The Ten Warning Signs of Cancer in Cats and Dogs

signs of cancer in dogs
Photo by Karin Hiselius

Pets have become members of our families, and we want to ensure that they live the longest and best lives they possibly can. As we have taken better care of our dogs and cats, they are indeed living longer; yet despite this, or perhaps because of this, cancer is one of the leading causes of death in pet dogs and cats. Some estimates suggest that greater than 50% of dogs over 10 years old will die of cancer.

The earlier you detect cancer, the better your chance of effective treatment. Below are ten warning signs of cancer in both dogs and cats. Please understand that these are just potential warning signs and should not panic you, but prompt a visit to your veterinarian.

Swollen lymph nodes:

These “glands” are located all throughout the body but are most easily detected behind the jaw or behind the knee. When these lymph nodes are enlarged, they can suggest a common form of cancer called lymphoma. A biopsy or cytology of these enlarged lymph nodes can aid in the diagnosis.

An enlarging or changing lump:

Any lump on a pet that is rapidly growing or changing in texture or shape should have a biopsy. Lumps belong in biopsy jars, not on pets.

Abdominal distension:

When the “stomach” or belly becomes rapidly enlarged, this may suggest a mass or tumor in the abdomen, or it may indicate some bleeding that is occurring in this area. A radiograph or an ultrasound of the abdomen can be very useful.

Chronic weight loss:

When a pet is losing weight, and you have not put your pet on a diet, you should have your pet checked. This sign is not diagnostic for cancer but can indicate that something is wrong. Many cancer patients have weight loss.

Chronic vomiting or diarrhea:

Unexplained vomiting or diarrhea should prompt further investigation. Often tumors of the gastrointestinal tract can cause chronic vomiting and/or diarrhea. Radiographs, ultrasound examinations, and endoscopy are useful diagnostic tools when this occurs.

Unexplained bleeding:

Bleeding from the mouth, nose, penis, vagina, or gums that is not due to trauma should be examined. Although bleeding disorders do occur in pets, they usually are discovered while pets are young. If unexplained bleeding starts when a pet is old, a thorough search should be undertaken.


A dry, non-productive cough in an older pet should prompt chest radiographs to be taken. This type of cough is the most common sign of lung cancer. Please remember there are many causes of cough in dogs and cats.


Unexplained lameness, especially in large or giant breed dogs, is a prevalent sign of bone cancer. Radiographs of the affected area are useful for detecting cancer of the bone.

Straining to urinate:

Straining to urinate and blood in the urine usually indicate a common urinary tract infection; if the straining and bleeding are not rapidly controlled with antibiotics or are recurrent, cancer of the bladder may be the underlying cause. Cystoscopy or other techniques that allow a veterinarian to take a biopsy of the bladder are useful and sometimes necessary to establish a definitive diagnosis in these cases.

Oral odor:

Oral tumors do occur in pets and can cause a pet to change its food preference (i.e., from hard to soft foods) or cause a pet to change how it chews its food. Many times a foul odor can be detected in pets with oral tumors. A thorough oral examination with radiographs or CT scans, necessitating sedation, is often necessary to determine the cause of the problem.

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