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

Signs of Cancer in Dogs

Updated: June 30th, 2021


Many people ask me what to look for to tell if their dogs have cancer. I thought I should give you a little summary of some of the biggies.

Many people ask me what to look for to tell if their dogs have cancer.  Well, I must confess it is a tough question since there are so many cancers, and they all can present a little differently.  I thought I should give you a little summary of some of the biggies.

First, statistically, cancer TENDS to affect older dogs.  So, more lumps and bumps on a young dog are benign than cancerous.  One hallmark of a cancer is it worsens over time.  Cancers you can see usually get bigger. A growth that stays very small for years is not likely to be a true cancer.  Again, this is on average and is not a guarantee.

Some cancers are visible, while others are internal. The visible ones can be blackish (melanomas), purplish (hemangiosarcomas), fleshy, inflamed and red (histiocytomas), look like a non-healing open sore (squamous cell carcinomas), be firm, hard and deeply attached (fibrosarcomas), or have any appearance (mast cell tumor, the great imitator).

The internal ones are invisible, so we have to look for overall signs in the dog. When they are far along, cancers usually cause weight loss (cancer cachexia) without an obvious external reason. They often will cause less appetite.  Many times dog owners will tell me they think their dog got tired of his or her food.  They can cause low energy, where the dog will just lay around a lot.

NOTE: If you are reading this article and worried about your dog, do yourself and your dog a favor and get Dr. Dressler’s The Dog Cancer Survival Guide. It’s the best-selling animal health book for a good reason: it’s helped thousands and thousands of dogs just like yours face and cope — and even beat — cancer. And if you really want to help your dog, get the Dog Cancer Kit we put together for you, based on what’s been most helpful for other people who have faced this terrible illness.

-The Dog Cancer Vet Support Team

(The Team of Dog Lovers Behind This Site Who Understand What It Means to Have a Dog with Cancer)

Internal cancer signs also depend on where the cancer is happening. For example, a bone tumor (osteosarcoma) might cause a limp, or a bladder tumor (transitional cell carcinoma) might cause straining to urinate, blood in urine, or urinating small amounts frequently.  A tumor found in the wall of the stomach might cause vomiting, and in the intestine, diarrhea.

Some cancers cause internal bleeding, like hemangiosarcoma of the spleen.  This bleeding causes sudden weakness and wobbly legs.  A nasal tumor like a squamous cell carcinoma might cause discharge or bleeding from a nostril, or sneezing that won’t go away. Lung cancers (bronchial adenocarcima) or tumors of the heart can cause coughing. Lumps in the breast with discharge from teats could be mammary carcinomas.

The good news is, not all of these signs point to cancer.  Lots of other things can cause each and every one of these signs.  The important thing to remember is to get it checked out by someone who knows what they are doing.  If we are dealing with cancer, moving early is the way to go.

For more information on all the ways cancer can come about and what you can do, you will definitely want to read the Dog Cancer Suvival Guide.

Best to all,

Dr Dressler

Leave a Comment

  1. Chris Landman on January 17, 2021 at 10:22 am

    What does it mean if the MTC develops a black dot on it?

  2. Trudy Spears on February 6, 2020 at 5:32 pm

    My 10 pd dog had a tumor removed down close to the bottom of her belly last year. About 4-5 months ago it gre back even larger. Now she cries when I pick her up or my other 12 pd dog partially lays on her. She is lethargic and I noticed today she isn’t playing and today I also noticed she doesn’t want to go outside and when she tried to potty she did it on the porch and she never does that. My porch is covered with snow and there was no yellow snow. I am worried to death. She is usually always wanting to play but not today

    • Dog Cancer Vet Team on February 10, 2020 at 10:51 am

      Hello Trudy,

      Thanks for writing and we’re so sorry to hear about your girl. As we’re not vets, we can’t give you medical advice. From what you’ve written, it sounds like a trip to the vet might be a good idea. They will be able to perform a fine needle aspirate on the lump to find out what it is, make recommendations on what you can do to help with her lethargy, and suggest strategies to help manage her pain.

      We hope this helps! Sending warm wishes to you both x

  3. Dottie on December 26, 2019 at 10:59 am

    I have a 13 year old Jack Russell who is in pretty good health. However in the past few months he has developed a sore that is getting bigger and continues to bleed. It is on his side and he keeps biting it making it bleed all the more. I am worried that it could be serious, but I cannot afford to get him tested. Any advice will be appreciated. Thank you.

  4. Mike Ford on May 17, 2019 at 2:48 pm

    My dog is going on 17 Years with us…She has had 1 puppy…My wife got her spaded…She cant have anymore puppies…This has been 10 years ago.At first we didnt think any thing about it…But she kept getting bigger around her stomack area…Now she weights 16 ##s she is a Cha wawa…Teii me what is making her swell like this…Please…-

    • Molly Jacobson on May 17, 2019 at 3:07 pm

      Hi Mike, thanks for writing. Please take your dog to the veterinarian so you can find out what’s going on. A selling stomach could mean lots of different things, and your vet will be able to find out what’s happening and tell you the best course of action. Best of luck and give your girl a snuggle!

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