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.

Disorders Related to Increased or Decreased White Blood Cells

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, or reduced formation 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. Adverse reactions to drugs may result in neutropenia or even pancytopenia (a reduction in red and white blood cells and platelets).

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, kidneys, lungs, or skin. A decrease in eosinophils is known as eosinopenia. It is a common reaction 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 certain hormones, stimulation of the immune system by infections, chronic diseases like arthritis, and leukemia, which is a cancer of the immune system. Lymphopenia is a decrease in the number of lymphocytes. It is most commonly caused by corticosteroids (either those naturally occurring in the body or given as treatment for a disease). Lymphopenia may also be caused by other conditions, such as decreased production of lymphocytes, some viral infections (such as equine viral arteritis), and hereditary diseases.

Monocytosis is an increase in monocytes and may be associated with longterm inflammation and the use of corticosteroids.

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. Lymphoma is a related cancer of certain white blood cells that begins in a lymph node or other lymphoid tissue. It is most likely to occur in horses that are 5 to 10 years old.

Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; Nemi C. Jain, MVSc, PhD; Susan M. Cotter, DVM, DACVIM (Small Animal, Oncology); Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM