In the US, there is widespread recommendation for early spay and neuter. But recently the association of spay/neutering and cancer in dogs has been in the news again. Specifically the concern is that spay/neutering increases the risk of cancer, which brings into question this recommendation to spay/neuter at 6 months of age. We are going to look into this complicated issue is this series.
I started to learn more about this when I joined Dr. Dressler and we began to prepare and edit the 2nd edition of The Dog Cancer Survival Guide. The studies were interesting, but contradicted what I learned in vet school at Cornell.
This was also just AFTER I had spayed my own seven-month-old Labrador puppy, Matilda. I began to question my decision as I explored the pros and cons of early spay/neutering. Let’s see why.
Spaying Female Dogs Lowers Breast Cancer Risk
We have known for awhile that hormones may promote or inhibit cancers, depending on the sex of the dog and the tissue or organ.
Less exposure to female sex hormones has been shown to be protective against breast cancers in dogs, which is important since mammary cancer is the second most common form of dog cancer.
Breast cancers can be nearly completely eliminated by spaying a female dog before the first heat. (On average, the first heat arrives at six months of age and recurs approximately every six months until late in life.)
Spaying provides less protection for canine breast cancer with every passing heat. Studies have shown that dogs spayed before the first heat have a 0.05% risk of developing mammary cancer, when compared to intact female dogs, which means the risk is almost completely eliminated by the surgery.
If the spay happens between the first and second heats, female dogs still have quite a bit of protection, with only an 8% risk, compared to their intact sisters.
If the spay happens between the third and fourth heat, female dogs have a 26% risk, compared to intact dogs.
Clearly, spaying a female dog reduces her risk of mammary cancer. It also removes all risk for ovarian and uterine cancer.
So based on this, which is what I learned in vet school, I should feel good about my decision to spay my Matilda, right? We will come back to that.
Neutering Males Lowers Testosterone-Related Tumors
What about male dogs?
Neutering definitely decreases the risk of benign perianal adenomas, which are stimulated by testosterone. As testosterone levels increase, glands called sebaceous glands get bigger, and so dogs can get these benign perianal adenomas. (Perianal means beside the anus.)
These benign butt tumors are seen most commonly in certain breeds, including Arctic breeds, Beagles, Cocker Spaniels, Shih Tzus, and English Bulldogs.
In addition to lowering the risk of developing benign adenomas, neutering your dog also eliminates his risk for testicular cancer. Testosterone-secreting tumors in the testicals can also lead to those benign butt tumors. Interestingly, perianal adenomas have also been reported in female spayed dogs who have with adrenal tumors (yep, you guessed it, they secrete testosterone).
The treatment of perianal tumors is castration. Remove the source of testosterone, and the tumors often resolve. If the tumor does not go away or significantly shrink, then we recommend removing the anal gland tumor. In fact, >90% of dogs are cured with castration and/or tumor resection.
Other Benefits of Spay/Neuter
In addition to the benefits above, spay/neuter also helps to reduce uterine infections (pyometras), and, in some cases, unwanted behaviors like humping, marking, and aggression.
Early spay/neuter is also important in population control and preventing the euthanasia of unwanted pets. If you just look at these factors, early spaying and neutering might seem like an obvious choice.
But … Spay/Neuter Increases Risk for More Aggressive Cancers
But recently, there is evidence that less exposure to sex hormones, while protecting against the cancers named above, actually increases the dog’s risk for other aggressive cancers, including osteosarcoma, bladder transitional cell carcinoma, prostate cancer, lymphoma, and mast cell tumors.
And to further complicate the mammary tumor and early spay recommendation, this recommendation was recently questioned in an article called, Effect of neutering on the risk of mammary tumours in dogs – a systematic review (Beauvais, JSAP, 2012).
In a review of the studies that were used to make the recommendation for early spay/neuter, the evidence in favor of the practice was judged to be weak.
The article stated: “Due to the limited evidence available and the risk of bias in the published results, the evidence that neutering reduces the risk of mammary neoplasia, and the evidence that age at neutering has an effect, are judged to be weak and are not a sound basis for firm recommendations.”
So … maybe what I learned in vet school about early spay/neuter was not so straightforward after all! Join me for my next blog to learn how sex hormone can be PROTECTIVE against certain cancers.
Live longer, live well,