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What are signs of cancer in dogs?

About half of dogs over the age of ten will develop cancer at some point in their lives. As in humans, cancer types seen in canines can vary, as can the severity and aggressiveness of the disease. However, in almost all cases, early detection is critical to improving outcomes.

Here we will highlight some of the most common warning signs that your dog may have cancer. If you observe any of these signs or symptoms in your animal, see your veterinarian as soon as possible. While no one sign is conclusive for the presence of cancer, your vet has the tools and technology required to make an accurate diagnosis and to create a treatment plan.

Common Cancer Warning Signs in Dogs

1. Any changing mass. Symptoms of canine cancer vary, but any observed lump should be taken seriously. A lump that grows or changes in size or shape should raise a red flag. While not all lumps are malignant, your vet will likely suggest that any growth be surgically removed and biopsied. Benign tumors such as lipomas (fatty skin growths) are common in older animals, so while an observed mass may not be cancerous, only a biopsy can refine the diagnosis. If your pet does require surgical removal and biopsy of a mass, your vet can go over the procedure with you, so you’ll know what to expect in terms of recovery and follow-up.

2. Sores that don’t heal. Another common cancer warning sign is the appearance of skin sores or wounds that are either slow to heal or do not fully heal at all, despite antibiotic treatment. Most such sores can be easily observed, though sores that occur in the mouth, gums, or throat may require more vigilance to detect. Oral tumors ort lesions are usually accompanied by other symptoms, such as foul breath, mouth tenderness, reduced interest in food, and a reluctance to eat hard foods like kibble.

3. Drastic changes in weight or appetite. While loss of appetite and chronic weight loss do not constitute the certainty of a cancer diagnosis, they are strong indicators that something is wrong and in need of assessment. Your vet can ascertain whether the weight loss is related to a cancerous growth or to some other cause and can suggest a course of treatment.

4. Enlarged lymph nodes. The lymphatic system plays an important role in the immune responses of both people and dogs. While enlarged or swollen lymph nodes can often indicate the presence of an infection, they can also occur as a result of lymphoma—a serious blood-borne cancer that can be fatal. Your vet can palpate the lymph nodes and perform blood tests to check for the presence of lymphoma. Your vet may suggest a biopsy or cytological (cell-based) test to confirm the diagnosis.

5. Swollen abdomen. Prolonged swelling of a dog’s belly can be a troubling sign. A distended or tender abdomen can indicate the presence of edema (fluid) or internal bleeding, both of which can accompany abdominal or systemic cancers. Your vet can perform an x-ray or ultrasound to determine the cause of the swelling.

Other Signs of Cancer in Dogs

Please be aware that there are other signs of malignancy in dogs—including loss of stamina, lameness (which may indicate bone cancer), loss of appetite, persistent trouble breathing, or difficulty defecating or urinating. Again, no one sign is definitive for a cancer diagnosis, but if you observe any of these signs in your animal, see your vet as soon as possible.

Editor’s Note: In the US, Cancer is the leading disease-related cause of death for dogs. Here we discuss common cancers in dogs.


Cecily Kellogg is a pet lover who definitely has crazy cat lady leanings. Her pets are all shelter rescues, including the dog, who is scared of the cats. She spent eight years working as a Veterinary Technician before becoming a writer. Today she writes all over the web, including here at Figo.

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