The easiest way to describe a leukemia is that it is a cancer of blood cells. Blood is made up of red blood cells, white blood cells, and platelets. The red blood cells are, as their name implies, red. They are what give the blood its color and their job is to carry oxygen to the body. White blood cells aren’t necessarily white, but they will look that way in a blood tube. The different white blood cells are neutrophils, eosinophils, basophils, monocytes, and lymphocytes. Each of these cells have a different job, but they all have a role to play in the immune system. Platelets are very small cells, whose job is to form clots to stop bleeding.
Leukemias start in the bone marrow, because this is where blood cells are made. To understand the different leukemias, we first need to understand how blood cells are formed. This is a process known as hematopoiesis, and the image below is a great visual of how this occurs. I know it looks confusing, but it really is simpler than it looks. One step that the chart is missing, is the formation of a pleuripotent stem cell, which can be considered the mother cell of all blood cells. This pleuripotent stem cell receives signals from the body that will tell it to change into a myeloid or lymphoid stem cell. From there, the stem cells will receive additional signals that tell them which specific cell they need to change into. Once they are mature, they stop growing/changing, and the clock starts on their, often short, life span.
There are a couple of ways to describe a leukemia. One way to say is that the leukemia is either acute or chronic. Acute leukemias arise very fast and progress quickly, while chronic leukemias get worse over a long period of time and can be present for many months, and sometimes years, before they are detected or cause symptoms. The acute leukemias arise from the stem cells or blast cells, shown in the image above, while the chronic leukemias arise from the more mature cells at the bottom of the image. Leukemias are also grouped by the cell lines that are affected. The two groups are lymphocytic leukemia, which arises from lymphocytes, or myelogenous leukemia, which arises from the other white blood cells, red blood cells, or platelets. Overall, lymphocytic leukemias occur more frequently than myelogenous leukemias.
The four main types of leukemia are acute lymphoblastic leukemia (ALL), chronic lymphocytic leukemia (CLL), acute myelogenous leukemia (AML), and chronic myelogenous leukemia (CML). There are other rare forms of leukemia that occur in humans, but as these are virtually non-existent in our pets, we will leave these types out of the discussion.
In our pets, leukemias occur rarely. The lymphocytic leukemias occur more commonly than the myelogenous leukemias, and they are more often chronic than acute. Also, leukemias affect dogs more often than cats. As for what causes leukemias in pets, the only true factor that has been identified is feline leukemia virus (FeLV) infection in cats. Dogs that are affected with an acute leukemia tend to be younger, while dogs that develop chronic leukemia tend to be older.
The clinical signs of leukemia in pets tend to be vague and non-specific. Vomiting, diarrhea, weight loss, a decreased appetite, and even enlarged lymph nodes may be present. Pets with acute leukemia are much more likely to be sick than those with chronic leukemia. In fact, many pets with chronic leukemia exhibit no signs of illness. Their disease is often found when routine blood work is performed for some other reason, such as a required or elective surgery, or during their senior wellness exam.
The main indication that a pet has a leukemia is a significantly increased white blood cell count (WBC) on a complete blood count, or CBC. Once this is found, the veterinarian must determine what the cause is. Possible causes include a viral or bacterial infection, other cancers such as lymphoma, or severe inflammation. If leukemia is the cause of the increased WBC, and it is severely affecting the bone marrow, the other cells in the blood may be decreased. This occurs when the leukemia cells take over the bone marrow and leave little or no room for the normal cells to be made.
Finding an increased WBC will not be enough by itself to diagnose a pet with leukemia. Once the increase is found, the abnormal cells must be identified. One of the best ways the veterinarian has to do this is with flow cytometry. A flow cytometry machine is able to determine the size and complexity of the cells present, as well as detect certain markers on the cells. Once these values are known, the machine will then be able to identify the cells. We know how many of each cell type should be present in the blood, and if the majority of cells present in the sample are of one specific type, or have a specific cell marker, we can then identify what type of leukemia is present. This is usually the absolute best way to diagnose a leukemia, and is essential in determining what the pet’s prognosis is as well as the best treatment for their disease. Often, a bone marrow sample will also be examined. This will allow the veterinarian to determine if adequate numbers of normal cells are still present in the marrow. If so, treatment can be initiated with less worry that the pet will not produce an adequate number of the normal cells. If not, patient treatment becomes more risky. If there aren’t enough normal cells being produced, the patient will quickly lose their normal cells as they die off and not be able to replace them. This will put the patient at a greater risk of anemia, bleeding problems, or infections.
Once the type of leukemia is known, the veterinarian will be able to determine the best treatment. Acute leukemias are very difficult to treat as they have a low chance of responding to chemotherapy treatment, and even with aggressive chemotherapy, the patient’s life expectancy is usually only a few months. Chronic leukemias may not need treatment at the time they are diagnosed. Treatment is usually indicated when their cell counts are increased to a certain level, they are sick from the disease, or they have decreases in the numbers of other normal cells such as the red blood cells or platelets. Chronic leukemias tend to respond well to less aggressive chemotherapy treatment, and the most widely used protocols are a combination of an oral chemotherapy drug given at home along with a steroid, such as prednisone. The life expectancy of many patients with chronic leukemias can approach two years.
The overall take home message I want you to get about leukemia in pets is that it can be a very treatable disease, and some pets will have a great quality of life for a long period of time. Pets with CLL tend to have the best prognosis, and pets with any of the other types of leukemias tend to have poorer outcomes. Routine blood work during a senior wellness exam is important, as this may allow your veterinarian to detect the disease earlier, which may lead to an improved outcome.