Oncology Update: Canine Leukemia

Oncology Update
Central Toronto Veterinary Referral Clinic
Kevin Finora DVM, Diplomate ACVIM (Oncology and Small Animal Internal Medicine)

Canine Leukemia

Leukemia is an immune system cancer and is the most common form of blood cancer reported in dogs. Many different types of canine leukemia have been reported. However, the most common form of leukemia is lymphoid leukemia. Lymphoid leukemia is defined as an abnormal expansion of the lymphocyte population in the blood. In most cases the site of the neoplastic lymphocyte production is the bone marrow, though in rare circumstances, the spleen may be the site of production. Leukemia is essentially “lymphoma of the blood.” Lymphoid leukemia is noted most commonly in one of two forms; acute lymphoblastic leukemia (ALL) and chronic lymphocytic leukemia (CLL).

ALL is the more common form. ALL is a disease of young to middle age dogs, with the median age of onset being 5.5 years. ALL is characterized by the sudden and rapid production of large lymphoblastic cells. Dogs with ALL are usually clinical ill, with a rapid onset of clinical signs noted. Clinical signs include weakness, lethargy, anorexia, vomiting, diarrhea and sometimes bone pain. In ALL the bone marrow is almost exclusively involved in lymphoblast production. As a result there may be decreased production of the other cell lines produced in the bone marrow potentially resulting in anemia, thrombocytopenia or neutropenia. The development of cytopenias secondary to bone marrow myelophthisis, is a complication which will carry additional risks for the patient. A diagnosis of leukemia is made by evaluation of the peripheral blood, and at times flow cytometry and/or bone marrow cytology. There are situations when it can be difficult to distinguish between ALL and CLL and in these circumstances flow cytometry can be used looking for CD34 expression. CD34 is an ALL marker and expression of CD34 is associated with a poorer prognosis.

canine leukemiaALL is difficult to treat. Aggressive therapy is absolutely necessary. A modified CHOP based protocol is usually recommended. The response to therapy is typically good but quite short lived. Initially the lymphocyte count will drop but will rarely ever return to normal levels. Very few dogs actually survive to complete a typical 25-week treatment protocol. The prognosis for ALL is grave to poor with median survival times surpassing 5 months rarely reported.

CLL is less much common compared to ALL. CLL is a slowly progressive form of leukemia. The neoplastic lymphocytes are well differentiated and appear to have identical morphology to normal lymphocytes. The neoplastic lymphocytes are small in size, as compared to the large blastic cells seen in ALL. CLL is typically a disease of older dogs, with the average age of onset generally between 10 and 12 years. CLL is often an incidental finding noted on routine blood work and the dogs are rarely clinically ill. On a CBC an elevated lymphocyte count may be accompanied by a mild decrease in the other cell lines. Pathology review of the blood smear will usually comment on the normal appearance of the circulating lymphocytes. Most cases of CLL are T-cell in origin and are slowly progressive. Dogs with CLL live much longer than dogs with ALL. Once diagnosed, dogs with CLL should have their CBC monitored on a monthly basis. The point at which therapy should be initiated is unclear and controversial. I will delay initiating therapy until the lymphocyte counts increase to over 60, 000, or until clinical signs of illness become apparent (despite the lymphocyte count). Many dogs may take a year or more for their count to increase to a level where treatment is recommended. Once therapy is started, oral therapy with prednisone and chlorambucil is recommended. Treatment of CLL is palliative and complete remission is not expected. The lymphocyte count will drop and in many cases can return to normal. The median survival time for CLL, once treatment has been initiated, is 12 months with 30% of dogs living beyond 2 years.

Dr. Kevin Finora is a Board Certified Oncologist and Small Animal Internist who is part of the Healthcare Team at the Central Toronto Veterinary Referral Clinic. He is available for referrals and consultations Monday to Thursday (including Monday and Tuesday evenings). Please contact him with any oncology questions or concerns.

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