Myeloproliferative Disorders in Dogs

Myeloproliferative disorders (MPD’s) in canines are abnormal tissue growths (or tumors) within the blood cells made in the bone marrow and are considered to be cancerous.  There are two types of MPD’s. Chronic myeloproliferative neoplasm occurs when there is an excess amount of blood cells (red, white) or platelets (used for blood clots) being produced. The second type is called acute myeloid leukemia, where there are too few healthy, mature, blood cells being produced in the bone marrow and released into the peripheral bloodstream to circulate through the central nervous system and other major organs of the body. The disastrous result of having either of these diseases is that the cells are unable to develop and mature into healthy formed blood cells, a process that is called hematopoiesis.

Myeloproliferative disorders are characterized by abnormalities within the bone marrow that result in excess quantities of specific blood cells, (red or white) and/or thrombocytes (platelets known as blood clotting cells). Acute myeloid leukemia is characterized by having typically an insufficient amount of healthy white blood cells (myelocytes and lymphocytes) within the liver, spleen, and bone marrow.

Myloproliferative diseases

• Lethargy
• Weakness
• Lack of appetite
• Repeated infections
• Abnormal growth of spleen and liver

Acute Myeloid Leukemia

• Anemia (low red blood cell count)
• Depression
• Neurological issues
• Fever
• Anorexia
• Thirst
• Frequent urination
• Recurrent infections
• Bruising easily 
• Hemorrhaging 
• Abnormal growth of spleen and liver
• Inflammation of lymph nodes
• Aggressive behavior

Types

There are two types of MPD’s, chronic myloproliferative disease and acute myeloid leukemia.  Both can be classified as conditions that result in abnormal hematopoietic functioning, where there are too many of a specific types of blood cells or platelets (CMD), or too many unhealthy and abnormal blood cellular components that are regenerated and released into the peripheral blood stream (AML).

Myeloproliferative diseases do not have any pre-determining factors that pertain to a specific breed that may cause susceptibility.  The conditions are considered to be very uncommon. With chronic myeloproliferative neoplasm, the cause is the proliferated production of a specific type of blood cell. In short, the bone marrow consists of too many blood cells.

• White (that fight off disease and infection) called chronic myelogenous leukemia
• Red (carry and circulate O2 throughout the body) called polycythemia vera
• Platelets (responsible for blood clotting) called thrombocythemia 

In the case of acute myeloid leukemia, massive amounts of hematopoietic stem cells (young, and immature cells) develop and regenerate abnormally.  Therefore, there is a deficiency of healthy blood cells within the bone marrow, and these large numbers are released into the peripheral bloodstream.

• Neutropenia (low white cell count that include lymphocytes and myelocytes)
• Anemia (low red blood cell count)
• Thrombocytopenia (low platelets)

A  complete blood count will be conducted to determine the level of platelets in your dog’s blood, and to determine if your dog has the ability to form blood clots.  A urinalysis and biophysical profile are routine procedure. Using a blood smear/film to analyze blood under a microscope is extremely useful in identifying neoplasm (abnormal tissue growth) cell growth. However, a more invasive and even more expensive measure is to conduct a bone marrow aspiration, whereby a small hollow needle and a syringe are used to take a liquid sample of bone marrow. A bone marrow biopsy or core is when a larger needle is used to evaluate the marrow itself. To have 30% or more blast cells (undifferentiated blood cells) contained within bone marrow is sufficient enough to be determined as having acute myeloid leukemia.

Treatment for MPD’s include:

• Hydroxyurea (Anti-cancer chemotherapy medication)
• Blood plasma transfusions
• Antibiotics to reduce infection

Treatment for Polycythemia Vera include trying to reduce the red blood count by thinning and diffusing the blood.

• Phlebotomy (where an IV is inserted to remove a certain amount of blood volume)
• Hydroxyurea medication (it slows bone marrow production of blood cells in the body)

Treatment for Thrombocytosis may include  the steps listed below.

• A partial or whole platelet blood transfusion
• Hydroxyurea medication

Treatment for AMC’s must be aggressive and it has been found that chemotherapy drugs such as Vinblastine, Vincristine, and Doxorubicin, may give a better chance  of remission for your pet though prognosis is usually poor, and outcomes are not typically successful. Chemotherapy drugs are typically given 4-6 times over a 3-4 week period and are usually given by injection subcutaneously or intravenously. 

Using the drug Chlorambucil (an immunosuppressant drug that weakens the ability of the immune system) is effective in treating cancer in that it prevents the abnormal tissue growth from replicating.  The therapy used to support and counter the disease also include supplemental nutrition, fluids, blood plasma transfusions (to counter conditions such as anemia), and antibiotics (for pets that have a low white blood cell count).

Prognosis for remission is not likely and rare.  Many veterinarians focus on the “quality of life” not “quantity of life”.  Most dogs that are diagnosed and given no treatment, or even those who have undergone surgery only live an additional 2 months. Dogs that have undergone chemotherapy have a 50% chance of living for up to one year, and that declines to 20% who survive 2 years. Expect to have regular visits and follow-up appointments to see if the medications and treatments are working. The side effects of chemotherapy medications include emesis, diarrhea, and loss of appetite. In addition, X-rays, CT-scans and ultrasound images will be conducted to screen your pet to ensure your veterinarian that the chemotherapy or procedures have worked. Rest assured that your veterinarian will take the utmost care with your pet to ensure that he is comfortable and free of discomfort as the treatment progresses.