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Cancer in Dogs and Cats

The dreaded big C is not something any loving pet parent wants to think about, but sadly it’s a fact of life. It’s estimated that 1 in 3 dogs and 1 in 5 cats will develop the disease at some stage, making it one of the biggest causes of death in older animals. The good news is that if caught and treated early, many cancers have a good rate of recovery.

Causes of cancer

If your pet has been diagnosed with cancer, it’s only natural that will want to know why. Drawing a direct link between cause and effect is not always easy, but there are some things we know about what causes cancer in pets.

Genetic factors often play a part in disease, and cancer is no exception. Certain breeds, while not certain to develop cancer, do have a higher instance of specific types of the disease. Golden retrievers, Boston terriers, and boxers have a higher rate of mast cell tumors, while large breeds such as Great Danes, Rottweilers, and Saint Bernards are more prone to bone cancer. There is no breed of cat that is more susceptible to cancer, but white cats are more at risk of skin cancer due to their coloring.

Beyond family history, environmental and lifestyle factors play a large part in the development of cancer. Animals are unfortunately exposed to a number of known carcinogens during their day-to-day lives, including weed killers, insect sprays, air pollution, and second-hand smoke. Lifestyle factors that are linked to cancer include overexposure to the sun, poor oral care, obesity, and poor diet.

Female dogs and cats that have not been spayed have a higher instance of mammary cancer and ovarian cancer. And spaying before the first heat for dogs, and in the first six months for cats, will reduce the risk even further. Male animals that have not been neutered have a higher risk of testicular cancer.

Illness and old age have a role too. As the body ages the immune system, which is the body’s primary defense against illness, becomes less efficient. For cats, the feline leukemia virus and feline immunodeficiency virus are thought to cause cancer.

Common cancers in pets

Cancer in animals, just like in humans, can take on many different forms. These specific types of cancer can be more prevalent in certain animals, and all affect the body in different ways. These are the most common, but not the only cancers seen in dogs and cats.

Cancers in dogs:

Lymphoma– this affects cells in the lymph nodes or bone marrow and can attack the immune system if left untreated. The first sign of lymphoma is often a swollen lymph node at the neck.

Hemangiosarcoma– a malignant cancer of the blood vessels that almost exclusively affects dogs. It is most commonly diagnosed in the spleen, liver or heart, but the lack of early symptoms means it is often not diagnosed until it is in the late stages.

Mast cell tumors– a form of skin cancer that can also be found in the intestinal or respiratory tracts. This is a very common cancer among older dogs, and are usually first detected as a lesion on the skin.

Melanoma – this is a particularly aggressive form of skin cancer that often starts around the lips or mouth, but can also be detected in the nailbed or eyes. If left untreated it rapidly spreads through the body to affect the vital organs.

Osteosarcoma– tumors that affect the bones, but can also be found in the joints or lungs. This cancer is very often malignant and is most commonly found in larger dog breeds.

Mammary cancer – the equivalent of breast cancer in humans. It is most commonly found in female dogs that have not been spayed, but can also be found in spayed females or male dogs. Around half of these tumors are malignant.

Cancers in cats:

Lymphoma– this cancer of the blood cells is the most common cancer to affect cats, but is more prevalent in cats between 2-6 years of age, or in those that have feline leukemia virus.

Fibrosarcoma – also known as soft tissue sarcoma, this cancer develops in the connective tissue. Owners may notice a firm lump under the skin that continues to grow. Fibrosarcoma has been known to develop after the administration of injections, where it is known as feline injection-site sarcoma.

Squamous cell carcinoma– a type of skin cancer that develops in areas of the body that lack pigment or are constantly exposed, such as the ears, eyelids, and nose.

Cancer symptoms

Lumps or swelling– lumps are the most obvious early sign of a number of forms of cancer. Any unusual masses under the skin should be examined by a vet, who will most likely recommend removal and biopsy to determine if it is benign or malignant.

Sores that won’t heal – if a lesion on the skin persists after treatment with ointment or antibiotics, it should be checked out by a vet.

Abnormal discharge or bleeding– bleeding from the nose or persistent discharge from the eyes or nose could be an early indicator of facial tumors.

Loss of appetite – while many things can cause a lack of appetite, this could be a sign of intestinal tumors.

Unexpected change in weight – if not explained by a change in diet or other illness, a sudden weight loss or weight gain should be checked out.

Lethargy or loss of stamina – a sudden change in energy levels could signify a number of health problems, including cancer.

Sudden and persistent lameness or stiffness – particularly in larger breeds of dogs, this can be a sign of bone cancer.

Difficulty breathing, urinating or defecating – if your pet has problems with their bodily functions, it can indicate pain or obstruction caused by cancer.


Diagnosing cancer in animals is not a straight forward procedure, as there is no definitive test that can confirm or rule out the disease. Vets will rely on the physical symptoms that your pet has been experiencing to start their diagnosis, then use a range of tests to check for particular types of cancer in the affected areas.

A physical examination will be used to check on any unusual lumps, sores or discharges that you as an owner may have noticed. At this stage, any lumps or sores can be biopsied. For other cancers or inaccessible parts of the body, your pet may require a combination of x-rays, ultrasounds, blood tests, urine tests, CT scans or MRI scans.

Treatment and prognosis

If your pet has had their cancer diagnosis confirmed, your vet will discuss the best treatment options available, and the change of a full recovery. The course of treatment taken will depend on the type of cancer, how advanced the cancer is, and the overall health of your dog or cat.

For localized cancers, surgery is the most straightforward option, with the aim being to remove all of the tumors in one go. This may be combined with chemotherapy or radiation therapy to ensure that all cancerous cells are eradicated.

Chemotherapy is used for non-localized cancers, such as cancer of the blood cells, but can also be used to stop tumors spreading to other parts of the body. Radiation therapy is used in the treatment of tumors that cannot be removed easily through surgery, such as tumors in the brain.

If detected early, many cancers can be removed or treated successfully, which is why keeping an eye on any unusual symptoms is so important for pet owners. Sadly, however, some cancers are uncurable or may have spread to such an extent that there is no chance of treatment. In these cases, your vet will offer palliative care, which aims to reduce the suffering caused by cancer without treating the disease.

Cancer in Dogs and Cats

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