Chronic Lymphocytic Leukemia in Dogs

By Krista Williams, BSc, DVM, CCRP

What is chronic lymphocytic leukemia?

Chronic lymphocytic leukemia (CLL) is a slow-progressing, uncommon cancer of lymphocytes (a type of white blood cell). It can originate in the bone marrow or the spleen and affect both cats and, more commonly, dogs. This disease can develop at any age but is more typically detected in middle-aged to senior dogs. It seems to be more prevalent in German Shepherds and Golden Retrievers.

There are three types of CLL: T cell origin, B cell origin, and atypical CLL.

What causes chronic lymphocytic leukemia?

The reason why a particular pet may develop CLL, or any tumor or cancer, is not straightforward. Very few tumors and cancers have a single known cause. Most seem to be caused by a complex mix of risk factors, some environmental and some genetic or hereditary. In the case of CLL, no direct cause is known.

What are the clinical signs of chronic lymphocytic leukemia?

Clinical signs are subtle and can include lethargy, reduced appetite, weight loss, fever, and increased drinking and urination. Often, there are no signs, and the disease is detected on a routine blood test. On physical examination, your veterinarian may detect mildly enlarged lymph nodes or an enlarged liver or spleen.

"...your veterinarian may detect mildly enlarged lymph nodes or an enlarged liver or spleen."

Rarely, patients with CLL will develop aggressive lymphoma called Richter’s syndrome. In these patients, severe lymph node enlargement, coughing, vomiting, weight loss, and neurologic signs can develop.

How is chronic lymphocytic leukemia diagnosed?

Although an elevation in lymphocytes in the blood is a hallmark of CLL, this can also be caused by many other diseases. If the elevation is persistent and other diseases have been ruled out by additional diagnostic tests, your veterinarian may recommend performing another test to see if the lymphocytes are identical, indicating that cancer is likely, or if they are varied, indicating the presence of an immune-stimulating factor or disease. This test is called PARR analysis.

Anemia (low red blood cell count) may be seen when CLL Is initially diagnosed and will worsen as the disease progresses.

Other tests involve obtaining samples from the enlarged spleen or liver or from bone marrow and sending them to a pathologist to assess for characteristics of cancerous lymphocytes. This is called cytology or histopathology.

How is chronic lymphocytic leukemia treated?

CLL is treated with immunosuppressive medications, such as chlorambucil (Leukeran®) and prednisone. Chemotherapeutic medications may be used depending on the severity of the clinical signs.

"Chemotherapeutic medications may be used depending on the severity of the clinical signs. "

What is the prognosis?

CLL generally progresses very slowly, and treatment is often only initiated after clinical signs are observed or when high lymphocyte numbers are detected on blood work. The response to treatment is usually quite good, although remission is rare. The average survival time after treatment is initiated has been shown to be one to three years. Dogs with the T-cell type have a better prognosis than dogs with the atypical type of CLL.

Unfortunately, dogs diagnosed with Richter’s syndrome have a poor prognosis for survival.

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