Spay, Neuter, and Cancer: Revisiting and Old Trinity

Perhaps no aspect of pet ownership in the U.S. elicits as passionately supportive emotions as the subject of spay and neuter. In fact, this orientation is so well established that saying anything that questions the procedure is akin to blasphemy. However, just as women were routinely relieved of their reproductive organs with a "La de da, you'll never miss 'em" attitude until studies exploring the nonreproductive effects of reproductive hormones made human physicians rethink this position, so veterinarians and other animal-care professionals are making tentative moves to rethink wholesale sterilization of companion animals, too.

To understand what difference this may make in our attitudes about the procedure, let's consider the subject of cancer. Most dog owners have heard that spay and neuter prevent testicular and mammary (breast) cancer: however is that the whole story relative to cancer or is there more to it?

Obviously, if we remove a dog's testicles, there's no way he'll develop testicular cancer. On the other hand, most dogs who develop testicular cancer respond well to castration, so the advantages of preventive surgery are perhaps not as great as one might expect. Although intact (unsterilized) females have a higher incidence of mammary cancer, the dog's weight plays an important role in the process: intact females who are lean at one year of age have a lower incidence of the disease compared to their chunky cohorts.

In an interesting article in the August Veterinary Practice News entitled "Can we neuter cancer in dogs?" veterinary oncologist Kevin Hahn opens by saying that, after reviewing studies reported over the last 30 years, he's not sure what to recommend to his clients. Like most veterinarians, Dr Hahn mentions the higher incidence of testicular and mammary cancer in intact animals, but also notes that spayed females have a 4 times greater risk of cardiac hemangiosarcomas, and neutered males also show a significant increased risk for this cancer compared to intact ones.

Another cancer Dr Hahn discusses that deserves mention is prostate cancer because a lot of people erroneously believe that castration prevents this. In reality, it does not. In fact, castrated dogs have up to a 4 times greater risk of developing prostate cancer than intact animals. At the same time, spayed or neutered dogs have a 1.5 to 3 times greater chance of developing bladder cancer. Because of this, rectal examinations and abdominal palpation should always be part of a routine veterinary physical examination.

The link between sterilization and osteosarcoma (i.e. bone cancer) is also troubling: Spayed and neutered animals are twice as likely to develop this cancer. Those spayed or castrated before their first birthdays had a roughly 1 in 4 lifetime risk for osteosarcoma and were significantly more likely to develop a tumor than intact dogs.

The article then goes on to discuss the role of hormones and genetic controls in cancer. All agree that there is a connection, but no one knows exactly what it is. However, in his article Dr Hahn discusses a study done by Dr David Felman (and published in the June Nature) that I find intriguing because of how it may relate to the role the animal's behavior and his/her relationship with the owner plays in cancer. In a very tiny nutshell, the study looked at two gene mutations that lead the stress hormones cortisol and cortisone to trigger the growth of later stage cancer cells.

Because cortisol is also one of the hormones that's elevated when stress results in animal behavioral problems which, in turn, may result from human-animal relationship ones, it would seem that avoiding such elevations of this hormone by treating bond and behavioral problems could conceivably lower the probability of cancer in some animals, or improve the survival chances of those already afflicted with the disease. Although such a hypothesis might seem to require too great a leap of credibility for those who associate cortisol and cortisone with those drugs that counter inflammation and itching, another effect of these hormones is that they undermine the immune response. So while they may benefit animals who encounter occasional stresses of brief duration, these same substances may seriously undermine the health of those who daily live in stressful environments. In that case, not only will these animals have a higher probability of developing stress-related behavioral and medical problems (such as aggression or separation anxiety displays, irritable bowel syndrome or chronic or recurring urinary tract conditions), these animals' taxed immune response may experience more difficulty recognizing and dispatching mutant cells before they multiply and form cancers.

Currently the exploration of the nonreproductive effects of sex hormones is in its infancy and, unlike the rise of feminism which challenged the philosophy underlying hysterectomy and ovariohysterectomy in women, many of those who normally claim to speak for the animals are usually quiet about how sterilization may affect companion animals. Like Dr Hahn, I, too, have reviewed the literature and am not sure what to tell clients. However, I do know that unless we can free the subject from the emotional cocoon that has protected spay and neuter from objective scrutiny all these years, our pets won't be able to benefit from the knowledge that is slowly, but surely, being generated on this subject.

If you have any comments regarding subject matter, favorite links, or anything you'd like to see discussed on or added to this site, please let me know at mm@mmilani.com.

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