Spay Your Dog — Or Risk a 1 in 4 Chance of Mammary Cancer

The reason for this frightening number is that a female dog’s hormones drive the cancer.

When she's lying down, run your hands across her belly to feel for any lumps or bumps. By:
When she’s lying down, run your hands across her belly to feel for any lumps or bumps. By: Loren Sztajer

Everyone who has a female dog should read this article.

“Mammary cancer” is the term for breast cancer in a dog. Unfortunately, this serious condition is all too common and accounts for half of all cancers in female dogs. You read that right. Half.

At the risk of scaring you further, I’ll add that the statistics don’t improve when you look at them more deeply.

Research shows that 4 out of every 10 cases of mammary cancer are malignant, which means the cancer is at high risk of spreading to other parts of the body, such as the liver, lungs or bone — and could be responsible for ending the dog’s life.

All of which means you need to be vigilant and regularly check the mammary area.

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Horrible Hormones

Why does mammary cancer happen?

The reason for these frightening figures is that the dog’s hormones drive mammary cancer (especially if she has her womb and isn’t spayed). The mammary glands contain receptors that are sensitive to female reproductive hormones. Each time your dog comes into heat, those hormones activate the receptors and increase the risk of mammary cells tipping over from normal to cancerous.

The long and short of it is that spaying your dog (especially at an early age) can help protect her against this potentially life-threatening condition.

I worked for 5 years at a clinic for people with low incomes. Very few dogs were spayed, and the numbers with mammary cancer were heartbreakingly high. This was in stark contrast to private practice, where more people could afford to get their pets spayed, and mammary cancer was the exception rather than the rule.

2 More Risk Factors

  1. The dog’s age. This is mainly because of the repeated assault of hormones on the mammary tissue as time goes by. The average age for a female to be diagnosed is around 9–11 years. The condition is relatively rare in those younger than 8, although I have seen a 2-year-old dog, a Rhodesian ridgeback, with multiple mammary tumors.
  2. The dog’s breed. Sadly, some genes make the dog more prone to cancer, such as in Cocker Spaniels, poodles, terriers, Beagles, Greyhounds and Boxers.

Check Regularly

If you have a female dog and I’ve scared you witless, then I apologize. That is not my intention, but I do want to raise your awareness and prompt you to check your dog regularly, and perhaps give spaying some serious consideration.

As with so many health problems, early detection gives your dog the best chance of recovery and a long life. For mammary cancer, this means you should check your dog’s chest regularly — at least once a week. It is as easy as paying attention when you stroke her tummy when she lies down.

Female dogs have 2 “chains” of mammary glands, running from just behind their armpit to their groin, on both sides of the belly.

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Here’s how to check:

  • Use the flats of your fingers to feel for lumps or bumps in the skin or just below the surface.
  • Any round swellings (other than nipples) or firm lumps are abnormal.

This video of a pointer named Sasparilla shows what mammary tumors can look like:

Diagnosis and Treatment

If the vet confirms your suspicion, he may recommend tests to check that the cancer hasn’t spread and then surgically remove the lump.

More aggressive mammary cancers can spread (metastasize) to the lungs early, so the vet may X-ray your dog’s chest to ensure she is a good candidate for surgery.

Surgical removal of the lump is your dog’s best option for making a full recovery. If there are several lumps, the vet will remove all the glands on that half of her body to reduce the chances of the cancer popping up again on that side.

Unfortunately, few drugs are both safe and effective at treating mammary cancer in a dog.

Would you prefer a 1 in 4 risk of mammary cancer, or a 1 in 25,000 risk? By:
Would you prefer a 1 in 4 risk of mammary cancer, or a 1 in 25,000 risk? By: Humane Society of Jefferson County


The frightening statistic is that an adult dog with a womb has a 1 in 4 chance of developing mammary cancer.

However, the risk falls to 1 in 25,000 if a female puppy is spayed before her first season, and 1 in 10,000 if done between her first and second season. Unfortunately, after 2 seasons this protective benefit against mammary cancer is lost.

Knowing this, if you have a young female dog, give serious thought to spaying early in order to protect her (and you) from the heartbreak of mammary cancer.


This pet health content was written by a veterinarian, Dr. Pippa Elliott, BVMS, MRCVS. It was last reviewed March 12, 2015.

Dr. Pippa Elliott, BVMS, MRCVS

View posts by Dr. Pippa Elliott, BVMS, MRCVS
Dr. Pippa Elliott, BVMS, MRCVS, is a veterinarian with nearly 30 years of experience in companion animal practice. Dr. Elliott earned her Bachelor of Veterinary Medicine and Surgery from the University of Glasgow. She was also designated a Member of the Royal College of Veterinary Surgeons. Married with 2 grown-up kids, Dr. Elliott has a naughty puggle called Poggle, 3 cats and a bearded dragon.

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