Proponents of spay/neuter claim that spayed and neutered dogs enjoy a longer and healthier lifespan than their intact counterparts. Based upon the scientific information presented on this website, it seems unlikely spay/neuter would extend our dogs’ lives. Rather, it seems clear to us spay/neuter not only shortens the lives of our dogs, but diminishes their ability to enjoy their lives in happiness and good health. A review of the most current body of evidence with respect to human and canine aging and longevity follows; it confirms the legitimacy of comparing dogs and humans, and substantiates our position that spay/neuter shortens our dogs’ lives.
Why is the dog an ideal model for aging research? The dog has already become a key model system in which to evaluate surgical techniques and novel medications because of the remarkable similarity between human and canine conditions, treatments, and response to therapy… Just as the dog offers a natural model for human conditions and diseases, simple observation leads to the conclusion that the canine aging phenotype also mimics that of the human. 114
Initially, we chose to look at results in the Nurses’ Health Studies, among the largest (and longest follow up-now at 40 years) investigations into the risk factors for major chronic diseases in women. It was found, compared with ovarian retention, bilateral oophorectomy (i.e., removal of the ovaries) at the time of hysterectomy for benign disease is associated with a decreased risk of breast and ovarian cancer. However, there is a corresponding increased risk of all-cause mortality, fatal and non-fatal coronary heart disease, and lung cancer. In no analysis or age-group was oophorectomy associated with increased survival.115
Other more limited studies confirmed the findings of the Nurses’ Health Study:
“Bilateral oophorectomy performed before age 45 years is associated with increased cardiovascular mortality, especially with cardiac mortality. However, estrogen treatment may reduce this risk.” 116
Next we looked at a specific canine study designed to determine whether lifetime retention of ovaries was associated with exceptional longevity. The authors found:
“Our results mirror the recent findings from more than 29,000 women in the Nurses’ Health Study. In that study, women who had elective hysterectomy with ovary sparing had lower overall mortality than those who underwent hysterectomy with ovariectomy. Taken together, the findings from dogs and women support the hypothesis that early life physiological influences, such as ovarian hormones, lay the foundation for adult health outcomes including longevity.” 117
Questions have persisted as to whether presence of the ovaries conferred longevity, or perhaps the actual process of reproduction and/or the number of offspring or other variables conferred longevity. We found studies of both humans and dogs designed to answer those questions:
The human study concludes:
“… we analyzed the relationship between longevity and menopause, including other factors that impact “ovarian lifespan” such as births, oophorectomy, and hormone replacement therapy. We found that later onset of menopause was associated with lower mortality, with and without adjusting for additional factors (years of education, smoking status, body mass index, and marital status). Each year of delayed menopause resulted in a 2.9% reduction in mortality…
Our findings suggest that maintenance of HPG [hypothalmic-pituitary-gonadal(ovary)] axis homeostasis is a predictor of mortality…We also found in our preliminary regressions that surgical and natural menopause at age 40 resulted in identical survival probabilities, as might be expected given that both result in the dysregulation of the HPG axis…These results support the maintenance of the hypothalamic–pituitary–gonadal axis in homeostasis in prolonging human longevity, which provides a coherent framework for understanding the relationship between reproduction and longevity.” 118
The study of dogs states:
“To better understand the potential trade-off between female reproductive investment and longevity in an emerging model of human healthspan, we studied pet dogs to determine whether intensity of reproduction (total number of offspring) encumbered the likelihood of exceptional longevity. Instead, independent of reproductive investment, the duration of lifetime ovary exposure was significantly associated with highly successful aging. Moreover, our conclusions are consistent with the hypothalamic–pituitary–gonadal (HPG) axis hypothesis of longevity proposed by Atwood and colleagues, in which longer lifetime gonad activity (i.e., delayed onset of menopause), rather than number of offspring, predicts longevity in women.” 119
It seems abundantly clear that extensive research supports the finding that the lifespan of female dogs is shortened by spay (removal of ovaries and uterus) or even the removal of the ovaries alone, for the sole purpose of preventing reproduction. However, the veterinary community at large has chosen to rely upon other studies over time to justify their position of advocacy for spay/neuter. The most recent of these studies was published in the highly referenced open access scientific journal produced by the Public Library of Science (PLOS). The study concludes that the average age at death for dogs that had not been spayed or neutered (i.e., reproductive organs remain intact) was 7.9 years versus an average age at death of 9.4 years for spayed or neutered dogs. However, the study does contain an admission that there is no ability to conclusively state that failure to spay or neuter causes a shortened lifespan. The study states:
“Although a retrospective, epidemiological study such as this cannot prove causality, our results suggest that close scrutiny of specific causes of death, rather than lifespan alone, will greatly improve our understanding of the cumulative impact of reproductive capability on mortality.” 107
Let’s examine the causes of canine death as discussed in the PLOS study. With respect to spayed/neutered dogs, the study’s findings are consistent with those we have outlined on this website, i.e., dogs that had been spayed/neutered were more likely to die from cancer or autoimmune diseases.
The PLOS study also concluded that intact dogs were more likely to die from any combination of five specific infectious diseases (i.e., distemper, parvovirus, heartworm, intestinal parasites and blastomycosis) and/or trauma. The conclusions of the PLOS study regarding intact dogs are not supported by the evidence.
The first three infectious causes of death in intact dogs utilized in this study are distemper, parvovirus, and heartworm. Distemper and parvovirus can effectively be eliminated by vaccination. Heartworm can be eliminated by preventative medication taken on a monthly basis. There is no information in this study to tell us whether intact dogs who died from these illnesses were vaccinated or medicated to prevent these diseases or not. If intact dogs were not vaccinated or medicated, how could one conclude they died because they were not spayed/neutered? In fact, it seems much more likely the dogs died because they weren’t vaccinated. We find it next to impossible for the study to draw any relevant conclusions regarding reproductive status until the obvious questions about vaccination and premedication are answered. Therefore, the statistics cited by the study regarding intact dogs dying from these three infections are meaningless in the context of reproductive status and must be discarded.
As to the two remaining infectious causes of death in intact dogs (i.e., intestinal parasites and blastomycosis), it does appear geographical location is very significant. Intestinal parasites are more likely to be encountered in rural or mountainous areas rather than urban or suburban environments. According to a veterinary medicine reference article:
“Blastomycosis is a systemic fungal infection caused by the dimorphic fungus Blastomyces dermatitidis…Dogs at greatest risk for developing clinically apparent blastomycosis are 2- to 4-year-old intact male large-breed dogs living in Mississippi, Missouri, and Ohio river valleys and the Middle Atlantic states…This group of dogs has a greater tendency to roam and to sniff and dig in the soil, resulting in greater exposure to the organism. Sporting dogs and hound breeds are predisposed, most likely because of increased exposure to high-risk areas during hunting. Residence near a river or lake has been demonstrated to increase the risk of infection.”
Because blastomycosis is so specific to certain geographical locations, and even to certain recreational activities, the location of the dog’s residence is critical if one were to make legitimate comparisons of lifespans between spayed/neutered dogs and intact dogs. The failure of the PLOS study to provide any information as to the location of residency for the spayed/neutered vs. intact groups of dogs is disturbing and certainly limits our ability to accept their conclusions without great skepticism. Furthermore, the demise of dogs from this deadly disease at a young age will serve to skew the lifespan statistics markedly down for intact dogs.
The inclusion of “death by trauma” in the PLOS study is a red flag. Because this study is attempting to assess medical outcomes based upon reproductive status, there were certain dogs excluded from the evaluation. For example, congenital causes of death (i.e., a disease or physical defect one is born with) were removed from subsequent analysis because they would have been present before the time that sterilization was or was not elected.
Likewise, trauma should have been eliminated in the study with respect to the calculation of lifespan for all dogs as it has no clear relationship to the actual health of the dog at the time of its death. The longevity of the intact or altered dog can be dramatically skewed based upon the risks for injury or death that are lifestyle related. As an extreme example, would it be reasonable to include in the study bomb sniffing dogs serving in law enforcement or the military? Close evaluation of the PLOS study reveals a comparison of what appear to be two very disparate geographical populations with very different lifestyles. Rural dog guardians are less likely to spay or neuter their dogs, and would be less inclined to follow the dictates of the veterinary profession with respect to wellness exams and vaccinations. The lifestyle of a rural dog generally involves different activities (e.g., hunting) where the dog may encounter wildlife and venture into wild areas without continuous and direct owner supervision, thus increasing the risk of traumatic harm to the dog. On the other hand, dogs in most urban/suburban areas are required by law to be licensed, and licensure requires proof of vaccination. Doggy day care, boarding facilities and dog parks – commonplace habitats for the urban/suburban canine – all require proof of vaccination as well.
We have already presented extensive studies with respect to spay and female dogs’ longevity. With respect to male dogs, we feel it is most appropriate to provide other published scientific research which substantiates long held theories about aging and stress. The idea of a relationship between stress and aging has traditionally taken two approaches, as described in a Sage Science classic paper:
First, research (including this Sage paper) has frequently confirmed the concept that aging renders the individual less able to adapt to stress.106 We would point out it is undisputed that spay/neuter puts additional stress upon the adrenal glands to produce the sex hormones once produced by the testes and ovaries after they are removed in spay/neuter. The adrenals often compensate nicely when the dog is young. However advancing age or physical challenges such as surgery can render the adrenal glands unable to compensate and produce enough, if any, sex hormones at all.
The second concept is that chronic stress can accelerate the aging process. The Sage paper supports this concept as well.106 The authors describe the results of their current study which showed that aged male rats, when exposed to chronic stress develop runaway glucocorticoid production. As discussed in this Sage paper and our Hormones and the Endocrine System section, stress has a profound effect upon the adrenocortical axis. Consistent with the findings in the paper, we are currently observing an unexplained surge of spayed/neutered dogs with runaway glucocorticoid (cortisol) production, i.e., hyperadrenocorticism/Cushings. We believe stress upon the adrenocortical axis contributes directly to the endocrine disease syndrome (i.e., Cushings, Atypical Cushings, hypothyroidism, hyperestrinism and diabetes) we are seeing in the spayed/neutered dog population.
To summarize, we believe spay/neuter, by dysregulating the hormonal axes of our dogs (i.e., the hypothalmic-pituitary-gonadal and/or adrenocortical), can be shown to shorten their lifespan.