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Featuring Demian Dressler, DVM and Sue Ettinger, DVM, Dip. ACVIM (Oncology), authors of The Dog Cancer Survival Guide

Spay/neuter and the association with cancer in dogs: part one

Updated: May 13th, 2019

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

Discover the Full Spectrum Approach to Dog Cancer

Leave a Comment





  1. Laura Jackson on July 14, 2020 at 9:49 am

    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.

    • Molly Jacobson on July 14, 2020 at 10:24 am

      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.

  2. jake engles on November 29, 2019 at 1:33 pm

    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 ?

  3. Sally Sue on March 25, 2019 at 7:46 pm

    Where’s part 2?

  4. amy kirkpatrick-hartman on March 19, 2019 at 9:30 am

    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?

    • Molly Jacobson on March 20, 2019 at 4:54 pm

      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.

  5. Tareq Haddad on February 28, 2019 at 7:39 pm

    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!

    • Dog Cancer Vet Team on March 1, 2019 at 6:52 am

      Hello,

      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.

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