There are risks involved with any type of treatment for cancer. Some normal cells will be injured and killed by the chemotherapy drugs. Side effects may be apparent because of these normal cells being killed. However, these side effects are usually outweighed by the benefits of eliminating the cancer cells.
Dogs and cats generally tolerate chemotherapy much better than human patients do. The two side effects encountered most commonly in canine and feline patients receiving chemotherapy are toxicity to the gastrointestinal tract and toxicity to the bone marrow. Normal cells in both of these areas divide very rapidly, and as such, are more susceptible to the toxic effects of the chemotherapy. When the cells lining the gastrointestinal tract are affected, the result may be vomiting or diarrhea. Most patients will experience this side effect at least once or twice during their course of chemotherapy treatment, but the symptoms are usually mild and can be overcome with supportive care at home.
White blood cells are cells of the immune system responsible for fighting infection. The bone marrow produces these cells (called progenitor cells). If these progenitor cells are damaged, the patient’s white blood cell count may drop low enough to result in an increased susceptibility to infection. Even bacteria to which a patient would normally be resistant can cause serious illness in this situation. White blood cell counts of all canine and feline chemotherapy patients are monitored carefully; however, occasionally a cat or dog receiving chemotherapy will develop a life-threatening systemic infection. The only way to successfully treat these infections is to admit the patient to the hospital and administer intravenous fluids and antibiotics.
Hair loss in cats and dogs receiving chemotherapy is usually very minor, with some notable exceptions. If you have a Poodle, Old English sheepdog, Schnauzer, Puli, Lhasa Apso, Shih Tzu, Bichon Frise, Terrier, or Maltese, you should expect your pet to lose a significant amount of hair during the initial stages of chemotherapy. However, the hair that is lost will grow back after your dog’s course of chemotherapy has been completed or once treatments are being administered less frequently. Hair may need to be clipped frequently during chemotherapy to identify veins. This hair may be slow to regrow. Cats usually do not lose hair, although many will lose their whiskers.
Some chemotherapy drugs can be extremely irritating to the subcutaneous tissues if they are able to leak outside of the vein during injection. Examples include the chemotherapy drugs vincristine, vinblastine, adriamycin, and mustargen. Severe swelling, ulceration, and inflammation can be seen. However, this complication is rare because all chemotherapy drugs are carefully administered through catheters placed in the vein.