Nobody wants to think about the “C-word,” but unfortunately, cancer is the leading cause of natural death among dogs. According to veterinary oncologist Dave Ruslander, 50% of dogs over age 10 will develop a form of cancer.

As the body ages, it becomes more vulnerable to disease. Learn the early signs of canine cancer to keep your dog as healthy as possible for as long as possible—in addition to providing a wholesome diet, age-appropriate exercise, and mental stimulation, like interesting puzzle toys and new experiences.

If you do suspect a problem, consult with your vet. We also highly recommend the Dog Cancer Survival Guide, written by expert veterinarians to help you make the best decisions for your dog’s care.

Tumors, mysterious swellings, and unusual growths


Although it’s probably the first symptom you think of when you hear the word “cancer,” tumors are not always cancerous. As dogs age, they are more likely to develop fatty deposits and other benign lumps. But some growths can be malignant, and tumors can signal skin cancer, mammary cancer, and other types of disease.

You can perform a monthly “lump check” to keep track of your dog’s lumps and bumps. This is especially important for older dogs who develop benign growths all the time. With practice, you’ll probably learn to tell the difference between a benign fatty deposit and a more concerning growth. But if a new lump or bump develops, it’s a good idea to check with your expert veterinarian just in case.

Wounds that won’t heal


Like tumors, persistent wounds can be signs of abnormal cell growth in your dog’s system. Typically, a small wound or lesion should heal over time, with visible signs of healing (i.e. scabbing and skin and hair regrowth). If your pet has a recurring lesion or wound that just won’t heal, it’s time to see the vet.

Sudden lameness

“Lameness” is a change in your dog’s regular gait. It may present as tenderness and subtle pain, limping or favoring a limb, or in severe cases, the inability to place any weight on the limb. Basically, lameness = pain, and can be an indication of bone cancer, particularly in older dogs.

You don’t need to panic about every little hitch in your dog’s step (particularly if they’re an older dog with arthritis), but sudden, persistent lameness should be evaluated by a vet.

Sudden, persistent lameness should be evaluated by a vet

Rapid, unexplained weight loss or gain


Weight loss is a particularly common sign of canine cancer, and may indicate a gastrointestinal tumor that is otherwise undetectable from the outside. If your dog starts losing weight rapidly, whether their appetite changes or stays the same, get to the vet ASAP.

Sudden weight gain or bloating can also be a sign of cancer. If your dog maintains their regular appetite but seems to gain weight quickly, it’s time for a check-up.

Abnormal discharge or bleeding

Abnormal discharge or bleeding anywhere on the body is cause for concern, but this dog cancer symptom is most visible on the face. Funky eye discharge or a sudden bloody nose can indicate certain types of eye and skin cancers.

Similarly, sores and bleeding in the mouth can be a sign of oral tumors, which often go undetected because people assume the discharge and odor is a normal sign of aging. While bad breath is common in older dogs, unusual odor, discharge or bleeding is cause for concern.



Old dogs slow down. It’s an unfortunate but unavoidable fact of doggy life. However, a sudden, unexplained lack of energy—lethargy—can be a sign of illness or disease.

Lethargy is different from plain old tiredness in that it alters your dog’s enthusiasm level. They may suddenly lose interest in a favorite toy or activity, or fail to get up and greet you when you come home from work. Other signs of lethargy may included excessive sleep and delayed responses to visual and auditory stimuli.

Lethargy is a general symptom of a broad range of issues, so it doesn’t automatically signal cancer. But if your dog is suddenly a lot less active than usual, something could be going on.

Unusual poop

You know your dog, and you see her “output” every day. You probably have a sense of the difference between normal poop, somebody-got-into-the-cat-food-again poop, and something more concerning. Persistent diarrhea, hardened stools, and straining can all be symptoms of illness.

If you’re concerned about something in your dog’s output, don’t hesitate to call the vet. In particularly, watch for black, tarry stools, which can indicate ulcers, a symptom of mast cell tumors (source).

Difficulty breathing or going to the bathroom

One of the most common (and alarming) signs of illness or injury is when normal bodily functions become labored or painful. If your dog is having trouble breathing, straining to go to the bathroom, or otherwise seems to be uncomfortable in the course of normal activities, don’t hesitate to have her checked out. Sudden, extreme discomfort or pain are important warning signs.

Cancer is scary, but you don’t have to live in fear of it. Remember: modern dogs live a lot longer than their ancestors. The fact that dogs routinely live beyond age 10 is a great indication of how far pet care and veterinary medicine have advanced. So track your dog’s health, and see the vet if you notice something unusual. The rest of the time, enjoy life to the fullest with your four-legged best friend.


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