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Intact Dogs Most Often Die from Infection or Trauma

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.

Written by

Susan Chaney has been on the editorial side of publishing since 1990, starting her career as a newspaper features writer and editor. A lifelong lover of dogs, Susan has lived with German Shepherds, Labs, Yorkies, an Irish Setter, a Great Dane-Bloodhound mix, a Sheltie and currently a Chihuahua mix of unknown pedigree. She was the editor of Dog Fancy magazine, content editor of DogChannel.com and group editor of Dog World, Dogs USA, Puppies USA, Natural Dog, Cat Fancy, Cats USA and Kittens USA from March 2005 to December 2009 when she left her position to work at home, part-time. Susan lives in Long Beach, Calif., with her artist husband, Tim, that Chi mix and two big cats. As an editor and writer for Best In Show Daily, she is reveling in the amalgam of three loves: writing, editing and dogs.
  • Jenell Brinson April 24, 2013 at 4:56 PM

    Attempting to draw cause-effect conclusions from correlation is risky at best. My initial thought to the results of this study are that higher death rates of infectious disease, infection, and trauma are most likely the result of difference in behavior of both the animals and their owners. Intact animals are more like to be prone to straying, roaming, coming at greater risk of infection and traumatic injury. But also, while keeping pets intact doesn’t necessarily mean an owner is less committed to reponsible care and handling of their pet, there is no question that less committed, less responsible pet owners are less likely to spay neuter, and likewise less likely to take precautions such as vaccinating, regular veterinary care, to prevent or treat infections, and more likely to allow animals to roam, whether there is increased risk of traumatic injury.

    • Barb Bristol April 24, 2013 at 6:32 PM

      Jenell you took the words out of my mouth! Well said.

    • Susan Chaney
      Susan Chaney April 25, 2013 at 9:47 AM

      Thank you for commenting, Jenell. I believe the researchers who conducted this analysis of veterinary college records were attempting to make a first pass at determining the possible effects that sterilization has on dogs. I suspect that you are correct that many of the results may be due to owner behavior. If that is the case, it would be great to quantify it, in my opinion.

  • Helen Dorrance April 24, 2013 at 6:49 PM

    In some ways this study contradicts the recent UC Davis Study that showed intact golden retrievers lived longer than goldens spayed or neutered before a year of age. Kudos on another well-written article by Susan Chaney. I frequently hand out her informative articles to the clients at my boarding kennel.

    • Susan Chaney
      Susan Chaney April 25, 2013 at 9:52 AM

      That’s the conundrum of research, Helen. The U of Ga. researchers did track breed and adjusted for diseases known to be prevalent in particular breeds, so that may help explain the disparity between these two studies. Whenever I’m writing these articles, I can’t help but think about the criticisms of scientific research in general and its fallibility. But it’s all we have, so… Thank you for reading and for the compliment. It`s wonderful to know that dog owners can benefit from my work.

  • Mary Mahaffey April 25, 2013 at 3:30 AM

    I have the feeling that the authors of the above study are making conclusions while ignoring many variables such as dogs being allowed to run loose, etc. I might have missed it, but I did not find that they were even aware of previous research done by Dr. David Waters of the Gerald P. Murphy Cancer Foundation that found that female Rottweilers having their ovaries for at least 6 yrs was associated with exceptional longivity. Apparently, this also applies to women (keeping their ovaries until 50).

    See the links below:



    • Susan Chaney
      Susan Chaney April 25, 2013 at 9:59 AM

      Thank you for pointing our readers to that information, Mary. As Jenell pointed out as well, the average life span would certainly be affected by owner behavior. The researchers did say their results were for the “average” dog, but of course that’s the “average” dog who ended up at a veterinary teaching hospital for treatment, then died. Their data didn’t include all the dogs that are treated by private-practice veterinarians and never get to a veterinary college.

  • Susan Chaney
    Susan Chaney April 25, 2013 at 10:06 AM

    One of the reasons I wanted to report on this study is that whenever I’m writing about a veterinary topic I ask my sources: “Are there any statistics on how many dogs get x, how many survive and with what long-term effects, and how many die from x each year?” Much more often than not, these experts say, “No. We don’t have that information.” This is because private-practice veterinarians do not report the incidence of anything other than certain contagious, catastrophic diseases to any organization or agency that collects the info and analyzes it — unless they’ve been tapped by a researcher or are cooperating with, for example, an AKC CHF or a Morris Animal Foundation study. As frustrating as it is for me as a writer, I can’t imagine how frustrating it is for researchers who want to get to the bottom of canine diseases. So, when a study of this kind comes along with data from so many dogs, I think of it as a step in the right direction.

  • Linda Rehkopf April 26, 2013 at 1:42 PM

    The UGA study only included dogs that were treated at veterinary teaching schools/hospitals. According to my contacts in the field, and my own UGA-trained vets, most owners who take their dogs for treatment at vet schools already know their dog is very ill or injured, or hope that center’s protocols will help their individual dog. The owner(s) are also more likely to spend more time and money trying to treat the dog. That excludes most dogs right there, given the rates of economic euthanasia. So while the information in the UGA study is very important, I think that we cannot make generalizations or conclusions for the health or disease of the entire dog population.

    The UC Davis study (cited above) goes indepth into a breed, and might be much more valuable to other with similar breeds (including this Labrador retriever owner). Thanks for covering this topic, Susan.

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