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.
An increase in white blood cells may occur as a result of exercise or excitement. This response, which is known as physiologic leukocytosis, is caused by increased epinephrine (the hormone adrenaline). Excitement may double the total white blood cell count within minutes. In addition, contraction of the spleen releases white blood cells and red blood cells into the bloodstream. An increase in lymphocytes (lymphocytosis) may also be present, especially in young horses.
Neutrophilia is an increase in the number of neutrophils in the bloodstream and is usually caused by inflammation. Structural changes in neutrophils may occur during severe inflammation and are referred to as toxic changes. Neutrophilia can occur in horses with equine viral arteritis and in those being treated with corticosteroids.
Neutropenia is a decrease in the number of neutrophils in the bloodstream. It may occur due to the white blood cells sticking to the walls of damaged blood vessels, destruction of neutrophils, reduced formation in the bone marrow, or if the demand for neutrophils exceeds their production in the bone marrow. Neutropenia may occur in all species during overwhelming bacterial infections. Destruction of neutrophils due to an immune response occurs in animals, and tests have been developed to detect antineutrophil antibodies in horses. Injuries to developing blood cells in the bone marrow can also result in neutropenia or even pancytopenia (a reduction in red and white blood cells and platelets). This can occur due to drug reactions, radiation therapy, toxic plants, bone marrow cancer, and viral infections (such as influenza, equine herpesvirus, and equine viral arteritis).
Eosinophilia is an increase in the number of eosinophils, which are involved in allergic reactions and in controlling parasites. Increases are caused by substances that promote allergic reactions (for example, histamine) and by certain antibodies. Eosinophils increase during infections with parasites and sometimes during inflammation of the intestines, lungs, or skin. A decrease in eosinophils is known as eosinopenia. It may occur due to stress or treatment with corticosteroids in horses.
Lymphocytosis is an increase in the number of lymphocytes in the bloodstream. It can be caused by excitement (see physiologic leukocytosis, above), vaccination, and leukemia, which is a cancer of the immune system. Foals normally have more lymphocytes than adults. Occasionally, lymphocytosis can occur due to stimulation of the immune system by infections, longterm diseases, or certain hormones.
Lymphopenia is a decrease in the number of lymphocytes. It is most commonly caused by corticosteroids (either those released in the body due to stress or given as treatment for a disease). Lymphopenia is also rarely caused by other conditions, such as decreased production of lymphocytes, some viral infections (such as equine viral arteritis), and hereditary diseases (such as combined immunodeficiency disease of Arabian foals).
Monocytosis is an increase in monocytes and is associated with inflammation, especially longterm inflammation, and, sometimes, the use of corticosteroids.
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.
Also see professional content regarding white blood cell disorders.