June 12, 2025 – Many people either have had breast cancer or know someone who has been diagnosed with this common cancer. But what many pet owners don’t know is that dogs and cats get breast (mammary) cancer too, and it can be just as deadly in our pets as it is in people. Pet owners need to know the facts to keep their pets healthy and catch mammary cancer before it has a chance to spread.
What Is Mammary Cancer?
Before we dive into the specifics about mammary cancer in dogs and cats, it’s a good time for a basic anatomy lesson.
Cats and dogs have two chains of mammary glands running along the abdomen. Dogs typically have 8-10 glands (4-5 on each side) with smaller breed dogs tending to have fewer glands than larger breeds. Cats have 8 glands (four on each side). Occasionally a dog or cat may have an extra gland(s). The glands are connected to each other, and lymph nodes drain the region. Mammary tissue has a rich blood supply.
Both benign and malignant tumors can affect one or multiple glands. In dogs, roughly 50% of mammary tumors are malignant but in cats, ~85-90% are malignant. Cat mammary tumors are aggressive, and most tumors have spread (metastasized) at the time of diagnosis.
According to published statistics, mammary cancer accounts for 42% of all tumors diagnosed in dogs (more on this later) with a reported lifetime risk of 23-34% for an unspayed, female dog. Interestingly, almost all mammary tumors found in male dogs (rare but possible) are malignant.
The average age of dogs diagnosed with mammary tumors is 6-10 years of age. The average for cats is 10-12 years of age.
Causes and Risk Factors
There is no question among experts that spaying female dogs and cats decreases the risk of mammary cancer. The timing of spaying in relation to heat cycles in dogs is also clear, with earlier spaying decreasing risk. This risk increases with each subsequent heat cycle, and spaying dogs over 2 years of age no longer confers protection against later cancer development. Many pet owners have likely heard about this protection from their veterinarian.
However, in the last 15-20 years, there have been questions about the timing of spay in relation to joint health, risk of obesity, and other potential health impacts in female dogs. Current recommendations to wait to spay large breed dogs can also impact the development of mammary cancer. It’s important for all dog owners to discuss the best timing for their pet with their veterinary care team.
The current recommendation for cats remains to spay before 6 months of age.
Statistics suggest:
- Dogs: Spayed before 1st heat = <1% risk.
- Cats: Spayed before 6 months = 91% reduced risk.
Other risk factors include:
- Overweight/obese body condition – there is some evidence that being overweight or obese increases risk in dogs for mammary (and other) cancers.
- A possible breed predisposition for mammary cancer has been published but many experts are skeptical and remind owners that any breed of dog, including mixed-breed dogs, can develop this cancer.
- Age – older animals are at higher risk.
Signs of Mammary Cancer in Pets
Many of us do monthly or yearly cancer bump and lump checks on ourselves as part of our overall wellness efforts to catch any potential problems early – sometimes with lifesaving results. The same kind of check can be a lifesaving routine for our pets, too. It’s important for pet owners to do a thorough exam of their pets for any new lumps or bumps at least once a month and even if your female pet is spayed, or if you have a male dog or cat, don’t forget a good palpation of each mammary gland when you’re checking for lumps and bumps.
Mammary tumors typically are very firm and feel like BB pellets and most are not painful in early stages. Multiple tumors are common, so it’s important to carefully examine all mammary tissue.
Other signs owners need to note are:
- Any swelling, redness, discharge, ulceration of a gland(s).
- Changes in appetite, lethargy which could indicate cancer spread.
As always, any new lumps or bumps noted in a mammary gland, even if small, need to be checked by your veterinary care team.
Diagnosis
Most mammary tumors are diagnosed when a pet owner or veterinarian feels a lump within or near a mammary gland. Once a bump is found, it’s important to determine the tumor’s stage. A stage 1 tumor is typically characterized as a growth that is small, with no spread to local lymph nodes or metastasis to more distant sites. Stages progress in severity to Stage 5. Treatment and long-term survival are dictated by the tumor’s stage.
Diagnostic tests done to assess a newly found mass usually includes:
- Routine bloodwork and urinalysis to assess overall health and determine if other diseases are present
- Fine needle aspiration and biopsy to confirm the diagnosis and assess for local spread.
- Imaging (X-rays, ultrasound) to check for spread to distant sites.
Not all mammary tumors involve the same tissue, and several types of cancer have been identified. The type of cancer can dictate the treatment options available.
Treatment Options
Surgery remains the cornerstone of therapy for mammary cancer in both cats and dogs. If the cancer is caught early and is limited to one gland (and there’s no evidence of spread), surgery can be curative.
Surgery can involve removal of one or more glands in one chain. Sometimes both mammary chains are removed. While most pets recover well when the surgery is confined to one or only a few glands, more radical surgery can result in prolonged recovery times and more discomfort for the pet.
Because of the aggressive nature of mammary cancer in cats, many oncologists recommend chemotherapy after surgery. However, studies looking at this approach have found inconsistent results.
There is also controversy among surgeons whether to spay a patient at the same time as removing the tumor. There is conflicting evidence that this improves overall survival time or decreases the risk of tumor recurrence although it can reduce the risk of other diseases. One exception is a type of tumor called a secretory carcinoma, which is estrogen driven. For dogs with this type of tumor, spaying is recommended.
Unfortunately, if the cancer has spread, there are limited options for additional treatment. In cases of human breast cancer, there are many specific therapies available depending on the type of cancer present but because we don’t know the same level of detail in our veterinary patients.
With inflammatory mammary carcinoma, surgery isn’t associated with improved survival. In these cases, some oncologists recommend chemotherapy and anti-inflammatory medications.
If there is evidence of metastasis, sometimes chemotherapy can be used to slow progression, but results are inconsistent across studies.
As always, treatment plans must be tailored to a pet’s age, health, and cancer type.
Prognosis and Life Expectancy
The prognosis for dogs with small tumors can be excellent. However, statistics suggest that 50-75% of dogs with malignant tumors will die within 1-2 years due to recurrence, spread or both. It’s important to remember that there is a wide variation in outcomes based on the type of tumor, whether the tumor is localized, and the tumor’s stage.
The prognosis for cats with mammary cancer is very poor, with most cats living only a few months after diagnosis.
Morris Animal Foundation-Funded Studies
The Foundation funded its first study focused on mammary cancer in pets in 1983, and since then, we’ve funded 24 more studies looking at all aspects of the disease, from early diagnosis to treatment.
Two of our newer studies are in progress at the University of Saskatchewan and under the direction of Dr. Melissa Meachem. Dr. Meachem is looking for new ways to treat and diagnose mammary cancer in cats and dogs by taking a closer look at diseased tissue.
“I think mammary cancer is overlooked in veterinary medicine yet it’s a common cancer,” said Dr. Meachem. “We’ve seen huge strides treating human breast cancer but we’re not there yet in our veterinary patients. If we knew more about this type of cancer in pets, we could leverage some of the treatments used in people, or create new treatments, and I hope my research can lay the groundwork for more personalized strategies.”
In her dog-focused study, Dr. Meachem is looking at specific proteins in cells from mammary tissue in dogs with cancer and in dogs without. Some of these proteins are known as immune checkpoint inhibitors, which are molecules that help cancer hide from the immune system. They hope their research could lead to better ways to detect and treat cancer. The team is almost done with their research and hope to publish their findings soon.
In their feline-focused study, the Dr. Meachem and team will be looking at cancerous mammary tissue to see which genes are driving growth and spread of this aggressive disease. They hope their findings could not only give insight into the basic biology of this cancer but also uncover potential treatment targets.
Prevention and What Pet Owners Can Do
Spaying remains the best prevention for mammary cancer in both female cats and dogs. As mentioned, it is important to work with your veterinarian to decide the best timing for surgery.
Routine physical checks at home are a very good way to catch tumors early, which can have a big impact on long-term survival in both dogs and cats.
And, as always, yearly veterinary visits are also important in detecting not only mammary cancer but any type of cancer or other health issue. The prognosis for any disease is much better when caught early!
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