Spay/neuter and the association with cancer in dogs: part one
Updated: February 8th, 2021
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,
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
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.
I’m a practicing vet in Youngstown, OH, graduate of U. of Ill., 1980. I acquired a GSD female from the Monks of New Skete in April, after waiting 10 months (wait is 6 months to 2 years). They’ve been breeding GSDs for 40+ years in upstate NY.
For most of my career, I have advocated spay/neuter at 3-5 months, before coming in heat at 6 months+. This was from a study that showed spaying before first heat, less than 1% breast cancer later in life; spay after first heat = 1 in 100, and after 2 heats = 1 in 14. So, I texted the NS breeding coordinator and said I planned to spay my dog, “Questa,” next week. She immediately called me and asked me NOT to spay her early, due to growth plate issues and also referenced your study in 2020.
I copied the article, written by Amy Quinton, about your study from the UC Davis website. The first word concerns me; “Heavier.” Were these large-breed mixed-breed, or overweight mixed-breed? With 60% of our pets being overweight today, and nearly all of them spayed/neutered, “joint disorders,” i.e. arthritis, ACLs, luxating patellas, are primarily due to carrying too much weight for too long. Ununited anconeal processes and hip dysplasia are pretty much genetic issues that develop regardless of body score.
So, my question is, did the dogs in this study have body scores of “overweight” or “obese”? I am willing to change my advice toward pet owners to continue spay/neuter for small breed dogs between three and five months of age, but delay spay/neuter for larger breed dogs until much later. The breeding coordinator from NS advised to wait on Questa until after she has her first heat. I’m following her request.
(The above message I sent to Drs. Ben and Lynette Hart, authors of the study referenced. Below is his response.)
You raise a question that has been frequently asked. In our study we found no relationship between body weight (BMS) and which dogs get a joint disorder, although, in general, body weight tends to add to the risks of a joint disorder. For the GSD I would wait until into the second year. (You can also note that the larger mixed breed dogs in general are significantly affected by early neutering for joint disorders.)
A major point to keep in mind is that there are major breed differences in the effects of spay/neuter on joints as well as increased risks of mammary cancer. I suggest you look at the open-access papers on 35 breeds and the GSD—web link below.
(This link his their own study, so it just underscores their belief.)
You can see I am confused and now doubt my history of advising spay/neuter between 3 and 5 months. Their study was on mixed-breed dogs, which should have hybrid vigor in general, but the study does not indicate body score. Looking at OFA data on hip dysplasia, GSDs have an 22.4% incidence! Of course, theirs is a select population, in that they mostly get purebred dogs to check for breeding potential. Can you help sway me one way or the other? Unfortunately, I see bias in Dr. Hart’s recommendation, referring to his own study.
Late to the discussion, but Thank you for this, it’s such a difficult topic, and can certainly cause some anxiety and second guessing. Our female american field labs have all had ovary sparing spays. I’m happy they keep their hormones, yet I do worry about the mammary tumor issue. Going to read that study you sent off. Also, I’d be curious what the stats and percentages really mean?
My brother in law had his Great Dane male neutered early (at under one year of age I think) and when the dog was around 5 years old he developed an osteosarcoma of his leg. He died less than 6 months later. Veterinarians should probably be waiting for these giant breeds to be older before neutering them as there seems to be a link with early neutering with this extremely aggressive cancer.
I have a new pup (6 months) which I planned to wait to neuter after reading your articles about avoiding early spay/neutering. Unfortunately, one of his testicles has not descended. I understand that this puts him at a much greater risk for cancer. I am working with my alternative medicine vet who has had some success using herbal medicine in bringing the testicle down so she said to wait a while to see what happens. In you opinion, how long should I wait to have him neutered before the risk of cancer increases?
Hi Carol, the general recommendation Dr. Dressler makes is to wait until 18 months for a neuter. I hope you have success getting that boy to drop!
Spay neuter is oh so necessary. Try visiting the Poor Side of your Town, or any underdeveloped country and see how important a simple surgery might have been to alleviate animal suffering. Many of these studies on pediatric s/n has been done on purebreds, which are genetically flawed anyway. S/n reduces so many problem behaviors. Unclean air, an unclean environment, pesticides in foods and toxins in water assure that dogs are going to not live forever. None of us get out of this world alive. Please support spay-neuter.
The recommendation from veterinarians discussing the increased cancer risk to dogs who are desexed early is clear: to delay spay/neuter. The decision can be made on a case-by-case basis as for when and why to spay/neuter.
my female boxer mix is 14 & has never been spayed , her last heat was 3 months ago & went on for around 3 -4 weeks. how much am I risking her to cancer ?
Where’s part 2?
This is Part Two: https://www.dogcancerblog.com/articles/cancer-causes/spayneuter-and-the-association-with-cancer-in-dogs-part-two/
and this is Part Three: https://www.dogcancerblog.com/articles/cancer-causes/spayneuter-and-the-association-with-cancer-in-dogs-part-three/
Our four year old Lab just had a single mast cell carcinoma removed. For fear of spreading a gene related MCT pre-dispositon problem we neutered him. Was this a mistake? Have they found genetic evidence of MCT on any gene loci?
Hi Amy! In general, Dr. D and Dr. E recommend that if you neuter, you delay it until after 18 months, so that the dog has time to get the protection of adult sexual hormones. Since your boy was four when you neutered him, he had several years of those hormones circulating. The genetic connection for MCT is not clear, although there may be some connection to a mutation of the c-kit gene. However, not all dogs with MCT have that mutation. Labs as a breed are more prone to MCT, true — but the actual cause is not known for certain. Dogs of any gender, and any age, and any breed can develop it. There’s more in the book in chapter 30, the chapter on mast cell tumors. However, please don’t second-guess your decision. Every time we make a decision, we choose what’s best at the time, based on the information we have. You’ll never be able to prove a negative — you’ll never know whether your boy’s pups would or would not have developed MCT. And neutering late in life doesn’t guarantee something won’t happen later. Try to go easy on yourself and set aside questions that just don’t have answers. Focus on your boy and be well.
I have 2 females and 1 male dog. Now I’m confused whether to spay/neuter or not.
You article has increased my confusion and reluctance toward the surgery. Please HELP!
Thanks for writing. As we’re not veterinarians here in customer support, we can’t offer medical advice 🙂
In this Spay/Neuter series, Dr. Sue is basically trying to say that there are risks for spaying/neutering your dog too early, and too late in their life.
In the Dog Cancer Survival Guide, Dr. D writes that there are a number of factors that need to be taken into consideration when determining a date to spay or neuter your dog. In general, Dr. D’s recommendation is to spay females sometime between their 3rd and 4th heat, and neuter males sometime between the ages of 18-24 months.
He does write that there may be a reason why you should spay/neuter your dog earlier in life, but this would be made on a case-by-case basis in consultation with your vet.