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5 min read

Early Warning Signs of Cancer in Dogs


By Darlene Stott

Published: 08/23/2017, edited: 11/16/2022

Reviewed by a licensed veterinary professional: Dr. Linda Simon, MVB MRCVS

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No one wants to get the news that their pet has cancer. While cancer is typically seen in older dogs, some cancers can be found in younger dogs as well. Often, a dog will be brought into the vet for a simple growth or change in behavior, and it's usually quite a shock when the big 'c' word comes up. 

It can be difficult, if not impossible, to predict which dogs may be affected by this disease, but luckily for some pet parents, there are some symptoms that can alert you that something is wrong. Educating yourself on the early warning signs of cancer and watching your dog carefully can help your vet diagnose and successfully treat cancer in its early stages for a better prognosis.

Catching some cancers early by knowing the signs is the best way to help your dog fight the disease, so we've gathered the most commonly seen symptoms to look out for in your dog.

Bloodhound wrapped up in a blanket and not feeling well

What kinds of cancer most commonly affects dogs?

Just like in humans, cancer can occur in any area of a dog's body, from the skin to internal organs and lymph nodes. Some of the most common types of cancer in dogs includes:

  • Lymphoma - Describing over 30 different types of cancers in dogs, lymphoma starts in the lymphocytes, or white blood cells, and then can spread throughout the body and internal organs. Swelled lymph nodes often appear under the jaw. The most common type is multicentric lymphoma (most apparent in lymph nodes). Other common kinds are cutaneous lymphoma (skin), mediastinal lymphoma (chest organs), and gastrointestinal lymphoma (stomach or intestines.)
  • Mast cell tumors - These tumors originate in the mast cells, or white blood cells tasked with allergen response, and can migrate to the lymph nodes, then to organs and even bone marrow. Common symptoms of a MCT that has spread include vomiting, diarrhea and changes in appetite. 

  • Osteosarcoma - Though this type of cancer frequently affects the leg bones in large and giant breeds, it can appear in any dog and bone area, and can spread to the lymph nodes and other bones. Dogs often experience pain, swelling and lameness in affected limbs.

  • Melanoma - The most common oral cancer, these tumors are most frequently seen in dogs with dark gums and tongues. While they first develop in the mouth, they can spread to other areas of the body, including the skin and toes. Foul breath, changes in eating habits, rashes and lumps on the skin or swelling or toenail issues on the paws are common symptoms. 

  • Hemangiosarcoma - First developing in the cells that form the blood vessel walls, this cancer can attack the spleen, skin, liver and heart. While an affected spleen won't often show symptoms until a tumor rupture occurs, other forms can show as lumps or masses in the skin, weight loss, weakness, lameness or even seizures. 

  • Mammary gland tumors - Most common in unspayed dogs, these tumors appear as hard or soft lumps near the teats. They can often appear as small nodules near the nipple, and can easily be overlooked if they are flesh colored or covered by hair. 

There are many different kinds of symptoms that dogs can present when cancer is growing. Read on to discover the most common symptoms that should alert you to schedule a vet appointment for your pup. 

A closeup of a skin rash under a white fur dog coat - Early Warning Signs of Cancer in Dogs

Sores which don’t heal

One sign of cancer are sores that don't heal easily or at all. Wounds on the surface of the skin that scab over, appear to go away, and continue to come back is a sign that something is not quite right with your dog’s overall health and immune system. Skin cancer is often discovered by observing the lack of healing on the skin. Sores that do not heal quickly or at all could also mean your dog’s immunities are too weak to heal that part of the body. This usually means their immune fight is elsewhere within the body.

Swellings or lumps

Lumps or bumps under the skin are sometimes tumors. Petting your dog is the easiest way to discover if they have swelling or weird lumps forming that could potentially be cancerous. You can turn playtime into a cancer check by simply giving your dog affection. You may also notice swelling in areas of lymph nodes or around your dog’s neck and face. These are all signs to visit your veterinarian right away, even if only for peace of mind. Remember, many growths will not be cancers.

Tan dog panting outside

Difficulty eating, swallowing or breathing

Your dog could potentially have throat or mouth cancer if they are showing signs of pain when they try to swallow. Your dog will also probably show an interest in food without the desire to eat. And if cancer is affecting organs or muscles in the chest, this could lead to breathing issues. Likewise, a persistent cough can also be an alert to check those possibly affected areas. 

Unpleasant odor

Sometimes dogs smell like dogs. But if an unusual, unpleasant odor is emanating from your dog, it could potentially be a sign of a cancerous tumor. Tumors found within your dog’s mouth, nose, or other orifices can create an offensive odor that is quite unlike the smell of a dog in need of grooming.

Old gray and white dog lying down

Loss of appetite and weight loss

If your dog has lost interest in their food and is losing weight, they may be ill. Cancer is only one disease your veterinarian will want to look at and consider if your dog's appetite or weight changes. Loss of interest in activity and foods is a typical first warning of any health trouble from just not feeling well to something more serious like cancer.

Lack of energy

If your dog's energy is suddenly lower than normal, this is a sign they are not feeling well. A tumor pushing on the heart or lungs could lead to a lethargy, as could pain or swelling in joints, bones and limbs. Lethargy is often one of the first symptoms of illness.

Cream Golden Retriever showing lameness by favoring one front leg


Your dog may potentially be experiencing pain for various reasons. Lameness could result from cancer within soft tissue, nerves or bones. Have your dog checked out by your veterinarian if they are favoring a limb over others and walking with a lame gait.

Unusual bleeding or discharge

If your dog bleeds without explanation or visible injury, or if your dog is experiencing blood in their urine or stool, have them examined by your veterinarian. Unusual vomiting and diarrhea could also be signs of trouble within the body.

Tan dog trying to pee outside

Difficulty urinating or defecating

Several types of cancers can affect the urinary or digestive system which could put pressure on the bladder, kidneys, intestines, colon or anus, this making elimination painful or difficult. 

Early Detection and Prevention

If you are seeing any of these symptoms in your dog, schedule a visit with your vet as soon as possible. Don't wait, as the earlier cancer is detected, the more chance the cancer can be successfully treated and there will be a full recovery. 

Preventive care through your veterinarian with regular visits and watching for early signs are your best chance of catching something serious early. Providing a high-quality diet with added supplements or fresh fruits and vegetables for antioxidants, and keeping their calorie counts down to keep your dog at a healthy weight are preventive measures easily done from an early age. Cancer cells thrive in glucose, so dogs with diets high in sugar may be more susceptible to developing cancer than dogs who have been eating a healthy, lean protein diet with a reasonable fat content.

With a watchful eye and regular checkups, your pup stands their best chance to live a long and woofderful life!

While some breeds are more predisposed than others to certain types of cancers, all dogs are susceptible to this disease. Be sure your dog is covered when unexpected expenses appear by checking out our pet insurance comparison tool. See how insurance plans from leading companies like Figo and Healthy Paws compare and find just the right policy to ensure your pup gets the help they need.


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