There are any number of third-rail topics that occasionally electrify the conversationally unwary dog owner: Grains or grain-free? Dewclaw removal, cropped ears and tails, or leave well be? Raw or cooked? Flat collars or choke, pinch, or electronic?
But dog owners who came of age in the decades before spay/neuter practices became de riguer are sometimes shocked by the charge, increasingly popular, that sterilizing a dog is tantamount to condemning him or her to a shorter or more painful life. After all, if you are a respectable dog owner, nearly your entire adulthood may have been spent judging people with reproductively intact dogs – especially intact mixed-breed dogs – as supremely irresponsible. What gives? Why the reversal?
Veterinary practitioners have long dithered about what might be the “best” time in a dog’s life to undergo spay/neuter surgery, and some have theorized that coming to maturation without the benefit of secondary sexual hormones might possibly have deleterious effects on health. There have even been small studies looking for such. But there wasn’t any really impactful data that made the average dog owner question the wisdom of spay/neuter until a 2013 study (Torres et al) out of the University of California – Davis that found a link between neutering and the risk of certain cancers and joint disease in Golden Retrievers.
Since then, there has been a steady trickle of studies (with more on the immediate publication horizon) that examine some aspect or another of canine health and how it may be affected by spay/neuter – and the effect of this unending drip of evidence has been torture for many dog owners. We thought we were doing the right thing by sterilizing our dogs!
But is it really so bad to subject dogs to spay/neuter? Many of the studies that people cite to support their claims that the practice is unhealthy for dogs are based on statistically tiny samples, or dogs of a single breed. Extrapolating the results of highly limited studies to assert that spay/neuter is deleterious to all dogs is quite a reach.
To help you understand the modern claims that spay/neuter is bad for dogs, we’ve looked at dozens of studies examining some aspect of the possible health effects of gonadectomy – the removal of the dog’s gonads (sex organs, the testes in males and the ovaries in females). We’ll describe the evidence and discuss what it all means – but here is a little hint about our conclusions: You are still going to have to make your own choice about what’s “best” for you and your dog. And if your dog is already gonadectomized, that’s okay! The evidence is not so cut-and-dried as to support any across-the-board recommendations for all dogs.
WHAT DO GONADS DO?
Before we look at the studies that examine the effects of gonadectomy, it’s helpful to understand what functions the gonads have in addition to reproduction.
Normal male and female dogs each have a pair of gonads.
The male gonads – the testes – reside in the scrotum and produce the male reproductive cells (spermatozoa, sperm for short) as well as androgen hormones that promote male characteristics.
Sperm cells are formed in the seminiferous tubules in the testes; in between these tubules are groups of endocrine cells, called interstitial cells, which produce androgens in response to luteinizing hormone (LH, sometimes referred to as interstitial cell stimulating hormone [ICSH]) secreted from the anterior pituitary gland located in the brain.
The principal androgen produced is testosterone, which is responsible for the development of the male reproductive system and secondary male sex characteristics, such as male body shape and sexual behavior. Testosterone is a steroid hormone that has an overall anabolic effect on the body, promoting protein synthesis and growth of tissues, encouraging the growth of muscle mass and strength, increasing bone density and strength, stimulating linear growth, and supporting bone maturation – all of which results in the larger size and weight of male dogs compared to females of the same breed.
Testosterone also stimulates development of the penis at puberty, the functioning of the prostate (a male accessory sex gland), and activation of sperm formation. Testosterone reaches the highest level in male dogs around 6 to 12 months of age and then it begins to plateau. As soon as a male dog is castrated, testosterone production ceases.
The ovaries are the female gonads, producing the ova (reproductive cells) and the female hormones estrogen (a compound term for the estrus-producing hormones estradiol, estriol, and estrone) and progestin.
Estrogens are produced by the cells of the ovarian follicles and are responsible for female secondary sex characteristic development, contributing to the maturation of the reproductive organs, control of the reproductive system, and for the behavioral and physical changes that occur in preparation for breeding.
Progestins, and in particular progesterone, are produced by the corpus luteum, a mass of cells that develops from the empty follicle after ovulation; they help prepare the uterus for implantation of the fertilized egg, maintain pregnancy, and promote the development of the mammary glands.
The adult male dog’s testes produce spermatozoa and hormones continuously; in contast, adult female canines produce reproductive cells in cycles, occurring about once every six months.
The estrous cycle is controlled by follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. Coinciding with this short period of ovulation, estrogen levels rise, followed by a rise in progesterone levels. After ovulation, progesterone levels remain high for several weeks, even if the dog has not become pregnant. When a female dog is not in estrus, her estrogen and progesterone levels are low.
In addition to these reproduction-based tasks, the dog’s hormones function as chemical messengers with far-reaching and diverse tasks in the body – and likely include some that have yet to be identified. It should not be surprising, then, that researchers studying the effects of gonadectomy on dogs keep coming up with results that require further exploration.
LOOKING AT THE SPAY/NEUTER LITERATURE
The following is an overview of the major areas of concern regarding the possible adverse health effects of canine spay/neuter and findings from relevant studies. The studies mentioned in the text below (and referenced completely on page 22) are some of the most frequently cited in discussions in veterinary literature.
Overall it appears that spay/neuter is associated with an increased life-span. However, be aware that most of the studies that concluded this looked only at gonadectomy (as opposed to other methods of sterilization) and usually did not take into account the age of spay/neuter.
Furthermore, the occurrence of having spay/neuter performed may contribute to an increased likelihood of better husbandry and veterinary care, which theoretically has a positive effect on life expectancy.
In the retrospective study by Hoffman et al (2013), the records of more than 40,000 sterilized and intact domestic dogs listed in the Veterinary Medical Database (a collection of data from veterinary teaching hospitals) were analyzed for associations between gonadectomy and lifespans and causes of death. It was found that sterilized dogs lived on average 1.5 years longer than intact dogs and life expectancy increased by 13.8 percent in males and 26.3 percent in females.
The study also found that intact dogs were more likely to die of infectious disease, trauma, vascular disease, and degenerative disease, and sterilized dogs were more likely to die of neoplasia (including an increased likelihood of transitional cell carcinoma, osteosarcoma, lymphoma, and mast cell cancers) and immune-mediated diseases. No causal relationship was found; note that gonadectomized dogs live longer and cancer is more prevalent in older dogs.
The dataset did not include at what age a dog was spayed or neutered, or whether a dog had reproduced prior to gonadectomy.
The Hoffman study findings were supported by Banfield’s State of Pet Health 2013 Report, which looked at data from Banfield facilities across the nation and included 2.2 million dogs.
The literature review by Houlihan (2017) looks at the research on musculoskeletal diseases and possible associations with spay/neuter. Several studies have found gonadectomy to be a risk factor for development of cruciate ligament disease (CLD) and hip dysplasia (HD) in both male and female dogs.
HD has a high genetic component but is recognized as a multifactorial condition. Incidence of CLD tends to occur in young, active, large breed dogs from degenerative or traumatic causes, but it has also been correlated to aging, conformational abnormalities, and immune-mediated joint issues.
One focus for recent research is the assessment of the tibial plateau angle (TPA) – the slope at the top of the tibia. The steeper the TPA, the more stress on the ligament resulting in an increase in risk for CLD. Studies have demonstrated that the TPA is steeper in dogs that undergo gonadectomy before the closure of the tibial growth plates. The risk, however, may have a breed predisposition: Hart et al (2014) found that CLD risk increased in Golden Retrievers who were gonadectomized between the ages 6 and 11 months, but the risk for Labrador Retrievers did not increase when undergoing gonadectomy at the same age.
In a study of 759 client-owned Golden Retrievers, Torres et al (2013) looked at the effects of spay/neuter on joint disorders and cancer. The authors state, “An important point to make is that the results of this study, being breed-specific, with regard to the effects of early and late neutering, cannot be extrapolated to other breeds, or dogs in general.”
It is well documented that both testosterone and estrogen play an important role in the growth and maturation of bones. A decrease in bone density in spayed Beagles has been described in one study, but these results have yet to be reproduced in subsequent studies.
One heightened concern is whether gonadectomy affects the closure of growth plates (physes). Salmeri et al (1991) found that overall growth rates appear to be unaffected by spay/neuter, although prepubertal gonadectomy has been associated with delayed closure, resulting in lengthening of associated limb bones. While this can be statistically relevant, it is not readily visible or determined to be clinically relevant.
The age at which growth plates close is dependent on breed, genetic factors and disorders, physiological conditions, disease, and nutritional conditions (unbalanced or incomplete diets can result in growth abnormalities). Certain categories of breeds, such as working, herding, and sporting breeds, show greater susceptibility to orthopedic disorders in general; specifically dogs having large stature or great substance are at a greater risk for hip and elbow dysplasia (Oberbauer et al, 2019).
Spain et al (2004) found no specific correlation between age at spay/neuter and incidence rates of arthritis or long bone fractures, including physeal fractures. This retrospective study at the Erie County (New York) SPCA looked at 1,842 dogs who underwent spay or neuter surgery between 6 weeks and 12 months of age. Dogs who had the procedure before 5.5 months of age were found to have a higher incidence (6.7%) of hip dysplasia and were diagnosed at an earlier age when compared to dogs undergoing the procedure at the age of 5.5 months or older (4.7%).
However, those dogs who had spay/neuter surgery when they were older than 5.5 months were three times more likely to be euthanized due to hip dysplasia than those who had surgery when they were younger. This suggests that early gonadectomy may be associated with a less severe form of HD.
Estrogen has a number of metabolic functions and its effect on muscle, tendons, and ligaments has become the focus of more research. Chidi-Ogbolu and Baar (2019) found that, while estrogen improves muscle mass and strength and increases the collagen content of connective tissues, it decreases stiffness in tendons and ligaments, which can directly affect performance and injury rates. (Risk of cranial cruciate ligament [CCL]injury appears to increase with spay/neuter across the general dog population as well as in the individual breeds studied.)
Kustriz (2007) did not find any studies at that time that would implicate changes in physeal closure with subsequent asynchrony of long bone growth and abnormalities in joint formation as a cause of CCL rupture in dogs.
Behavior is the result of a complex interaction between genetics and environment. It has been noted that spay/neuter can mitigate some behaviors – and that’s about as far as the data can take you. The few effects that have been studied and found to be statistically relevant have generally been positive.
Studies generally report that spay/neuter reduces libido and decreases the associated reproductive behaviors. Spayed females tend not to engage in any of the behaviors associated with estrus and therefore do not seek out breeding opportunities.
Neutered males tend to show a decrease in roaming, intermale aggression, mounting, and urine-marking behaviors. There is consistent evidence that the frequency of urine marking does not depend on the age at gonadectomy.
Kustritz (2007) reported that neither reproductive status nor age at the time of spay/neuter has been found to affect the trainability of working dogs.
According to Duffy and Serpell (2006), behavioral changes are difficult to measure; the parameters with which they are measured are too subjective. Breed, sex, and individual differences need to be considered when examining the manifestation of behavioral changes following spay/neuter. As a result, there is not a clear consensus on what the real effects are.
Furthermore, “interpretation of the literature related to behavioral changes is further complicated by various definitions of aggression” (Houlihan, 2017); as a result, evidence for the influence of gonadectomy on aggressive behavior is inconsistent and sometimes contradictory.
Kustritz (2007) reported that several studies showed an increase among female dogs of heightened reactivity toward humans following spay. It is hypothesized that this may be due to a decrease in estrogen and oxytocin levels. Additionally, testosterone has been shown to increase confidence; this may be useful for timid dogs, but may not be for overconfident ones.
Spay surgery has been shown to have a beneficial effect on life-limiting diseases in female dogs. It not only eliminates the risk of pyometra and uterine and ovarian cancers, but also reduces the risk of mammary cancer.
Research into the influence of spaying on mammary cancer has been extensive. Dorn et al (1968) found that there is strong evidence that ovarian hormones are essential for the development of most cases of mammary cancer, so removal of the ovaries decreases this risk. Subsequent studies have continued to support the protective effect of early spay.
The greatest benefit occurs if spay takes place before the first estrus; the reported rates are .05% if before estrus, 8% if performed after the first estrus, increasing to 26% after second estrus cycle (Schneider et al, 1969). Mammary cancer rates increase greatly with age in dogs. Purebred dogs have been shown to have two times the rate of mammary cancer when compared to mixed-breed dogs of the same age. The incidence rate of mammary neoplasia is estimated to be about 3.4%, with about 50% being benign fibroadenomas and 50% malignant adenocarcinomas.
Castration eradicates the risk of testicular cancer (as the testes are removed) in males. Castration also reduces the risk of age-related prostate issues, benign prostatic hyperplasia (common but not generally life-limiting), and perianal adenomas.
Several studies that show testicular neoplasia is common in older (mean age of 10 years) intact male dogs; however, metastasis is uncommon and castration at time of diagnosis is curative. Benign prostatic hypertrophy is also common in intact male dogs (63.4% in one study). It tends to manifest in 50% of dogs aged 2-3 years, and in 75-80% by age 6. Castration results in a decrease in prostate size resulting in a reduction of clinical signs (Kustritz, 2007).
The literature review by Urfer and Kaeberlein (2019) reports that there are many studies that provide evidence for an increase in risk for cancer in dogs of both sexes that have undergone gonadectomy. Smith (2014) summarized that male dogs were at an increased risk after castration for developing cardiac tumors, osteosarcoma, prostatic tumors, transitional cell carcinoma, and lymphoma, while the risk decreased for testicular cancer.
In female dogs, there was an increased risk post-spay of cardiac tumors, cardiac and splenic hemangiosarcoma, osteosarcoma, mast cell tumors, and lymphoma, while the risk decreased for ovarian, uterine, and mammary cancers.
Many of the studies did not take age into account – which is arguably the most important factor for tumor development. However, when the studies did take age into account, increased age was found to be a higher risk factor than spay/neuter.
It’s been said that cancer is, ultimately, the result of failed immune surveillance. It is suspected, but not yet proven, that cancer-hunting immune cells depend to some extent on signals from the sex hormones to perform this surveillance. Researchers have speculated that the cancer-hunting immune cells may be less effective at this task in gonadectomized dogs due to the lack of hormonal signalling.
In the breed-specific retrospective study by Kent et al (2018), the timing of spay/neuter was not available for most of the Golden Retrievers. While the study assessed cancer as a cause of death, the association of hormonal exposure on lifespan or the risk of death by cancer couldn’t be evaluated, nor was it part of the evaluation for the risk of cancer development. Given that Golden Retrievers are known to be at a high risk for cancer, these results cannot be extrapolated to other breeds.
The study by Cooley et al (2002) found that Rottweilers of both sexes who had undergone early spay/neuter had higher risks for bone sarcoma (1 in 4) when compared to Rottweilers who were intact throughout their lifetime. However, the study acknowledges that it is not known how hormones affect the development of osteosarcoma. Makielski et al (2019) published a comparative review of osteosarcoma risk factors in dogs and humans and included this commentary on trending current hormonal studies:
“… associations between reproductive status and development of osteosarcoma have been inconsistent. Although several reports suggest that spayed and/or neutered dogs have higher incidence of certain cancers, including osteosarcoma, the relationship between reproductive status and cancer risk may be confounded by other variables, such as the documented tendency toward increased adiposity and body condition in gonadectomized dogs.”
In dogs, obesity is influenced by diet, breed, activity level, and age, but spay/neuter has also been reported to be a common predisposing risk factor for increased body weight. There is conflicting information as to whether gonadectomy alters metabolism (Reichler, 2009). It is speculated that gonadectomized dogs in general have lower metabolic rates (it has been estimated that they may require as much as 30% fewer calories) and tend to gain weight more than intact dogs, however, the cause and effect relationship is not well defined.
Spain et al (2004) conducted a population study that indicated that gonadectomy of dogs before 6 months of age is associated with a lower prevalence of obesity when compared to those undergoing gonadectomy after 6 months of age.
In 2019, Bjørnvad et al published a study of dog- and owner-related risk factors for obesity in Danish companion dogs. The research found castrated male dogs were at increased risk for obesity; it is suspected that this may be due to a reduction in testosterone and a subsequent lowering of basal metabolic rate. Female dogs were found to be at risk regardless of reproductive status. They also found that there was a complex association between the owner’s weight, the dog’s weight, and feeding habits.
Studies place the incidence of urinary incontinence in spayed female dogs at 4% to 20%, compared to a rate of 0.4% to 8% for intact females. Spayed dogs may develop incontinence within days of surgery or more commonly years later; it is typically controlled with treatment. Large and giant breeds appear to have a higher risk. Other factors that may contribute to the condition and need further evaluation are urethral length, resting position of the urinary bladder, breed, thyroid level, allergies, and level of obesity.
Studies are contradictory when it comes to determining a correlation between age at time of spay and the likelihood of developing incontinence. Spain et al (2004) and Thrusfield et al (1998) reported an increase in urinary incontinence in females who were spayed at an early age, yet other studies have failed to support this conclusion. More research is required, but in those studies that did find a correlation, it was associated only with pediatric (6-12 weeks) gonadectomy. Females spayed at an early age have also been reported to have had a slightly higher incidence of bladder infections, but these infections were easily treated and did not become chronic.
It has been theorized that it is the lack of estrogen that causes incontinence in spayed females, but this is controversial and not fully supported by research. Increased rates of incontinence are not reported in pregnant females even though they have extremely low estrogen levels during pregnancy.
Palm and Reichler (2012) report that incontinence in spayed dogs has been successfully treated with use of gonadotropin-releasing hormone (GNRH) superagonist implants. The implants work by suppressing the release of gonadotropins.
In contrast, surgical gonadectomy increases the release of gonadotropins. This suggests that an increased risk for incontinence is not caused by the lack of sex hormones, but rather by the increased levels of gonadotropins induced by removal of the ovaries.
Male dogs who have been castrated prepubertally tend to have a smaller penis and prepuce, but their urethral diameter and function are the same as dogs neutered later and no clinical significance or condition has been associated with this difference (Salmeri et al, 1991).
Findings from Sundberg et al (2016) suggest that spay/neuter is associated with an increased risk for certain autoimmune disorders. Six of the 11 immune diseases evaluated (atopic dermatitis, autoimmune hemolytic anemia, Addison’s disease, hypothyroidism, immune-mediated thrombocytopenia, and inflammatory bowel disease) showed an increased prevalence in gonadectomized dogs.
The study notes that even though the dataset included more than 90,000 dogs and expression of the diseases were statistically relevant, the actual incidence rate was not high and it declined over the 15-year evaluation.
Given that this was a retrospective study limited to dogs seen at a referral veterinary hospital, it may not reflect incidence rates within the population at large but rather may be biased to complex or more severe cases.
There has been limited research conducted on the risk that cognitive function may be altered as a result of spay/neuter. A comparison of the progression of cognitive dysfunction in intact and castrated male dogs was performed in the Hart study (2001) with a small sample size (6 dogs); it revealed a slowing of progression in the intact dogs.
In contrast, a 2000 study by Waters et al found that intact Beagles showed DNA damage to the neurons in the brain when compared to castrated Beagles (again, sample size was small, with only four dogs in each group). This is an area of research that is just beginning to be explored. Much more research is needed to understand the processes that influence cognitive function and how they may be changed by spay/neuter.
Statistically, puppies are less likely to die under anesthesia and recover faster from gonadectomy than their adult counterparts. Complications arising from the procedure are uncommon and the rates are consistent across ages.
WHAT (AND HOW) SHOULD YOU DECIDE?
Even a minimal survey of the research regarding the effects of spay/neuter reveals that the situation is extremely complex and, at times, ambiguous. There is evidence to support correlations for both beneficial and adverse effects, but even more important is that it demonstrates how much we still don’t understand about reproductive hormones and the consequences of spay/neuter.
When the time comes for you to make spay/neuter decisions for a dog that you do not want to reproduce, remember: There is no single course of action that is “best” for all dogs and all owners, and don’t let anyone make you feel bad for your decision, whatever it is – that is, as long as it doesn’t end with an accidental breeding and unwanted puppies.
Here’s the one time that we feel it makes the most sense for an owner to give more weight to published research than their own preferences: when the person owns a purebred dog of a breed that has been the subject of large, well-respected studies of the effects of spay/neuter on dogs of that specific breed, and the study found clear and significant statistical advantages to a certain course of action. In that case, we would strongly recommend reading the conclusion of those studies and discussing them with your veterinarian. Oberbauer et al (2019) determined that many canine health disorders reflect the dogs’ genetic heritage. Within breeds, there may be shared genetic susceptibility that increases risk for certain diseases within breeds and this risk may be enhanced with neutering.
However, you have to take single-breed studies with a grain of salt if your dog does not share any of the subject breed’s genes. Some of these studies are widely cited by people who think the studies should inform the decisions of all dog owners, but the findings often are contradicted when applied to another breed.
Some people strongly believe that it’s unethical to spay or neuter dogs, because the procedure irretrieveably alters the dog’s physiology and might might cause an adverse side effect, perhaps years in the future. As we have described, however, intact dogs are also prone to adverse health conditions; there simply isn’t a choice that doesn’t have consequences!
OWN YOUR DECISION – AND RESPECT OTHERS
As we’ve stated elsewhere in this article, it has become sort of politically correct today to maintain a dog in his or her intact state. But this isn’t something that everyone can manage in a responsible fashion! If there is a single weak link in a household, whether it’s a forgetful child, a distracted adult, or a less-than-super-secure fence, accidents can and will happen.
We know owners who swear their female dogs never left their sides and had zero contact with another dog, and yet – poof, a virgin pregnancy? Doubtful, and irresponsible, too.
And while some people will try to make you feel bad about it, it’s okay to admit that you do not enjoy living with an intact dog of either sex! If you have grown up in a time and place where literally all the dogs you’ve ever known were neutered, you might be quite alarmed at the personality change exhibited by your female dog when she comes into heat. You may not feel comfortable with some of the more strongly masculine attributes of an intact male dog, which may include more competitive urine-marking, humping, or overzealous sexual interest in female dogs.
Also, there are many people who are strongly committed to adopting only from shelters or rescue organizations, where spay/neuter is not only mandated but might also have been performed on very young puppies. Not only is prepubertal gonadectomy an important tool against pet overpopulation, it is likely to improve the odds that dogs will be retained by their owners. Studies have found that intact dogs are more likely to be relinquished than those that have undergone spay or neuter.
For intact dogs with homes, veterinarians and owners are challenged with making the best decision for that specific dog. An informed decision requires an evaluation reflective of our dogs and our risk tolerances. Every dog is an individual, including how they respond to gonadectomy or remaining intact. We always recommend consulting with your veterinarian to determine the best strategy for your dog based on age, body condition, breed, genetics, lifestyle, behavior, temperament, and reproduction management – and then taking responsibility for your choice.
Barbara Dobbins, a former dog trainer, writes about dogs and studies canine ethology. She lives in the San Francisco Bay area with her dogs, Tico and Parker.