Leukocytes, or white blood cells, in the blood of mammals include neutrophils, lymphocytes, monocytes, eosinophils, and basophils. These cells vary with regard to where they are produced, how long they circulate in the bloodstream, and the factors that stimulate them into going in or out of the intricate network of tiny blood vessels that branch out through the tissues of the body. 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.

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

Neutrophilia is an increase in the number of neutrophils in the bloodstream and is caused by inflammation. Structural changes in neutrophils may occur during severe inflammation and are referred to as toxic changes. 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. Adverse reactions to drugs may result in neutropenia or even pancytopenia (a reduction in red and white blood cells and platelets) in cats. Feline leukemia virus has also been associated with neutropenia.

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 such as heartworms or fleas. Eosinophilia also may occur with inflammation of the intestines, kidneys, lungs, or skin. Hypereosinophilic syndrome—with persistent and excessive levels of eosinophils, which accumulate in various organs—has been reported in cats. The cause is unknown. Diagnosis may require several blood tests. Less commonly, eosinophilia may be associated with cancer. In some cats, eosinophils collect in skin or mouth sores. A decrease in eosinophils is known as eosinopenia. It is a common reaction to stress or treatment with corticosteroids.

Leukemia and Lymphoma

Leukemia is a malignant cancer that is characterized by an increase in abnormal white blood cells in the bloodstream. Lymphoma is a related cancer of certain white blood cells that begins in a lymph node or other lymphoid tissue. Leukemia should be considered a potential cause when there is an increase in the number of white blood cells in the bloodstream (see Disorders Affecting Multiple Body Systems of Cats: Feline Leukemia Virus and Related Diseases).

Chédiak-Higashi Syndrome

This inherited syndrome occurs in Persian cats (and in humans). There is an increased susceptibility to bacterial infections due to impaired white blood cell function, an increased tendency to bleed due to platelet defects, and partial lack of color in the eyes and skin due to abnormal melanin (pigment) distribution. Diagnosis is based on abnormal skin color, presence of abnormal white blood cells, and increased susceptibility to infections.

Pelger-Huët Anomaly

This condition is characterized by the failure of certain white blood cells (granulocytes) to mature normally. White blood cell function is normal, and many cats do not have any signs of illness. In some animals, it is deadly and associated with skeletal deformities and increased susceptibility to infection.

Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; David J. Waltisbuhl, BASc, MSc; Michael Bernstein, DVM, DACVIM; Karen L. Campbell, MS, DVM, DACVIM, DACVD; Nemi C. Jain, MVSc, PhD; Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM