In recent posts, I discussed gastrointestinal, or GI, side effects resulting from canine chemotherapy treatment. In the next few articles, I’m going to go over chemotherapy and low white blood cell counts, specifically, bone marrow suppression, in dogs. There’s a lot of information to share, so let’s get going!
Reminder: Chemotherapy Kills Rapidly Dividing Cells
The reason we use chemotherapy in dog cancer treatments is chemo agents target and kill rapidly dividing cells, and cancer cells divide rapidly. Unfortunately, some normal cells in the body also divide rapidly, which means they may also suffer damage.
Which cells beside cancer cells divide quickly? The most commonly affected cells are those in the GI tract, like the lining of the mouth, esophagus, stomach, and intestines. The white blood cells in the bone marrow are also very commonly affected. Interestingly, a common side effect of chemotherapy in humans is hair loss, but that doesn’t usually happen in dogs. It can happen, but it doesn’t in always.
So let’s look at white blood cells in more detail.
White Blood Cells
Bone marrow, that spongy material at the core of bones, produces three types of blood cells: red blood cells, white blood cells, and platelets.
These cells are being born in the bone marrow, which is why they are rapidly dividing and likely to be affected by chemotherapy. That said, of all these cells, the white blood cells being born in the marrow are the most affected. (Already-developed white blood cells that are actively circulating in the bloodstream are not dividing, so they are not affected.)
Chemotherapy and Low White Blood Cell Counts
When chemotherapy agents destroy cancer cells, they can also destroy these rapidly dividing white blood cells in the bone marrow. But you might not even be able to tell!
Unlike GI side effects, which can show up as unmistakable diarrhea, vomiting, and nausea, a low white blood cell count is undetectable by behavior. Your dog won’t act like anything is wrong.
The only way to know if the white blood cell counts have been affected is by getting a blood test. This is why most oncologists, including myself, will test your dog’s blood before and after treatments.
What We’re Looking For
There are five types of white blood cells, and their numbers make up the total white blood cell count. Each of these white blood cells has a different function in the body. But when it comes to chemo treatments, we’re really only worried about one type of white blood cell: the neutrophil.
When we oncologists talk about the “white blood cell” we are almost always really referring to the neutrophil. Neutrophils fight infections in the body, so they are very important for overall health. If the neutrophil count is low, your dog is immuno-compromised and at risk for an infection.
A decreased neutrophil count, called neutropenia, is one of the signs we monitor to decide about dosing decisions. We decide how much to give of most drugs by looking at how low the neutrophil goes after treatment. If the dose from the last treatment caused a big plunge in your dog’s neutrophil numbers, we will change the dose to a lower one or skip that agent in the future.
This is a really important thing for you to understand: there is no “one dose” that we always use for every cancer case. Your dog’s own response to the treatment helps us decide how to treat next!
We are constantly balancing. We want to give an effective dose of chemo, AND we don’t want the white blood cell (neutrophil) count to get too low. That’s the balance of chemotherapy.
It’s important to understand that this test, the white blood cell count, is the main thing we use to find your dog’s limit for chemo treatments. The technical way to say this is that neutrophils are the “dose-limiting toxicity” for most chemotherapy drugs.
In the next article, I will discuss how and when I monitor the white blood cell count during treatment. I will also cover ways to minimize the chance of infections in dogs getting chemotherapy.
Until then, be sure to check out more about chemotherapy and its side effects in the Dog Cancer Survival Guide.
All my best,
Dr. Sue, Cancer Vet
Sue Ettinger, DVM. Dip. ACVIM (Oncology). Dr. Sue is a boarded veterinary medical cancer specialist. As a Diplomate of the American College of Veterinary Internal Medicine (Oncology), she is one of approximately 400 board-certified veterinary specialists in medical oncology in North America. She is a book author, radio co-host, and an advocate of early cancer detection and raising cancer awareness. Along with Dr. Demian Dressler, Dr. Sue is the co-author of The Dog Cancer Survival Guide: Full Spectrum Treatments to Optimize Your Dog’s Life Quality and Longevity.