How can I tell my pet has cancer?

NOTE: This is a free excerpt from my upcoming book about cancer in pets!

Many pet owners think we can diagnose cancer by looking at routine blood work, when that is very rare.

Exceptions include liver and thyroid cancer.

So how might you suspect your pet has cancer?

Here are 7 findings that should put you on alert.

1. Skin mass

The most common type of mass is directly visible on or under the skin. Sometimes, the mass can burst open and bleed, or ooze.

Don’t make the classic mistake of thinking “It’s just a cyst” or “It’s just a fatty tumor.”

Never procrastinate in those situations. Early intervention can make the difference between success and failure. Removing a small mass is understandably less invasive and cheaper than removing a big mass.

In some cases, removal of a mass has become impossible and the best option is to amputate a leg.

2. A big belly

Of course not every cat or dog with a big belly has cancer!

Rather, it’s about a belly that becomes big quickly.

That can be a sign that a mass has grown inside an organ (for example the spleen).

Occasionally, such a mass can rupture and cause internal bleeding, which can also cause a big belly.

Tumor in the spleen of an 11-year-old Husky

3. Unexplained weight loss or gain

A mass in the spleen can weigh several pounds, so it would make sense that the pet would gain weight despite having the same appetite.

In other cases, it’s the other way around. Cancer can cause weight loss.

Why? Because cancer cells need specific nutrients to grow and thrive, so they rob them from the patient.

4. Bleeding

Bleeding from anywhere in the body should be a cause for concern – and a veterinary consultation.

  • Bleeding from a mass or a wound as mentioned above.
  • Bleeding from the mouth, the ear, the bladder, or the rectum.
  • Internal bleeding, causing an enlarged belly as mentioned above, which may be difficult to tell.

5. Difficulty breathing, eating, walking

Any mass can affect a body part, and its function.

A mass in the throat can cause difficulty breathing.

A mass in the jaw, the gum, or around the tongue can cause pain and difficulty eating.

A mass in a leg or the armpit can cause difficulty walking.

6. Stubborn vomiting or diarrhea

Tumors in the stomach or the intestines can cause vomiting or diarrhea.

They typically don’t stop after the usual treatments provided by your family vet.

As they grow, these tumors affect the lining or the thickness of the stomach or the intestines, which can affect the way they work.

This can cause bleeding, so vomit or poop/diarrhea can contain blood, either red (“fresh”) or blackish (“digested”).

Sometimes, a mass grows so big, that it can cause a blockage of the stomach or the intestines.

Cancerous mass (myxo-fibro-sarcoma) in a 13-year-old cat’s foot)

7. Something just doesn’t make sense

A bladder infection that keeps coming back despite multiple rounds of antibiotics may very well be masking a bladder tumor.

And then sometimes, my experience shows that when something just doesn’t make any sense, or defies a logical explanation, cancer should be considered as a possibility.

If you ever notice any of the signs above, please get an appointment with your family ASAP.

If cancer is suspected, time is of the essence.

The sooner we intervene, the better off your pet will be.

Procrastination never helps.

Quite the opposite. Procrastination may mean there is no hope to help your pet.

So the sooner you act, the better it will be for your pet.

If you would like to learn how we can help your pet with safe cancer surgery and anesthesia, please contact us through

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Phil Zeltzman, DVM, DACVS, CVJ, Fear Free Certified

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Dr. Phil Zeltzman

Dr. Phil Zeltzman is a traveling veterinary surgeon in Pennsylvania & New Jersey. An award-winning author, he loves to share his adventures in practice along with information about vet medicine and surgery that can really help your pets. Dr. Zeltzman specializes in orthopedic, neurologic, cancer, and soft tissue surgeries for dogs, cats, and small exotics. By working with local family vets, he offers the best surgical care, safest anesthesia, and utmost pain management to all his patients. Sign up to get an email when he updates his blog, and follow him on Facebook, too!