Chemotherapy is something we generally associate with human cancer patients, not pets but when faced with your pet’s cancer diagnosis it is good to know exactly what chemotherapy can do for him or her.

If chemotherapy is recommended for your pet, it’s natural that you’ll have questions and concerns. When do we recommend chemotherapy? How does it work? What are the risks, and what can you expect? Here are some insights from cancer specialist vet, Dr Sue Ettinger.

In the world of veterinary oncology, chemotherapy is often recommended to control a pet’s cancer, prolong survival and maintain a good or even excellent quality of life.

Chemotherapy and pets: what happens?

Chemotherapy drugs are compounds that are toxic to cancer cells, which multiply very rapidly. Most chemotherapy drugs work by damaging the ability of cancer cells to divide and replicate. Therefore, the goal of conventional chemotherapy is to arrest cancer cell growth and to kill the cancer cells.

When is chemotherapy recommended for my pet?

Chemotherapy may be recommended for your pet:

1. To reduce or eliminate your pet’s cancer. For tumours sensitive to chemotherapy (such as lymphoma, myeloma or leukaemia), chemotherapy is usually the most effective single treatment. Depending on the cancer being treated, multiple chemotherapy drugs may be given.

2. To prevent or delay metastasis (spread) of your pet’s cancer. Chemotherapy is typically administered after the primary tumour has been controlled locally with surgery, radiation therapy or both. If your pet’s tumour has a high likelihood of metastasis, chemotherapy will likely be part of the treatment plan. For example, bone cancer (osteosarcoma) in dogs has a very high metastatic rate, so chemotherapy is recommended and most effective after the bone tumour has been removed.

3. To make your pet more comfortable. Occasionally, chemotherapy may be recommended as a palliative treatment for tumours that cannot be removed with surgery or treated with radiation therapy. The goal is to make your pet more comfortable and alleviate problems associated with the tumour, such as pain or pressure.

4. To increase the sensitivity of your pet’s tumour to radiation therapy. Some pharmaceuticals have been shown to increase the ability of radiation therapy to kill cancer cells without increasing the toxicity to normal tissue.

Does chemotherapy make pets sick?

The surprising answer is no! Chemotherapy is very well-tolerated in most dogs and cats. In my experience, 80% of pets have no side effects; 15 to 20% will have mild to moderate side effects, but the side effects only last a few days and will improve on their own.

Cats tend to tolerate chemotherapy even better than dogs, and both dogs and cats handle chemotherapy better than people. Most of my clients tell me the good days definitely outweigh the bad days while being treated. Many clients are in disbelief their pet is getting chemotherapy because the side effects are so rare and the dog or cat is so “happy and healthy.”

Plus, we have effective medications to minimise any side effects that do happen, and they help your pet get through the side effects more quickly.

Still, most chemotherapy drugs have the potential for side effects, so it is important to be aware of them when making the decision to treat, and so we can identify them early when they occur. However, they would not be used if their potential benefit of killing cancer cells did not outweigh the possible toxicity. Most pets that we recommend chemotherapy to are likely to live longer and live better with treatment than without.

Why do we see side effects during my pet’s treatment? What will they look like?

In addition to killing rapidly dividing cancer cells, the chemotherapy will injure or damage some normal cells. Some normal cells that divide rapidly are susceptible to the toxic effects of chemotherapy. The three most common side effects we see in pets are:

1. Gastrointestinal toxicity. This can manifest as nausea, vomiting, diarrhoea or loss or changes of appetite. These side effects most commonly occur for one to five
days after the chemotherapy treatment and last two to three days. Severe gastrointestinal side effects are uncommon. Generally, the side effects are mild and resolve on their own without additional treatment, but we can minimise the impact on our pets by being proactive and preventing and identifying effects early and treating when they occur.

Keep in mind that pets with nausea can be hard to figure out. A nauseous pet tends to approach food to eat but then turns away without eating. They may also salivate or appear anxious.

2. Bone marrow toxicity. This toxicity typically causes low blood counts (white blood cells, red blood cells and platelets). If the cells that produce the white blood cells are damaged, the patient’s white blood cell levels may fall low enough to increase susceptibility to infection. The good news is that white blood cells recover quickly—typically within days—and antibiotics may be prescribed to decrease the risk of infection while they are low. If your pet acquires a systemic infection, you may notice severe lethargy or fever. Please call your veterinarian if you note either.

3. Hair loss. Usually mild in dogs but can show up unexpectedly. It’s not nearly as common as it is in people, because your dog’s hair does not grow continuously throughout their lives. Exceptions are Old English sheepdogs, poodles, Westies and other breeds whose hair does continue to grow—in other words, dogs that need periodic hair clipping. The hair will regrow after chemotherapy is stopped. And remember that dogs don’t worry about how they look, so the psychological impact is minimal compared with people.

It’s important to note that in any breed, the hair will be slow to regrow in areas that we need to shave for access to veins or for other procedures, such as abdominal ultrasound. In addition, when the hair regrows, it may be of a slightly different colour or texture.

Cats may lose their whiskers when they are undergoing chemotherapy.

4. Other effects. Some intravenous chemotherapy drugs can be extremely irritating to the tissues if they leak out of the vein when we administer the drugs. This can result in swelling, inflammation, ulceration and tissue damage.

We are very careful about minimising this by administering these chemo-therapeutic drugs through catheters and giving the treatments in a quiet, dedicated room.

In addition to the above side effects, other drugs may have unique potential toxicities. Your vet will review this information with you.

When the platelet count is low, clotting can become an issue, and you may notice more bruising than normal or nosebleeds. It is very important that you stay in contact with your vet.

Severe side effects from chemotherapy are rare. Only about 5% of chemo patients have severe side effects that require hospitalisation, and typically for just 24 to 48 hours. Most patients feel better with IV fluids, injectable nausea medications and antibiotics. In addi- tion, with chemotherapy dose reductions and medications to prevent further side effects, most of my patients can receive that same chemotherapy drug again without any further issues.

How might my pet’s chemotherapy given?

Chemotherapy treatment may consist of one drug or a combination of several drugs. Most drugs are given by intravenous (IV) injection or orally. Less commonly, chemotherapy is given in the muscle (intramuscularly, or IM), under the skin (subcutaneously, or SQ), or into the chest or abdominal cavity. The therapy will be planned by your vet based on your pet’s cancer and other medical problems that may impact drug choice.

Chemotherapy is usually given on an outpatient basis. Typically, your pet will stay for a short time to allow for a physical examination, pre-chemo blood work and chemotherapy treatment.

Most pets are awake for their treatment, and sedation is not required. The length of the actual treatment depends on the drug, but most drugs are given intravenously over five to 30 minutes. Some of the newer immunotherapies do require a full day at the clinic, but this is not the norm.

For some cancers, you may be administering oral chemotherapy at home.

Giving chemo to your pet is not a contract. If you do not like how your pet is handling it, you can stop. I always encourage people to try a dose or two, and most of them are so surprised and pleased, they return with their pet for more treatments. Meet with your vet and learn about the options. Even if you decide not to treat, you will have made an educated decision.

Remember, chemotherapy would not be used if the potential benefits of killing cancer cells did not outweigh the possible toxicities. Most pets tolerate chemotherapy extremely well. Your pet is quite likely to have normal activity and energy and continue its routine.

Cancer can be scary, but we can do this together.

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