It’s long been acknowledged – and in some cases it’s quite obvious – that sterilized dogs and bitches won’t get certain diseases. After all, if a dog has no testicles, it’s not going to get testicular cancer.

But now, as many are questioning the practice of sterilizing pet dogs prior to 4 to 6 months of age, a new study says the average dog or bitch will live one and a half years longer if it is neutered or spayed, and whether it lives its life intact will help determine what will end its life.

A recent study from University of Georgia researchers determined that sterilized dogs live on average one and a half years longer than intact dogs. Photo © Can Stock Photo.

According to the study results published in PLOS One on April 17, 2013, “By electing whether or not to sterilize their dogs, dog owners have inadvertently carried out a large-scale epidemiologic experiment on the consequences of effectively eliminating reproductive capability.” PLOS One is an international, peer-reviewed, open-access, online publication of scientific research.

Kate E. Creevy, D.V.M., M.S., assistant professor of internal medicine at the University of Georgia College of Veterinary Medicine, and her colleagues reviewed information from the Veterinary Medical Database, which compiles information from virtually every North American veterinary college,from 1984 through 2004 on 40,139 canine deaths. They found that sterilized canines lived an average of 9.4 years, while intact ones lived 7.9. Not surprisingly, sterilization increased life expectancy in bitches almost twice as much as in dogs: 26.3 percent in females and 13.8 percent in males.

Bitches’ life expectancy is increased almost twice as much as is dogs’ when both are sterilized. Photo © Can Stock Photo.

That doesn’t sound like the average life span of most dogs, considering lots of smaller breeds live well into their teens, and many large-breed dogs live to be more than 10. Creevy explains: “The overall average life span is likely shorter than what we would observe in private practice because these were dogs seen at teaching hospitals, but the difference in life span between sterilized and intact is real. The proportionate effects on causes of death are translatable to the global dog population, and it will be interesting to see if explanations for these effects can be found in future studies.”

While Creevy and her co-authors, Daniel E.L. Promislow, Ph.D., and graduate student Jessica M. Hoffman, B.S., acknowledge that plenty of research has been done to show that, if creatures, such as nematodes, fruit flies and mice, reproduce, they don’t live as long, no one had answered the question, “why would you die younger if you have offspring?” Their study explains: “While invertebrate species…serve as powerful model systems for genetic and molecular investigations, we know little about actual causes of mortality in these species. Studies on worms and flies are unlikely to explain whether reproduction itself and the physiology associated with reproductive capability affect all causes of mortality, or only certain ones. To address this question, we need a model system that is not only well characterized genetically, but is equally well characterized medically, so that we can investigate the underlying disease states that lead to mortality.”

For this, the team turned to dogs, whose physiology is so similar to that of humans that, as most of us know, many studies on new treatments and medical procedures are tried on dogs before they are tested in people.

Dogs that are neutered or spayed are more likely to die from an autoimmune disease or cancer, while intact dogs more often die from an infectious disease or trauma. Photo © Can Stock Photo.

The researchers discovered that dogs that had undergone a gonadectomy – the veterinary term for aspay or neuter procedure – were more likely to die from autoimmune diseases or cancer, while those with functional reproduction systems at death were more likely to die from infectious disease and trauma.

“Intact dogs are still dying from cancer; it is just a more common cause of death for those that are sterilized,” Hoffman said.

Creevy added, “At the level of the individual dog owner, our study tells pet owners that, overall, sterilized dogs will live longer, which is good to know. Also, if you are going to sterilize your dog, you should be aware of possible risks of immune-mediated diseases and cancer; and if you are going to keep him or her intact, you need to keep your eye out for trauma and infection.”

According to the study’s abstract, “Life span is a composite variable of myriad causes of death, and it has not been clear whether the consequences of reproduction or of reproductive capability influence all causes of death equally. To address this gap in understanding, we compared causes of death. Beyond the impact of reproduction on when individuals die, we must investigate its impact on why individuals die, and subsequently must identify the mechanisms by which these causes of death are influenced by the physiology associated with reproductive capability.”

To analyze the data from the Veterinary Medical Database, the team used only a single diagnosis as the cause of death and categorizedeach as congenital, degenerative, infectious, immune-mediated, metabolic, neoplastic (abnormal grown of cells, as in cancer), toxic, traumatic or vascular (related to the circulatory system). If a dog’s diagnosis wasn’t sufficient to allow its categorization, the dog’s info was excluded from the study. In a second analysis, dogs that died of congenital causes were also eliminated.

“We found a striking effect of sterilization on cause of death,” the study reads. “Sterilized dogs were dramatically less likely to die of infectious disease, trauma, vascular disease and degenerative disease,” dying more commonly from neoplasia and immune-mediated disease. “We saw effects of sterilization both on common causes of death, as well as on more rare causes (e.g., vascular disease).” The analysis accounted for breed variabilities in cause of death and also considered the age at which each dog in the analysis died, finding that even within a single age category or “bin,” as Veterinary Medical Database information is divided, “there were visible differences in causes of death for sterilized and intact dogs.”

Although the study identified major differences in the cause of death in intact versus sterilized dogs, the team admits that the link between sterilization and cause of death is not known. “A direct cause-and-effect relationship between reproduction and cause of death is possible, but the actual relationship is likely more complex. In mammals, removal of gonadal hormones has been shown to alter hematological and coagulation parameters, the pituitary-adrenal axis, satiety, neurotransmitters, thymic tissue and behavior,” the study reports. “Any or all of these factors” could explain the difference in causes of death. “Documentation of these outcome differences now creates the exciting opportunity to investigate the possible causal mechanisms in dogs and other species.”

The team concluded that “further and more detailed studies of reproduction and mortality in companion dogs could shed considerable light on this problem.” The fact that the U.S. contains two easily accessed sets of study subjects – dogs that are intact and dogs that are sterilized – provides “an unparalleled opportunity to evaluate outcome differences between the groups.”

Data analyzed did not include whether the female dogs were ever pregnant or had puppies during their lifetimes. Nor did it look at whether male dogs had mated or if those matings resulted in pregnancies.

The researchers’ interest in this topic reaches beyond the canine world: “Shifting the focus from when death occurs to why death occurs could also help to explain contradictory findings from human studies.”

Creevy said: “There is no other species where we can even begin to study cause of death as closely as we do with dogs. They model our own disease risk because they live in our homes, sleep in our beds and eat our food. All of the things that impact us and our health impact them.

“There are a few studies of people who are sterilized, specifically among men who are castrated for cultural or medical reasons. Interestingly, there was a difference in their life spans too, and the castrated men tended to live longer. The men in that study who were not sterilized also got more infections, supporting the idea that there is a physiological reason for this.”

According to Promislow, a genetics professor at the university’s Franklin College, “when researchers have looked at the effect of reproduction on survival rates in humans, the results have varied from one study to the next. Our findings suggest that we might get a clearer sense of potential costs of reproduction if we focus on how reproduction affects actual causes of mortality rather than its effect on life span.”

This article is based on the study results published by PLOS One and a University of Georgia College of Veterinary Medicine press release.