Several different types of white blood cells, or leukocytes, are found in the blood of mammals, including neutrophils, lymphocytes, monocytes, eosinophils, and basophils. These cells vary with regard to where they are produced and where and how long they circulate in the bloodstream. The normal numbers of each type of white blood cell also vary among species. Leukocytosis is an increase in the total number of circulating white blood cells; leukopenia is a decrease. Leukopenia in horses occurs in equine herpesvirus infections, equine ehrlichiosis, influenza, and sometimes during the early stages of equine infectious anemia.
In addition to an overall increase or decrease in white blood cells, increases or decreases in each type of white blood cell can lead to—and help diagnose—-disorders. Leukograms are blood tests that count the number of different white blood cells circulating in the bloodstream. By counting the cells and examining their form your veterinarian gains valuable information that can help diagnose a wide variety of disorders.
Leukemia and Lymphoma
Leukemia is a malignant cancer that is characterized by an increase in abnormal white blood cells in the bloodstream. It is rarely seen in horses. The severity of leukemia varies. Acute (short-term) leukemia often causes body-wide signs of illness and has a poor outlook. These animals have abnormal, immature white blood cells in the blood, as well as decreased numbers of other normal types of blood cells (red blood cells or platelets). In contrast, a chronic (long-lasting) leukemia often causes few if any signs and may be discovered by chance, have a longer course, and be more responsive to treatment. These horses usually do not have deficiencies of other types of blood cells. Your veterinarian may be able to estimate your horse's outlook based on the findings of laboratory tests and recommend an appropriate treatment.
Lymphoma (lymphosarcoma) is a related cancer of certain white blood cells (lymphocytes) that begins in a lymph node or other lymphoid tissue (such as the spleen). However, lymphocytes are present in all organs, and lymphoma can develop anywhere in the body. The cancer involves the spleen in as many as 37% of affected horses and involves the liver in 41% of cases. Lymphosarcoma is the most common blood cell cancer in horses. If laboratory tests show the presence of immature white blood cells in the blood, your veterinarian will likely want to look for cancer in other parts of the body. Signs of lymphosarcoma vary, but nonspecific signs (weight loss, decreased appetite, and lethargy) may be seen early on. Lymphoma is most likely to occur in horses that are 5 to 10 years old.
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Also see professional content regarding white blood cell disorders.
