Chemotherapy in Pets
The diagnosis of cancer is stressful for pet owners, and the prospect of chemotherapy treatments can be equally difficult. However, the fear that animals will spend most of their time sick from chemotherapy treatments is unwarranted. Knowing how anti-cancer chemotherapy drugs work and what to expect from the treatments can help pet owners decide on whether such therapy is appropriate for their pets.
When do we use chemotherapy to treat animals with cancer?
Chemotherapy may be used as the main treatment for certain cancer types, or may be used in combination with other treatments such as surgery and radiation therapy. In some cases, chemotherapy can be used to try to shrink large tumors prior to surgery, or to help eliminate microscopic cancer cells that cannot or have not been completely removed surgically. For cancers that are at high risk for spread, chemotherapy can be used after surgery to help delay or prevent the appearance of cancer in other parts of the body.
How does chemotherapy work?
Most chemotherapy drugs attack cells in the process of rapid growth. Individual drugs may work through many different mechanisms, such as damaging a cell’s genetic material (DNA) or preventing the cell from dividing properly. However, all rapidly dividing cells (including some normal ones) are potentially affected by chemotherapy. Damage to normal, rapidly growing tissues in the body is the reason for most of the side effects seen with chemotherapy. Fortunately, these tissues continue to grow and repair themselves, so injury caused by chemotherapy is rarely permanent.
What are the typical side effects of chemotherapy?
Compared to people receiving chemotherapy, pets experience fewer and less severe side effects because veterinary oncologies generally use lower doses of drugs and do not combine as many drugs as in human patients. The normal tissues that are most sensitive to chemotherapy are the intestinal lining, the bone marrow (which makes new blood cells), and occasionally hair follicles. Toxic effects to the digestive tract are responsible for decreased appetite, vomiting, and diarrhea. When we see them, these signs are mild and usually resolve on their own or with oral medication given at home. Although uncommon, some animals may develop severe vomiting or diarrhea requiring hospitalization and fluid therapy. In many cases, the digestive side effects from chemotherapy are not seen on the day of treatment — they may occur 2 to 5 days later.
Suppression of the bone marrow by chemotherapeutic drugs may cause a drop in the white blood cell count, leading to increased susceptibility to infection. Severe infections (rarely encountered) may require hospitalization for intensive supportive care, including intravenous fluids and antibiotics. When a chemotherapeutic drug that is known to have the potential for bone marrow suppression is used, a complete blood count (CBC) is checked before the treatment is given and at a pre-determined time after a chemotherapy dose (usually 7-10 days). If the white blood cell count is low but your pet is feeling well, treatment may be postponed, and antibiotics prescribed as a preventative measure. Subsequent doses of chemotherapy may be reduced.
Hair follicle cells in dogs that are wire-haired or non-shedding may be particularly susceptible to chemotherapy. Certain breeds of dogs, such as terriers and poodles, will experience variable amounts of hair loss. Cats may lose their whiskers during treatment.
There are many different types of chemotherapy agents and each has a different likelihood of causing side effects. If your pet is treated with drugs known to cause certain side effects, we will prescribe medications to help prevent these complications, such as anti-nausea medications. In addition, we will give you instructions on what to do if and when a problem arises. We seldom see severe side effects; it is estimated to be less than 5% of all pets receiving chemotherapy. With proper management, most animals recover uneventfully within a few days. Please keep in mind that any animal can have an unexpected reaction to any medication.
How is chemotherapy given?
How a chemotherapeutic drug is administered, how often it is given and how many treatments are given varies from case to case. The type of cancer, the extent of disease, and general health of the patient help the oncologist to formulate a treatment protocol (type of drugs, dose, and schedule used) appropriate for your pet. Some drugs are oral medications (pills) that you give at home. Others are brief injections that require an outpatient appointment. In a few instances, slow infusions or repeated treatments throughout the day may require an animal to spend the day in the hospital. The treatments are typically repeated from weekly to every third week. Blood tests are often needed to monitor the effects of chemotherapy during the weeks between drug treatments.
The duration of chemotherapy depends on the type of cancer and the extent of disease. Certain animals may need to receive chemotherapy for the rest of their lives. In others, treatments may be spread out or discontinued after a period of weeks to months provided that the cancer is in remission, i.e., there is no detectable evidence of cancer in the body. Chemotherapy can be resumed if the cancer relapses.
We usually recommend that every patient receive at least 2 cycles of chemotherapy and then be evaluated for response before we decide to continue the treatment, change drugs or discontinue chemotherapy.
What can be expected from chemotherapy?
In many cases, we are unable to cure our veterinary cancer patients. Our goal is therefore to maintain a pet’s quality of life while trying to prolong survival. To this end, chemotherapy can sometimes be used to minimize the discomfort caused by a tumor, or to slow down or stop the progression of the disease. For most (but not all) types of tumors, the oncologist will provide information on expected outcome with and without treatments. The decision of whether to pursue chemotherapy for your pet can be complex. Medical information, practical concerns (such as need for repeated visits, your pet’s temperament, etc.), and financial responsibility all play a part in this decision. We encourage you to discuss your concerns with the oncologist and your regular veterinarian when making this decision.
Do I need to change my pet’s diet or at home medications while on chemotherapy?
It is not strongly advised to change a pet’s diet during chemotherapy induction, unless medically this is necessary (due to organ dysfunction or a lack of interest in a diet). If your pet has a reaction to a new food causing gastrointestinal signs, it can be difficult to determine if the illness is due to the food change or chemotherapy side effects. Raw diets are strongly discouraged during treatment with chemotherapy.
Please give a thorough list of all medications and supplements to your oncologist so that they can advise if there are any potential drug interactions or need to stop any medications during cancer treatment. It is recommended to continue to use flea/tick and heartworm prevention during chemotherapy. As oncologists, holistic medications and supplements are not our area of expertise and we will do our best to guide you but may refer you to a veterinarian more familiar with these treatments for guidance. It is advised to wait to add in any new supplements until your pet has shown no side effects to chemotherapy to make sure we can determine if any new symptoms are secondary to chemotherapy or the supplements. Overall, due to the lack of information regarding most supplements, they are “use at your own risk” in combination with cancer treatment.
Can my pet receive vaccines while on chemotherapy?
This is often determined on a individual basis pending the health of the patient, risk of exposure, type of cancer treatment, and timing of chemotherapy. While it is not harmful for your pet to receive vaccines during chemotherapy, it may not be an advised time to stimulate your pet’s immune system. If vaccines are advised against in your pet, a rabies exemption form can be provided to you for documentation for your municipality.
Handling chemotherapy at home:
Chemotherapy drugs have the potential to cause damage to normal cells. Exposure to chemotherapy drugs or patient waste can theoretically pose a risk, despite the fact that the concentrations of drug are very low. Clients who are pregnant, attempting to conceive, or lactating should not be tasked with drug administration or cleaning of any patient excreta. Client consultation with their personal physician is recommended before initiating treatment to discuss any potential contact risks. The presence of small children in the home also should prompt a frank conversation about potential exposure to chemotherapy. Handling of pets after chemotherapy administration should be with caution; a pet’s interaction with children in the home should be supervised, avoiding contact with excreta, and with thorough hand washing afterwards. If inadvertent direct contact with chemotherapy or contaminated urine or feces of a patient occurs, the skin should be rinsed with water and washed using dishwashing detergent for a minimum of 5 minutes, in accordance with recommendations for chemical exposure and any additional instructions from a personal physician followed.
Chemotherapy-rated gloves (or doubled latex gloves) should be worn when giving chemotherapy or targeted therapy to a pet at home, and hands should be washed after each administration. The pet’s medications should not be stored with medications for humans, near food, or where accessible by children. Child-proof containers are recommended if there are small children in the home. Pills should not be split or crushed, capsules should not be opened, and swallowing of medications by the pet should be confirmed. Liquid formulations are therefore not recommended due to the potential for environmental contamination during administration. If a pet commonly spits out a pill after it has been concealed in food or a treat, the medication should be administered alone.
Animals receiving chemotherapy should remain in a controlled environment and not be allowed to urinate or defecate in community areas, areas where children may be exposed, or areas that cannot be easily cleaned for at least 48 hours after drug administration. Ideally a low-traffic, sunlit area would be preferred for elimination. If excreta are found in the house, the area should be cleaned, and hands should be washed afterwards.
Recommendations for cleaning excreta include wearing gloves, avoiding high-pressure sprays, and using disposable towels. Solid items should be removed with gloved hands and double-bagged with impermeable disposable bags for household wastes. Liquid wastes should be blotted dry. The area should be cleaned with dilute bleach once gross contamination is removed. Cat litter boxes should be cleaned daily and litter should be double-bagged and discarded with household trash. Any soft items (eg, bedding, towels, toys) should be washed twice, separately from other laundry, after exposure, and ideally bleached.