MANY dog owners have their pets spayed or neutered to help control the overall pet population, but new research from the University of Georgia suggests that the procedure could add to the length of the animals' lives and alter the risk of specific causes of death.
Looking at a sample of 40,139 death records in the Veterinary Medical Database from 1984 to 2004, the researchers determined that the average age at death for intact dogs -- dogs that had not been spayed or neutered -- was 7.9 years versus 9.4 years for sterilized dogs. The results of the study were published April 17 in PLOS ONE.
"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 Dr. Kate Creevy, an assistant professor of internal medicine at the University of Georgia's College of Veterinary Medicine. "The question that raises is, Why would you die younger if you have offspring?"
Historically, studies on how reproduction affects life span have been done in model systems like mice, nematode worms and fruit flies, where it is difficult to figure out the eventual cause of death. For the first time, researchers have been able to measure costs of reproduction in terms of the actual causes of death and found that the causes of death differed between sterilized and intact dogs, the announcement said.
Dogs that had undergone a gonadectomy (a spay or castration) were more likely to die from cancer or autoimmune diseases. However, those in the sample that still had functional reproduction systems at death were more likely to die from infectious disease and trauma, the Georgia researchers reported.
"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 University of Georgia doctoral candidate in the Franklin College of Arts of Sciences who co-authored the study.
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. If you are going to keep him or her intact, you need to keep your eye out for trauma and infection."
She said some of the reproductive hormones, particularly progesterone and testosterone, could suppress the immune system, explaining why there is an increased risk of infection among dogs that have been sterilized.
The researchers noted that the average life span seen in their study is likely lower than what would be observed in the population of dogs at large. The dogs 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 (animals) 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."