We’ve been told over and over: “Spay your bitches and do it early for the best health benefits.” Spaying (ovariohysterectomy) before the first heat greatly decreases the risk of breast cancer – the most common tumor of female dogs, with an overall incidence of 3 percent. About a third of all cases are malignant. The health benefits of spaying decrease the more heat cycles a bitch undergoes prior to the spay. Removing the ovaries and uterus also removes the possibility of pyometra, a potentially fatal infection of the uterus. Pyometra is fairly common; one study showed 23 percent of intact bitches were affected by 10 years of age1.
But medical decisions don’t occur in a vacuum, and spaying is no exception. Claims of obesity following spaying have circulated for decades, and the topic is controversial to this day. In cats, spaying has been shown to decrease metabolic rate. No similar reports exist for dogs, however.
Reports do exist for associations between spaying and other health problems. Breast cancer might be reduced in spayed bitches, but the chance of developing another type of cancer might increase. According to one study, the risk of hemangiosarcoma (a cancer of the blood vessels with a poor prognosis, and one of the three most common cancers in dogs) is five times higher in spayed bitches compared to intact bitches2. Osteosarcoma (a bone cancer with poor prognosis) might also be more common in spayed bitches. A study of Rottweilers found that bitches spayed before 1 year of age were significantly more likely to develop osteosarcoma of the limbs than dogs that were sexually intact3.
Spaying might also be associated with skeletal and joint changes. Bitches spayed before puberty tend to grow slightly taller because the growth plates at the ends of the long bones, such as the ulnae, in part depend on exposure to estrogen and testosterone to close. Spayed bitches have a significantly higher risk of anterior cruciate ligament rupture4.
Spaying has also been associated with an increased rate of urinary incontinence5. Even some behavioral problems, most notably noise phobias and sexual behaviors, seem to increase with spaying before 3 months of age6.
Such findings make it difficult to make sweeping statements about the health benefits and risks of spaying, partly because they must be weighed against the seriousness and prevalence of the disease, as well as the disease’s prevalence in intact versus spayed bitches. It’s difficult to make generalities because different breeds and different-sized dogs have different disease risks. For example, large dogs are more prone to urinary incontinence, ACL rupture and osteosarcoma. Even among large breeds, osteosarcoma is more prevalent in some (for example, Rottweilers and Greyhounds) than others.
New longevity study
But what if spaying affected overall longevity? It’s generally claimed that spayed bitches live longer than intact ones. But most of these claims are based on “snapshot” studies: a sampling of a population at one point in time. These types of studies fail to consider that with increasing age, more dogs in a population are likely to be spayed, thus unfairly weighting the data so that almost all old dogs are spayed.
However, a recent study side-stepped this methodological flaw by looking at longevity as a function of age at spaying rather than lumping bitches into “spayed” or “intact” groups. This new study found that the longer bitches kept their ovaries, the more likely they were to reach very old age7. The study, from the Center for Exceptional Longevity Studies at the Gerald P. Murphy Cancer Foundation in West Lafayette, Ind., has veterinarians and dog owners taking note.
The researchers compared the medical histories of 119 “oldest-old” Rottweilers (defined by the researchers as those reaching 13 years of age, 30 percent older than the average Rottweiler life span) to histories of 186 Rottweilers that had what was considered usual longevity for the breed (8 to 10 years).
Female Rottweilers were more likely than males to reach exceptional old age, unless they were spayed before they were 4 years old. If females kept their ovaries beyond 4 years, the chance of reaching exceptional old age was more than three times that of males. Bitches that were not spayed until 6 to 8 years of age were more than three times more likely to reach exceptional longevity than bitches spayed before 2 years of age.
What does this mean?
This begs the question: Could the differences in longevity among bitches in the Rottweiler study be explained by differences in susceptibility to particular diseases?
In Rottweilers with usual longevity, the major cause of death is cancer. So the investigators repeated the analysis after excluding all cancer deaths. The new result was the same as before: Keeping ovaries longer was associated with an increased likelihood of reaching exceptional longevity.
Looking at other factors that might have influenced age at spaying and longevity, none could account for the strong relationship. The researchers said their results revealed a “robust ovarian association with longevity that was independent of cause of death.”
The results agree with those of a human study that found women under the age of 50 who underwent elective hysterectomies, including ovary removal, had a higher mortality rate (mostly attributable to cardiovascular and cancer deaths) than a similar group of women who had hysterectomies, but left their ovaries in place8.
The lead investigator of the Rottweiler study, David Waters, D.V.M., Ph.D., Dipl. ACVS, is the director of the Gerald P. Murphy Cancer Foundation, professor of the Department of Veterinary Clinical Sciences and associate director of the Center on Aging and the Life Course at Purdue University. Waters says that, like other veterinarians, he was taught the benefits of ovariohysterectomy while in vet school. But now his stance on the matter is changing.
“Over the last decade, our research on aging has forced me to let go of the illusion that we really know a lot about spaying and longevity,” Waters says.
How do these findings affect the decision of if and when to spay? Waters thinks of ovaries as “information that the female body uses to reach its biological potential and respond to environmental challenges. Removing ovaries early in life takes away information, muddles messages.” He says such muddled messages might translate into adverse effects on health outcomes, such as shortened longevity.
Waters’ research raises many questions about the influence and critical timing of spaying as it affects health and longevity, but the degree to which the findings translate to other breeds is unknown. In fact your choice of whether or not to spay might depend in part on your dog’s breed – which is the topic of next month’s column.
D. Caroline Coile, Ph.D., is a breeder, owner and handler of top-winning Salukis.