Spay/neuter and the association with cancer in dogs: part one
Updated: February 8th, 2021
Summary
Spay neuter dog cancer: despite its widespread popularity and the very good reasons to spay/neuter, it’s not without long-term risks to dogs.
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.
For more helpful information and tools for dog’s with cancer, get a copy of the Dog Cancer Survival Guide
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,
Dr Sue
Other Articles in This Series
Spay/neuter and the association with cancer in dogs: part two
Spay/neuter and the association with cancer in dogs: part three
Further Reading
Beauvais W, Cardwell JM, Brodbelt DC. The effect of neutering on the risk of mammary tumours in dogs–a systematic review. J Small Anim Pract. 2012 Jun;53(6):314-22. doi: 10.1111/j.1748-5827.2011.01220.x. PMID: 22647210.
Sue Ettinger, DVM. Dip. ACVIM (Oncology). Dr. Sue is a boarded veterinary medical cancer specialist. As a Diplomate of the American College of Veterinary Internal Medicine (Oncology), she is one of approximately 400 board-certified veterinary specialists in medical oncology in North America. She is a book author, radio co-host, and an advocate of early cancer detection and raising cancer awareness. Along with Dr. Demian Dressler, Dr. Sue is the co-author of The Dog Cancer Survival Guide: Full Spectrum Treatments to Optimize Your Dog’s Life Quality and Longevity.
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My Atlas is a seven year old male Great Dane diagnosed with Osteosarcoma 4-19-13 and front left leg amputated 4-26-13. He is an in tack male so contrary to this discussion. we believe his is a case of genetics. He just underwent his second round of carboplatin and is doing great, only symptom is fatigue. X-rays are all clear!
Patricia,
Sorry to hear about Atlas. I am not saying that intact dogs will not get cancer. As you said, genetics also plays a role, as can environmental differences. Cancer and its causes are complicated, but I think there is some interesting new information that we are all learning. Wishing continued health to Atlas!
All my best, Dr Sue
Sight Hounds tend to be a little different as they mature at a much later age. I waited till 18the months old to neuter my Saluki.
I get Greyhounds from the track and adopt them out. I spay and neuter each one before I adopt them out. The females get small amounts of testosterone at the track to control heat cycles. I wonder what role this plays in future cancers? The biggest cancer I see in greyhounds is osteo.
Carl,
We have a lot to learn about sex hormones and cancer. I am just tying to get the conversation started.
Thanks for reading and sharing.
All my best, Dr Sue
Dr dressler it was good reading your letters and web sight my dog Keneau past away may 1 2013 he survived cancer for more than 2 years thanks to holistic medication not my regular vet . Michelle Yasson from hol vet .net and other holistic vets helped, cancer in dogs is. Crazy keep up the great work they need your help.
Byron,
i’m sorry to hear about the passing of your loved dog, but at least we were able to do some good. All my best, D
It is good to finally see more vets addressing this issue. Used to be they all pushed spay/neuter at 6 months, for any and all dogs. Now they are finding that not only does the dog need to finish growing, but that spay/neuter has other, negative implications. If it is the right thing to do, for that particular pet, and the decision has been arrived at mutually by the vet AND THE OWNER, and the dog has finished growing, fine. Cookie cutter methods definitely don’t work best in this scenario.
Dear Joe,
Ditto!
Dr D
Exactly Joe. This is a complicated, complex issue, and we really must consider the pros and cons for each pet. And I am hopeful we will have more studies.
Next part coming out soon! Stay tuned!
All my best, Dr Sue
Please Dr ETTINGER, what can I do to HEAL my female dog’s cancer lumps on either side of her throat/neck? PLEASE answer urgent!!!
THANKS, SILMY 85 years old in Hongkong
Silmy,
I am sorry but I cannot diagnose or treat through the internet. I recommend that you take your dog to a veterinarian and get a diagnosis first (aspirate, biopsy).
Good luck!
All my best, Dr Sue
Thank you so much for addressing this “hot button” issue. As a first time breeder but long time owner/trainer, I researched this issue at great length when writing up the contract for the beautiful puppies that my health-checked champion bitch produced. The champion sire was also extensively health checked. As a result of my reading, puppies were not to be neutered or spayed before 12 months. This was not only due to cancer “implications”, but also to allow growth plates to close and allow the puppies to mature fully before such surgery. I kept the pick male and pick female and will keep them intact as required during their show careers and then I will decide which is the healthiest way to go.
. I have recommended your book, written with Dr Dressler, and intend to buy the 2nd version as my “pre-edition” version(8/11!!) has been loaned out/referred to so many times it is getting worn out(smile). The guidance for preventing cancer is worth the price alone!
Thanks for reading and sharing, Ann!
Dr Sue
Hi, Dr. Ettinger:
Your newsletter and The Dog Cancer Survival Guide are both interesting and informative. I realize that in this column you are providing some information about the pros/cons of spaying and neutering so those of us with dogs can decide for ourselves. I think it’s great that you provided a link to the abstract that exposes the earlier studies for bias. However, a little knowledge is a dangerous thing: You neglected to provide any research support [i.e., link(s)] to the statement that affected me the most: “…there is evidence that less exposure to sex hormones…actually increases the dog’s risk for…mast cell tumors.” In my opinion, you should not make a statement like this unless you also provide sources that support it. I lost my sweet corgi, Lupine (13 yrs) last fall to a very aggressive mast cell tumor. His oncologist excised the tumor but it returned before Lupine completed his first round of chemo. As a broken-hearted dog parent, I did all I could do to locate and read any research I could find on mast cell tumors; during my search for information I didn’t come across any journal articles that stated the correlation (cause?) that you stated. Could you please follow up your column with links or a bibliography to your sources? I’d really like to read them. Thank you.
What about the risk of being hit by a car because of being in heat or a neighboring dog being in heat?