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Leukemia is a cancer of blood cells, and it can show up within the circulatory system as well as in the bone marrow, where blood cells develop. Leukemia symptoms occur in both dogs and cats, and is especially common in cats, accounting for one-third or more of all feline cancer cases. Most --but not all -- cases of leukemia in cats are associated with infection by the feline leukemia virus (FeLV). Leukemia can affect dogs and cats of any age, but as with most cancers, it is more common in middle-aged to older pets.

Leukemia can affect any of the cells in the blood, including the red blood cells (which provide oxygen), the white cells (which fight inflammation and infection), and the platelets (which help with clotting).  However, the vast majority of cases arise directly from white blood cells.  Leukemias are generally divided into two classes based on the type of white cell involvement.  The predominant cells involved with lymphocytic leukemia are the lymphocytes, whereas granulocytic leukemias involve granular white cells such as neutrophils.  

In leukemia, cancerous blood cells in the bone marrow multiply out of control. In some cases, these cells remain confined to the bone marrow, but in many cases they can escape into the circulating blood and other tissues, such as the liver and spleen. This massive multiplication of one type of cell disrupts the normal immune system, and can crowd other blood cells out of the bone marrow, leading to anemia and/or bleeding problems.  

The level of illness produced by leukemia depends largely on how rapidly the cancerous blood cells are multiplying. Acute cases (sometimes referred to as "blastic" forms) are associated with rapid cell production and serious illness. Pets with acute leukemia usually have a rapid onset of vague signs of illness, including loss of appetite, fatigue, weight loss, breathing difficulty, and (possibly) lameness or gastrointestinal disturbances. The gums of affected pets are often pale because of anemia, and the liver, spleen, and/or lymph nodes may be enlarged. In the chronic form of leukemia, the cancerous blood cells multiply slowly and affected pets often go for long periods with few signs or symptoms. However, this can change rapidly if the chronic form shifts to a more acute form of the disease.

Leukemia is usually diagnosed by blood tests, but bone marrow samples may also be needed.  White blood cell counts can be many times higher than normal, with malignant cells sometimes visible within the circulating blood, although in some cases, the white blood cell count can be normal or even low. Your veterinarian may also want to perform other tests, such as x-rays, to look for signs of involvement in the abdomen or chest. All cats should also be tested for FeLV infection.  

Pets with the chronic form of leukemia may need little immediate treatment aside from vitamins and other medications to promote bone marrow and immune function. Repeated blood tests are usually performed to monitor the status of chronic leukemias, and chemotherapy may be useful if symptoms develop. Chronic cases often survive for months to a year (or more) after diagnosis.  Acute leukemias may respond initially to chemotherapy, but long-term control is difficult. Most acute cases have a guarded to poor long-term prognosis.  


What is leukemia?

Leukemia is a cancer in which blood cells multiply out of control.

Can any blood cell be involved?

Yes, but most cases involve white blood cells such as lymphocytes or neutrophils.

How is leukemia diagnosed?

Leukemia is diagnosed by examining blood cells in the circulatory system and bone marrow.

How is leukemia treated?

Chronic forms of the disease can often be handled by watchful waiting, with no treatment required unless symptoms develop. Acute forms of this cancer may respond to chemotherapy, although long-term control is difficult.  

Credit: Written and reviewed by John A. Bukowski, DVM, MPH, PhD and Susan E. Aiello, DVM, ELS
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