Mammary Cancer in Cats: Signs Every Cat Parent Should Know

A small lump along a cat’s belly can be easy to miss, but it is never something to wait on. Mammary tumors in cats are often aggressive, and the earlier a lump is evaluated, the more options a cat may have. This guide explains what cat parents should watch for, what a veterinary workup may involve, and why prompt care matters.

Justice Cats is sharing this information because mammary cancer has changed our own lives through Juniper’s diagnosis. It is educational only and cannot replace a veterinarian’s examination or advice for your individual cat.

What is mammary cancer in cats?

Cats have two mammary chains, with four mammary glands on each side of the underside of the body. A mammary tumor is an abnormal growth in or near one of those glands or nipples. In cats, most mammary tumors are malignant, meaning they can invade nearby tissue or spread to other areas of the body.

That is why a new lump, no matter how tiny it seems, needs a veterinary appointment as soon as possible. A cat can feel and act completely normal while a tumor is present. Waiting for pain, weight loss, or a dramatic change means missing the chance to evaluate the mass early.

Signs of mammary tumors in cats

The most common sign is a firm pea-sized or BB-sized lump under the skin along the belly, often near a nipple. There may be one lump or several. Other warning signs can include:

• A firm nodule, swelling, or thickened area along the mammary chain
• A mass that grows, feels fixed in place, or appears near a nipple
• Red, warm, irritated, ulcerated, or bleeding skin over a lump
• Excessive licking at the area, discharge, odor, or obvious discomfort
• Loss of appetite, weight loss, weakness, or lethargy
• Breathing changes, coughing, or difficulty breathing, which need urgent veterinary attention

Many of these symptoms can have causes other than cancer. That does not make them safe to ignore. Mammary tumors cannot be diagnosed by sight or touch alone.

Who is at risk?

Mammary tumors occur most often in female cats, especially middle-aged and older cats, though a cat of any age can develop them. Male cats can develop mammary tumors too, although it is uncommon. Hormonal exposure is an important risk factor. Spaying before the first heat cycle greatly reduces future risk, but a cat who was spayed young can still develop mammary cancer. That is why routine checks matter for every cat.

Some breeds, including Siamese cats, may be overrepresented in studies. The exact cause of an individual cat’s tumor is usually not known. Cancer is not proof that someone failed their cat.

How veterinarians diagnose and stage mammary cancer

After finding a mass, a veterinarian will examine all of the mammary glands and nearby lymph nodes. They may recommend a fine-needle aspirate to collect cells, although a biopsy or surgical pathology is often needed for a definitive diagnosis and to determine how the tumor is behaving.

Because feline mammary tumors can spread, staging is an important part of planning care. Depending on the case, testing may include bloodwork, urinalysis, chest X-rays, abdominal ultrasound, and sampling of lymph nodes. These tests help the veterinary team look for cancer that may have spread and evaluate whether a cat can safely undergo treatment.

What treatment can look like

Surgery is commonly the main treatment for mammary tumors in cats. Because cats can develop multiple tumors within the same chain, veterinarians often recommend removing an entire affected mammary chain rather than only the visible lump. In some situations, surgery on both sides is performed in stages with a recovery period in between.

Pathology results, tumor size, lymph-node involvement, and imaging results all influence the next steps. A veterinary oncologist may discuss chemotherapy after surgery or as part of treatment for advanced disease. There is no one-size-fits-all plan, and the right choice will depend on the cat’s diagnosis, overall health, quality of life, and family’s circumstances.

Why early action matters

With feline mammary cancer, tumor size and whether disease has spread are important factors in prognosis. Tumors found and treated when small generally offer a better chance of long-term control than larger tumors or tumors with spread. The practical takeaway is simple: do a gentle check of your cat’s underside regularly, and call your veterinarian promptly about any new lump.

Do not squeeze or try to “watch” a lump at home. Take a photo and note the date and location if that helps you explain it, but let a veterinarian evaluate it.

A simple monthly check at home

When your cat is relaxed, gently run your fingers along both sides of the belly from the chest toward the groin. You are looking for any new pea-sized bump, firm patch, swelling, or area that makes your cat flinch. Keep it calm and brief. This is not a substitute for an exam; it is simply a way to notice changes sooner.

When to seek urgent veterinary care

Contact a veterinarian promptly if you find a mammary lump. Seek urgent care if a mass is open, bleeding, infected-looking, painful, rapidly changing, or if your cat is having trouble breathing, has stopped eating, is severely lethargic, or seems distressed.

You are not alone

A mammary cancer diagnosis is frightening, especially because so much can happen quickly. Ask your veterinary team to explain the staging, pathology, treatment choices, expected side effects, and the signs that should prompt an immediate call. Write questions down. Bring support to appointments if you can. Most importantly, do not delay because the lump seems small or because your cat is behaving normally.

Early attention is an act of love. A quick examination can provide answers, a plan, and the best possible chance to help your cat.

For further reading, see the NC State Veterinary Hospital guide to feline mammary tumors, Cornell Feline Health Center, and the Merck Veterinary Manual.