the sex of the animal being sterilized, which is unfortunate, according to Urfer and Kaeberlein, since studies in mammals have shown that population growth (or decline) is tied to the number of fertile females in a population, not the number of males.  If this is true for feral dog populations, narrowing the focus to females could impact the costs of these programs.  Moreover, giving males vasectomies, instead of providing more traditional neuterings, will cause the sterilized males to continue competing for females, which also results in a lower population growth (see, pp. 6-8).

Methods

Note that in this post, I am only summarizing sterilization methods covered by Urfer and Kaeberlein that have been used on dogs in the general population.  I am not summarizing methods that have only appeared in specialized research settings, even if they were touched-on in the article.

Surgical.  Here in the U.S., the most common forms of sterilization are neutering (known as orchiectomy) for males and, for females, spaying (which is usually an ovariohysterectomy, meaning the removal of both the ovaries and the uterus, in the U.S., but an ovariectomy or oophorectomy, the removal of the ovaries only, in western Europe). 

Despite these clear preferences, there are alternative methods available.  For males, vasectomies are another surgical option, which does not impact the dog’s testosterone levels and behaviors related to testosterone.  For females, other options are a hysterectomy or cutting the dog’s fallopian tubes.  Like a vasectomy for a male dog, these methods do not impact the female dog’s hormone levels (see, pp. 3-4).

Hormonal.  Progesterone is used, on a limited basis, in both male and female dogs to cause reversible infertility.   Testosterone and estrogens are also used.  These methods tend to be more reliable in females than in males. 

A more recent hormonal treatment, administered via subcutaneous implants, is known as GnRH superagonists or (gonadotrophin releasing hormone).  This is a key hormone of reproductive function in mammals.  GnRH agonists cause infertility by first increasing certain hormones, which after ongoing use, triggers a desensitization in the pituitary and a subsequent collapse in the dog’s sex hormone levels, thus preventing fertility.  After six to eight weeks on GnRH, a dog will have undetectable levels of sex steroids.  This method is widely used in Europe and Australia, but not in the U.S (see, pp. 4-5). 

Behavior

Many people look to sterilization as a means of eliminating undesirable behaviors in their dogs, and Urfer’s and Kaeberlein’s review indicates that this is warranted in some cases, but even then there are risks.  Methods of sterilization that remove the sex organs do impact behavior, but only some of the behavior changes that have been documented are beneficial to humans.  Others are not. 

It is also hard to predict which of the possible behavior changes will result from sterilization when you are looking at an individual dog.  Research looks at a population of dogs and determines what percentage of that population is impacted by a procedure.  If a large percentage is impacted, then data supports an effect, but your dog may be one of the outliers.  If sterilization is contemplated solely for behavior modification, Urfer and Kaeberlein suggest GnRH superagonist implants because this form of sterilization is reversible.

Documented behavior changes fall into four categories: libido and related behaviors, urinary marking, bite risk and aggressive behaviors, and cognitive function.  Most behavior changes are seen in male dogs and, in areas where both sexes show an effect, the effect in the male dogs is stronger (see, pp. 13-17). 

Behavior	Males	Females
Libido	Decreased mounting and roaming.	______
Urine marking	Reduced marking.	______
Bite risk and aggressive behaviors	Decreased bite risk and boldness.  Decreased aggression towards other male dogs.  Increased resource guarding against both dogs and humans.  Mixed findings on fearfulness and / or reactivity and aggression.	Decreased bite risk and boldness.  Mixed findings on fearfulness and / or reactivity and aggression.
Cognitive function	Increased risk of cognitive dysfunction.	Increased risk of cognitive dysfunction.

Libido.  Testosterone controls libido in dogs, as in humans.  Many undesirable behaviors, from the human point of view, are impacted by testosterone, including copulation, mounting, and roaming.   According to the authors, multiple studies found that neutering reduces “mounting [ ] and roaming in 60% to 90% of male companion dogs” (p. 14); however, a study involving free-roaming dogs did not see an effect.  It may be that age at the time of neutering matters: there is evidence that sexual behaviors will persist after neutering if a male is already sexually experienced (see, p. 14).

Urine marking.  Neutering does seem to reduce marking.  The effect of neutering on marking may be more pronounced when the procedure is done at an earlier age (see, p.p. 14-15). 

Bite risk and aggressive behaviors.  A variety of population-based studies in the US and Canada consistently show that sterilization reduces bite risk, but, according to the authors of the review, it is hard to quantify these findings because of how the studies were structured and because sterilization acts as a proxy for better husbandry overall.  

Additional studies look at boldness and the aggressive behaviors that exist on the far end of the boldness spectrum.  For example, an Australian study found that (1) males rate higher in boldness traits than female dogs and (2) sterilized dogs of both genders score lower on the boldness scale than intact dogs.  The problem is that several other studies have linked sterilization with the other end of the boldness spectrum—fearfulness and reactivity!

Moreover, there are studies that reveal a “small but significant increase in aggression towards strangers” (p. 16) in dogs who are sterilized under 12 months of age, and that also correlates to an increase in the length of time that a dog lives with increased fear and aggression.  Two more studies link sterilization with increased resource-guarding behaviors toward other dogs and humans.  A Hungarian study showed that intact males were less obedient than either neutered males or females, but that neutered “males who fall into the less-obedient category are more likely to show signs of owner-directed aggression“ (p. 16).

The strongest finding, according to Urfer and Kaeberlein, is that neutering male dogs is likely to reduce dog-directed aggression (see, pp. 15-16).

Cognitive function.  There is some evidence that sterilization increases the risk of cognitive dysfunction in both sexes; the finding is somewhat stronger in females.  There is preliminary evidence that ovariectomies may impact spatial learning in females (see, pp. 16-17).

Impact on health and life span

The first thing that Urfer and Kaeberlein note is that most published studies on the impact of sterilization on health fail to account for lifespan or the age at which a condition is diagnosed.  This matters because age remains the most important risk factor for many of the diseases that are impacted by sterilization—so, if lifespan increases, this alone could increase the prevalence of a disease linked to sterilization.  Also, because lifespan is not considered in most studies, results may show increases in disease without alerting us that the lifespan increased as well (see, p. 8).  Urfer and Kaeberlein argue that research is needed that uses “lifespan as a relevant measure of positive or negative health outcomes associated with [sterilizing] dogs, regardless of the ultimate cause of death” (p. 8), and this needs to be done despite the existence of several confounding variables that are hard to account for, such as the following:

1. sterilization functions as a proxy for better veterinary care, and

2. to be sterilized, an animal must live to the age of sterilization, which skews the data towards older dogs (p. 8). 

Most of the few studies looking at life span show that sterilization results in longer lifespans for female dogs and, in some studies, males, but the effect was more pronounced in females in all the studies.  One study found that intact males live longer than intact females, but spayed females live the longest.  Only one international study showed no difference in lifespan between intact and sterilized dogs (see, pp. 8-9). 

Other weaknesses in these studies, according to Urfer and Kaeberlein, include the following (see, pp. 8-9).

1. Age at sterilization is not accounted for.
2. The studies only look at surgical sterilization.
3. The strong effect seen in female dogs in many instances may overshadow the effect in males when considering sterilization across both sexes.
4. All but one of the studies used data from deceased dogs.

Disease risk

According to Urfer and Kaeberlein, most studies looking at individual disease risk have relied on data gathered from only a single breed.  Since risk can vary according to breed, the overall risk to your dog might vary significantly from what these studies find (see, pp. 9-13). 

Diseases/disorders	Males	Females
Reproductive sys. tumors	Eliminates testicular tumors and prostatic hyperplasia; increases risk of prostate tumors; decreases risk of venereal tumors	Eliminates conditions, including ovarian cancer, uterine cancer, and pyometra; reduces risk of breast cancer; decreases risk of venereal tumors
Other tumors	Increased risk of cardiac and splenic hemangiosarcoma, appendicular osteosarcoma, lymphoma, and transitional cell tumors of the bladder	Increased risk of cardiac and splenic hemangiosarcoma, appendicular osteosarcoma, lymphoma, transitional cell tumors of the bladder, and mast cell tumors
Joint disorders	More common disorders include CCL injury, hip dysplasia (HD), elbow dysplasia (ED), and osteoarthritis generally	More common disorders include CCL injury, HD, ED, and osteoarthritis generally
Urinary	___	Limited risk of increased incontinence and urolithiasis, esp. in larger dogs and those spayed under 3 mos. Of age
Obesity	Increased risk, plus an obesity associated increased risk of diabetes	Increased risk
Other	Increased risk of immune-mediated diseases, reduced risk of several congenital or early onset disorders, longer survival after onset of epilepsy	Increased risk of immune-mediated diseases, reduced risk of several congenital or early onset disorders, longer survival after onset of epilepsy

Reproductive system tumors.  The beneficial effect of sterilizing female dogs is strong overall, particularly for reducing pyometra and, to a lesser extent, breast cancer.  For males, the strongest effect is on benign hyperplasia, which is not life threatening, but the effect of sterilization on male reproductive-system tumors generally is mixed (see, pp. 9-10). 

Other tumors. Sterilization increases the risk of several tumors in this category in females and, for some tumors, in males as well.  Other than perianal adenomas in males, there are no decreases in the risk of tumors in the “other” category shown for either sex.  For many tumors not specifically mentioned in the review, the evidence regarding risk is inconsistent and varies widely depending on a dog’s breed, age, and gender (see, pp. 10-11). 

Joint disorders.  Sterilization affects the risk of joint disorders generally and among specific breeds.  The most common reported disorder with an increased risk in both sexes after sterilization is a craniate cruciate ligament (CCL) injury, with a subset of studies showing an even greater risk for dogs sterilized before puberty.  Because of variations in risk according to breed, sex, and age at sterilization, more evidence is needed for other listed disorders.  One study found an increased risk of low bone-density, but the result has not been reproduced, and Urfer and Kaeberlein believe that sterilization does not increase risk of bone density loss in females (see, pp. 11-12).

Other.  Sterilization increases the risk of immune-mediated diseases, including among other conditions, atopic dermatitis, Addison’s disease, hypothyroidism, epilepsy (although sterilized dogs lived longer after first seizure), and IBS.  For all but a couple of these disorders, the effect is stronger in females than males.  In contrast, there is actually a reduced risk of several congenital or early onset disorders (see, p. 13).

This was a lot of information! —with full credit to Urfer and Kaeberlein.  But as you can see, it doesn’t, on its own, clarify what you should do with the puppy in front of you. 

So what should you do?

After reading through all this information,

1. Identify your specific concerns; for example, is a condition listed here prevalent in your particular breed?  Was your breed used in a given study? Is your puppy fearful or bold?  Do you have multiple dogs? 
2. Consider what current science says regarding your areas of concern.
3. Last, take all the information that you have gathered to your veterinarian and ask them to help you wade through the process of balancing the pros and cons of sterilization for your particular situation and, most importantly, for your individual dog.