Cancer – the dreaded “C” word, and how it can affect our pets

Cancer is a term used to describe a disease that is caused by a tumour, (or neoplasia) – a collection of abnormal cells within the body that continue to grow and divide without control. This usually results in the development of masses (growths or lumps), which are mainly composed of the dividing abnormal cells.

Some tumours do not spread to other parts of the body and tend not to invade other surrounding tissues – these are termed “benign” tumours. In contrast to this, the term cancer is generally used to describe “malignant” tumours which often do invade surrounding normal healthy tissue, and may spread to other sites in the body (“metastasis“), via the bloodstream or lymphatic system. The speed with which a cancer can spread and the severity of the disease it causes depends on the type of tissue cells affected.

As many as one in five cats and dogs are likely to develop some form of cancer at some stage of their lives, with the risk of developing cancer increasing with age. This means that the number of pets with cancer has increased in recent years with advanced veterinary care meaning longer life expectancies for our pets.


As with human cancers, the causes of cancer in pets are still not well understood. Possible causes include:

  • Toxic chemicals (carcinogens) or exposure to harmful radiation
  • Abnormalities of the immune system that usually protects against infectious diseases
  • Abnormal genes
  • Feline Leukaemia Virus (FeLV) – a very common cause in cats

The signs of cancer are very variable and depend on the type of tissue cells involved, the site of the cancer and the stage of the disease. Animals with advanced cancer will often show weight loss and loss of appetite. Depression, vomiting, diarrhoea or constipation, and fever may also be seen. Other effects of the cancer such as anaemia, may also result in tiredness and lethargy.


Most forms of cancer can be treated, but this depends on the type of cancer involved and whether the disease has spread. The outcome of treatment can be very variable, producing a complete cure in some cases, or at least improving your pet’s quality of life and significantly increasing life expectancy.

There are 3 basic options for treating cancers, though not all are appropriate for every case and sometimes a combination of treatments has the best chance of success:

  • SURGICAL REMOVAL – this is usually the best choice for most cancers of solid tissue, especially if a more malignant cancer does not appear to have yet spread to other parts of the body.
  • CHEMOTHERAPY – drug treatment is generally the best option for cancer affecting the blood or multiple areas of the body. Chemotherapy may also delay the appearance of secondary tumours in other organs following surgical removal of the original mass. In pets, chemotherapy is used to improve quality of life, and the side effects seen in people are rarely experienced in cats and dogs.
  • RADIOTHERAPY (x-rays) – radiation is delivered by a machine in a radiation unit at a specialist centre, and is most effective on cancers of the extremities (such as the limbs and head) where a beam of radiation can be delivered directly to the affected area and is less likely to damage normal tissue before reaching the tumour. In some cases, it may be possible to treat the cancer by injecting radioactive material into the body.

As cancer advances, discomfort and related pain can become severe, but can often be controlled. A gentle painkiller may be prescribed initially, moving on to more powerful medications as required. It is important to remember that quality of life for your pet should always be of the most importance, rather than prolonging life, as our pets live in the “here and now“.


Sadly, as with human cancer, it is impossible to give a confident answer to this question. Survival times depend not only on the type and stage of disease, but also on your pet’s general health status. It is understandable to feel frightened for your pet’s future, but discussing this with your vet and agreeing on an appropriate treatment plan will help to reassure you that what you are doing is in your pet’s best interests.

Although this article has discussed cancer in cats and dogs, this disease can occur in any species, however treatment options will often be much more limited in smaller species, with surgical options or palliative care being offered.