Why haven’t previous dog studies showed this potential downside of ovary removal?

No previous studies in pet dogs have rigorously evaluated the association between ovaries and longevity. Two frequently cited reports^1,2 provide limited guidance because: (1) longevity data are presented as combined mean age at death for a relatively small number of individuals of more than 50 breeds of different body size and life expectancy; and (2) ovarian status is reported as “intact” or “spayed”, rather than as number of years of lifetime ovary exposure. Comparing female dogs binned into the categories of “intact” versus “spayed” introduces a methodological bias that might lead one to conclude that ovaries adversely influence longevity, i.e. ovary removal promotes longevity. Because the reasons for ovariectomy (e.g., uterine infection, mammary cancer) increase with increasing age, it is expected that a large percentage of the oldest-dogs are binned as “spayed” despite having many years of ovary exposure. For example, a dog who at age 12 undergoes ovariohysterectomy for pyometra would be binned as “spayed”, despite 12 years of ovary exposure. In our study, we employed a more stringent study design — restricting the study population to AKC registered, pure-bred dogs of one breed, carefully quantitating the lifetime duration of ovary exposure — in order to lessen the likelihood of such bias. And we reasoned that studying veterinary teaching hospital-based populations of dogs with artifactually low life expectancies (for example, 3.5 years is median age at death for Rottweilers in the Veterinary Medical Data Base) was an inappropriate vehicle to describe the influence that ovaries have on aging. So we cast a wider net and collected data from Rottweiler owners nationwide, focusing our attention on exceptional longevity, not average age at death, as our study endpoint.

1. Bronson RT: “Variation in age at death of dogs of different sexes and breeds.” Am J Vet Res 43: 2057-9, 1982.

2. Michell AR: “Longevity of British breeds of dog and its relationships with sex, size, cardiovascular variables and disease.” Vet Rec 145: 625-629, 1999.