Spayed and Neutered Dogs Live Longer

By Jennifer Coates, DVM on Jun. 20, 2013

We talked recently about a study that revealed an increase in the incidence of some significant diseases in neutered male and female dogs in comparison to intact individuals. Disease incidence is important, but the statistic that is of greatest interest to most pet owners is survivability, in other words, “what effect will a particular decision (e.g., neutering) have on my dog’s lifespan.”

Research published on April 17, 2013 in the online journal PLoS ONE looked at the decision to neuter dogs with that endpoint in mind. Based on the debate that surrounded my previous post, the results of this study might surprise some of you.

Looking at a sample of 40,139 death records from the Veterinary Medical Database from 1984-2004, scientists from the University of Georgia determined the average age at death for dogs that had not been spayed or neutered was 7.9 years versus 9.4 years for sterilized dogs. Dogs that had been spayed or neutered were more likely to die from cancer or autoimmune diseases while those that were not 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," said Jessica Hoffman, a UGA doctoral candidate who co-authored the study.

Researcher Kate 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."

The authors offer potential explanations for these observations in the PLoS ONE paper:

Sterilization increased the risk of death due to neoplasia, but did not increase risk for all specific kinds of cancer. Female dogs sterilized before sexual maturity are unlikely to develop mammary cancer because of the decrease in cumulative estrogen exposure associated with the absence of the estrus cycle[30]. However, it is not clear why the frequency of some cancers outside the reproductive system, including lymphoma and osteosarcoma, is influenced by sterilization, while the frequency of others, such as melanoma and squamous cell carcinoma, is not. The increased risk of death due to cancer observed in sterilized dogs could be due to the fact that in both sexes, dogs sterilized before the onset of puberty grow taller than their intact counterparts[31] as a result of reduced estrogen signaling [32]. Recent studies in humans suggest that growth is a risk factor for a number of different cancers[33].
Conversely, sterilized dogs had a decreased risk of death due to infection, and avoidance of infection may partly explain their longer lifespans. The relationship between sterilization and infectious disease could arise due to increased levels of progesterone and testosterone[34] in intact dogs, both of which can be immunosuppressive[35], [36]. Studies in humans, mice and rats reveal patterns of infectious disease morbidity and mortality associated with testosterone and estrogen exposure. However, these patterns vary with host species, type of pathogen, and chronicity of infection[37]. Additionally, sterilization and disease risk might both be correlated with specific canine behaviors. Given the opportunity, intact male dogs are more likely than sterilized dogs to roam, and to fight with other dogs, and intact female dogs show more dominance aggression than spayed females[38],[39]. These behaviors might increase the risks of both infectious and traumatic causes of death among intact dogs.

The authors note that the average life span seen in this study is likely lower than what would be observed in the population of dogs at large. Animals included in the study had been referred to a veterinary teaching hospital and represent a population of sick animals.

"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," Creevy said. "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."