Signs of Cancer in Dogs and Cats

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Older dogs and cats are at high risk for developing cancer. In fact, estimates reveal that as many as 50% of pets die because of the disease. Early diagnosis is essential to effectively managing or curing cancer, so it is important that owners be aware of the common signs of the disease and understand some basic facts about cancer in dogs and cats.

"In most cases, cancer can be successfully managed for a period of time and potentially even cured if it is caught early enough."

Types of Cancer in Dogs and Cats

Cancer is usually classified in one of two ways:

  • By the organ that it affects – liver cancer, brain tumor, skin cancer, etc.
  • By the type of cell involved – hemangiosarcoma (a cancer of blood vessels), mast cell tumor, adenocarcinoma, etc.

Usually, both classifications are necessary to fully understand a pet’s condition because different types of cancer can affect the same organ yet have dissimilar clinical signs, prognoses, and treatment protocols. For example, two types of skin cancer may look different, be treated differently, and tend to have different outcomes.

Signs of Cancer in Dogs and Cats Infographic

Symptoms of Cancer in Dogs and Cats

Because there are so many different types of cancer, no one clinical sign is unique to the disease. Nevertheless, if a dog or cat develops any of the following symptoms, cancer is certainly a possibility, and the pet should be examined by a veterinarian as soon as possible:

  • Abnormal masses – Some types of cancer form discrete tumors or cause organ enlargement (e.g., lymph nodes) that can be seen or felt. Often, these masses will grow or change over time.
  • Persistent sores – Cancer affecting the skin or mucous membranes can look like a wound, but the lesion does not heal in a typical manner.
  • Weight loss and poor appetite – Cancer requires energy and other nutrients, which takes away from what is available to the rest of a pet’s body. Also, pets with cancer generally don’t feel good and may not eat as well as they normally do. Weight loss associated with cancer frequently involves both fat and muscle tissue.
  • Poor coat quality – Cancer can cause pets to stop grooming themselves and/or grow dry and brittle fur.
  • Unexplained bleeding or discharge – Cancer may cause blood vessels to rupture or be associated with secondary infections resulting in abnormal discharge from the mouth, nose, anus, genitals, or other body openings.
  • Abnormal odors – Cancer disrupts the body’s normal protective mechanisms that keep infection at bay, and most infections are associated with a foul odor.
  • Difficulty eating or swallowing – Cancer of the oral cavity or esophagus can make eating and swallowing difficult and/or painful.
  • Lethargy, weakness, or exercise intolerance – Cancer can make pets anemic (have low red blood cell counts), decrease energy levels, and adversely affect the musculoskeletal, cardiovascular, pulmonary, nervous, and other body systems making animals unwilling or unable to be as active as normal.
  • Persistent lameness or stiffness – Cancer of the musculoskeletal or nervous system can adversely affect a pet’s gait.
  • Difficulty breathing and/or coughing – Cancer affecting the cardiovascular system or lungs often causes dogs and cats to cough and breathe rapidly or with greater effort than is normal.
  • Abnormal urination – Cancer of the urinary tract and other body systems can cause pets to strain to urinate, urinate a greater or lesser volume than normal, urinate more or less frequently than normal, or have blood in their urine.
  • Vomiting and diarrhea – Cancer can directly involve the gastrointestinal tract or alter the functioning of other organ systems resulting in an adverse effect on the gastrointestinal tract. In either case, pets may vomit and/or have diarrhea.
  • Constipation – Tumors that block the lower gastrointestinal track can cause pets to strain or be unable to defecate.
  • Chronic sneezing – Tumors of the nasal passages typically make dogs and cats to sneeze.
  • An enlargement or swelling of any body part – Tumors or abnormal fluid accumulations (e.g., blood in the abdomen) that develop as a result of cancer can cause parts of the body to enlarge.
  • Behavioral changes – Unexplained aggression, altered mentation, or other abnormal behaviors can be caused by a tumor in or around the brain, altered body chemistry caused by cancer elsewhere in the body, or pain.
  • Paleness or yellowing of the mucous membranes or skin – Cancer that results in bleeding, abnormal red blood cell destruction, poor red blood cell production, or liver disease can result in anemia or jaundice.

In most cases, pets with cancer will have more than one of the aforementioned symptoms. For example, a dog that is losing weight, is lethargic, is straining to urinate, and has blood in its urine is more likely to have cancer than is a dog that only has blood in its urine.

Diagnosing Cancer in Dogs and Cats

To definitively determine that cancer is responsible for a pet’s clinical signs and identify the type that is involved, a veterinarian will take a tissue sample from the abnormal area, either via a needle and syringe or through a surgical biopsy. Sometimes, the veterinarian can reach a diagnosis by looking at cells under the microscope in the clinic, but it is usually best to send the sample to a veterinary pathologist for a complete evaluation. Additional diagnostic tests including blood work, a urinalysis, x-rays, and ultrasounds may be necessary to rule out other diseases, find the cancer, determine how widespread or advanced the disease is, and plan appropriate treatment.

Treating Cancer in Dogs and Cats

In most cases, cancer can be successfully managed for a period of time and potentially even cured if it is caught early enough. Treatment options aimed directly against cancer include surgery, chemotherapy, and radiation. Symptomatic treatment is also important and can include pain control, nutritional intervention, antibiotics, anti-nausea medications, and more. A pet’s primary care veterinarian and/or a veterinary cancer specialist will design a treatment protocol specific to the patient’s condition and the owner’s wishes.


Top 10 Warning Signs of Cancer in Pet Animals. Colorado State University, Animal Cancer Center. Accessed 1/31/13.

Mortality in North American dogs from 1984 to 2004: an investigation into age-, size-, and breed-related causes of death. Fleming JM, Creevy KE, Promislow DE. J Vet Intern Med. 2011 Mar-Apr;25(2):187-98.

Handbook of Small Animal Practice. R Morgan, et al. Saunders Elsevier. Philadelphia, PA. 2008

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