Sterilized dogs live longer but are more likely to die from cancer, according to University of Georgia researchers. The study, published Wednesday in the online peer-reviewed journal PLOS ONE, found that intact dogs—those not neutered—lived an average of 7.9 years, compared to 9.4 years for sterilized dogs. The discovery was based on a sample of 40,139 death records contained in the Veterinary Medical Database, a collection generated by North American veterinary medical colleges. "There is a long tradition of research into the cost of reproduction, and what has been shown across species is if you reproduce, you don’t live as long,” said Kate Creevy, DVM, MS, Dipl. ACVIM, an assistant professor at Georgia’s College of Veterinary Medicine. "The question that raises is, why would you die younger if you have offspring?” The researchers learned that neutered dogs were more likely to die from cancer or autoimmune diseases. Intact dogs were more likely to die from infectious disease or 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 Georgia doctoral candidate who co-authored the study. The types of cancers documented in the sample puzzled the researchers. "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,” they wrote. Reproductive hormones, particularly progesterone and testosterone, may suppress the immune system, Dr. Creevy said. "There are a few studies of people who are sterilized, specifically among men who are castrated for cultural or medical reasons,” she said. "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.” Previous studies that looked at the effects of reproduction on human survival rates showed mixed results, said Daniel Promislow, Ph.D., a Georgia genetics professor and co-author. "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,” he said. Dog owners should take note of the study, Creevy added. "Our study tells pet owners that, overall, sterilized dogs will live longer, which is good to know,” she said. "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 noted that the life spans reported in the study are likely lower than those in the canine population at large. The database sample was based on dogs referred to a veterinary teaching hospital. They used statistical formulas to adjust for the increased likelihood of disease in older animals. "The overall average life span is likely shorter than what we would observe in private practice ... 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.” Compared with other pets, dogs were the right choice for the study, Creevy said. "There is no other species where we can even begin to study cause of death as closely as we do with dogs,” she stated. "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.”
