Chemotherapy – What My Pet and I Should Expect?

By: Ashley Bennett DVM, DACVIM (Oncology) – SAGE Dublin

After learning that a beloved pet has cancer, one of the scariest things to consider is whether or not chemotherapy may be involved in the treatment plan.  Sadly, many of the families that I meet with have gone through cancer treatment themselves or witnessed a human family member experience the negative side effects of chemotherapy.  This can be daunting when deciding how to pursue treatment for a four-legged family member.   Fortunately, chemotherapy tends to be much better tolerated in pets compared to people.  This is typically due to the fact that in veterinary medicine the doses tend to be lower and the medications are given less frequently. That being said, we can still see side effects in pets due to the way that these chemotherapies work. 

What is chemotherapy?

Chemotherapy is a drug that is utilized to kill certain types of cancer.  Each drug works a little differently in the way that it can kill cells.  For most of the chemotherapies that are used in veterinary medicine, the drugs target rapidly dividing cells.  Most cancers are rapidly dividing (growing) meaning they are targeted by the chemotherapy.  The reason that side effects can be seen is that there are also normal cells in the body that are always rapidly dividing and can be affected by the chemotherapy as well.  These cells are generally found in the gastrointestinal tract, the bone marrow and in some breeds the hair follicles.

When do we use chemotherapy?

Chemotherapy is generally used for cancers that are found in multiple places, such as lymphoma, or have a high potential for spreading (metastasis).  Even if we do not see signs of cancer spread at the time of diagnosis, for some cancers we know it has likely already spread microscopically. 

How do we give chemotherapy?

Many of the chemotherapies used for pets are given by IV (intravenous) injection.  Some are given in a matter of seconds and some are given over a more prolonged period of time (30 minutes to multiple hours).  For longer infusions, an IV catheter will be placed.  As soon as the treatment is completed this will be removed.  For some patients, we may recommend a mild sedative to reduce any stress during this time.  These treatments are generally given every 1,2 or 3 weeks.  The specific protocol depends on the chemotherapy used and the type Some chemotherapies are given by mouth.  These are sometimes still administered in the hospital, but there are also options that can be administered at home as well.  If chemotherapy is being administered at home you will be instructed to wear gloves when handling the drug.  It is also important that these tablets or capsules not be crushed or dissolved to minimize any exposure to chemotherapy. 

What side effects can happen?

Gastrointestinal tract: In 20% of pets treated with chemotherapy we can see an upset stomach.  Typically, this happens 2-4 days after treatment.  These signs can include nausea (drooling, decreased appetite, lethargy), vomiting and/or diarrhea.  Some pets quickly improve with the addition of an anti-nausea medication called maropitant (Cerenia) or an anti-diarrheal medication/probiotic.  Some pets may have more significant vomiting and diarrhea that may need to be treated with supportive care at the hospital (fluids and injectable medications).  Fortunately, even in pets that do experience these side effects they typically pass in 1-3 days. 

Bone marrow suppression: In 20% of pets treated with chemotherapy we can see suppression of the bone marrow. Typically, this happens 7-10 days or 3 weeks after treatment.  We will recommend a complete blood count for most pets to help monitor this.  About 5% of pets can experience significant suppression of their white blood cell count, which can make it difficult to fight infection resulting in a fever.  Signs of a fever in a pet could be lethargy, shivering, or lack of appetite.  If a fever does occur, we would recommend IV antibiotics in the hospital.  We also occasionally see a decrease in platelets, which can help us clot our blood.  Rarely we can see bruising due to a low platelet count.  These changes tend to resolve within 1-3 days.    

Hair loss: Because of how the coat grows differently in dogs and cats compared to people, most pets do not lose their hair. Breeds that do require routine grooming (Maltese, Yorkshire Terrier, Bichon Frise, etc) can lose some hair and having thinning of their coat, but do not tend to lose their hair completely.  We also notice a loss of the whiskers in some patients.  This will improve once chemotherapy is done. 

There are unique side effects that can also be seen associated with each specific chemotherapy. These are usually less common and your oncologist will help advise you of what to watch for and what we will monitor for here in the hospital. 

What are our goals with chemotherapy?

Our goal in treating dogs and cats with chemotherapy is to extend a good quality of life for as long as possible.  Sadly, many cancers are not cured with this form of treatment, but it can be very effective in extending life.  If a pet does experience a negative side effect from treatment then we will plan to reduce the dose of chemotherapy, change the frequency of treatment or change the treatment plan completely to make sure that we are preserving a good quality of life.

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