Karen B. London
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In a new study called Reproductive Capability is Associated with Lifespan and Cause of Death in Companion Dogs , researchers report on the links of reproductive status (intact or spayed/neutered) with both lifespan and cause of death. Previous studies have suggested that sterilization increases the risk of certain cancers. However, if spaying and neutering actually does increase life span, then any cancers that are more common in older dogs may only appear to be more common in sterilized dogs because sterilized dogs live longer.
The overall conclusions of this new study are that there is a link between lifespan, cause of death and reproductive status. Sterilization was associated with longer lifespan. The mean age of death for intact dogs was 7.9 years and for sterilized dogs was 9.4 years. Sterilization increased life expectancy 13.8% in male and 26.3% in females.
In this study, researchers found differences in the cause of death between the reproductively capable group and the sterilized group. Compared to reproductively capable ones, dogs who were spayed and neutered were more likely to die of cancer and immune-related diseases, but less likely to die from infectious diseases, trauma, vascular disease and degenerative diseases. These differences in causes of death were consistent when the data were compared between dogs of the same age.
Data are from 40,139 dogs in a veterinary teaching hospital who died from 1984 to 2004. Juvenile dogs, dogs with a congenital issue that caused death, and dogs whose reproductive status, cause of death, or age were unknown were eliminated from the over 80,000 dogs originally considered for inclusion in the study. Reproductive capability was defined in this study as intact versus spayed or neutered, and does not mean the sterilized dogs had not reproduced. It’s unknown if they had reproduced prior to being sterilized. There were no data on how many times intact dogs had reproduced, only that they were still reproductively capable.
Though the results of this study are intriguing, it is important to recognize the limitations in the data and therefore in the conclusions. Though the groups—intact or sterile—are assumed to differ in no other way, that may not be the case. It is possible that the members in the sterilized group have received more regular or better medical care throughout their lives, for example. The sterilized dogs may come from different sources such as rescue groups or shelters rather than from pet stores or breeders. In other words, differences in life span or cause of death may not relate to reproductive status, but to one of these other factors. This study shows links of reproductive status with lifespan and cause of death, but we cannot assume that reproductive status is the cause of these differences. They may be correlated for some unknown reason.
Some other concerns I have about the data are that the dogs had all been referred to a veterinary teaching hospital for medical reasons, which means that the dogs in the study may be largely dogs with serious health issues rather than typical dogs. This means that conclusions based on this study may not be applicable to dogs in general.
I’m glad that studies are expanding on the question of lifespan and reproduction by looking at causes of death instead of just reproductive capability. I think this study is a great start at exploring questions that are of interest to all of us who love dogs, but I do think we need to exercise caution in order to make sure that we are distinguishing between studies that show correlations between various factors and those that demonstrate a causal relationship between those factors.