Can my animal catch cancer from another animal or transmit it?
For the vast majority of cases the answer to both questions is “No!”
Animals can be infected with viruses or other microorganisms some of which cause cancer. Some are infected from their mother before or at birth but infection can also be acquired by contact with other animals. Some can be transmitted through bites of ‘vectors’ such as fleas or ticks. Feline leukemia virus, for example, can cause cancers of the blood and lymphoid system in cats. The virus is occasionally transmitted from an infected queen to her kittens before birth but is more commonly acquired from close contact with infected cats that shed the virus in saliva, urine and feces. If your cat is infected, it can pass the infection to other cats.
How does cancer affect my pet?
The most obvious effect of most cancers is an enlarging lump, but most lumps are not malignant. This lump may ulcerate, bleed or have physical effects (pressure, displacement, etc.) on the surrounding tissues. Benign cancers only enlarge locally by smooth expansion but malignant cancers may invade regionally with tentacles (i.e. “cancer the crab”). In fact, we use these differences in growth habit to help decide if the cancer is benign or malignant spreading widely through the body. Widespread distribution of a cancer occurs when cancer cells break away from the original tumor and seed in body cavities (such as the pleural cavity of the chest or peritoneal cavity of the abdomen) and when cancer cells invade the blood. The cells may then be carried to distant tissues where the cells lodge and start new tumor masses (metastases).
Weight loss due to loss of body fat and muscle is common in malignant cancer and unexplained weight loss can be an important sign. A few tumors induce clinical signs that are not readily explained by local or distant spread of the tumors. These are known as paraneoplastic syndromes. Some syndromes are due to abnormal hormone production by the cancer. (Hormones are internal secretions that pass into the blood and stimulate other organs to action.) Examples of paraneoplastic signs and symptoms include loss of hair, increase or decrease in blood glucose, and increased blood calcium levels.
How is cancer diagnosed?
Cancer is often suspected from clinical signs (a lump, loss of appetite and energy, loss of weight for example). X-rays may be useful in detecting internal tumors, including metastases. Blood tests can help indicate some tumors. In order to identify most tumor types, it is necessary to obtain a sample of the tumor itself. Surgical invasion may be needed such as needle aspiration, punch biopsy, full excision or exploratory surgery. The simplest approach in many cases is the aspiration (suction removal) of tumor cells with a syringe and needle. It does not require general anesthesia or surgery. Microscopic examination of the cells obtained is called cytology.
A more reliable test however, requires surgical removal of a tissue sample under anesthesia. The preparation and microscopic examination of stained sections from the removed tissue is called histopathology. This is done at a specialized laboratory where the slides are examined by a veterinary pathologist.
The histopathology report typically includes words that indicate whether a tumor is ‘benign’ (non-spreading, local growth) or ‘malignant’ (capable of spreading to other body sites). Malignancy is often shown by tumor names ending in “carcinoma” or “sarcoma”. These, together with the origin or type of tumor, the grade (degree of resemblance to normal cells) and stage (how far it has spread) indicate how the cancer is likely to behave.
The veterinary pathologist usually adds a prognosis (what will probably happen). This may include a prediction about the probability of local recurrence or metastasis (distant spread).
Can cancer disappear without treatment?
Cancer rarely disappears without treatment but as development is a multi-step process, it may stop at any stage. The body’s immune system can kill cancer cells using mechanisms that specifically target tumor cells that are recognized as “foreign”. These mechanisms include immune system cells such as cytotoxic lymphocytes and macrophages and antibody production. Not all tumors are recognized as foreign and even when they are, the immune system is rarely 100% effective in eliminating the cancer. Rarely, loss of blood supply to a cancer, by pressure on its own supply for example, will result in tumor cell death but the dead tissue will probably need surgical removal.
